Guide to Evaluating Wind Loads

SCI PUBLICATION P286

Guide to evaluating

design wind loads

to BS 6399-2:1997

C G Bailey B Eng, PhD, CEng, MIStructE

Published by: In association with:

The Steel Construction Institute Silwood Park Ascot Berkshire SL5 7QN Tel: 01344 623345 Fax: 01344 622944

The Building Research Establishment and The British Constructional Steelwork Association Ltd

P286: Guide to evaluating design wind loads to BS 6399-2:1997

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2003 The Steel Construction Institute

Apart from any fair dealing for the purposes of research or private study or criticism or review, as permitted under the Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the UK Copyright Licensing Agency, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organisation outside the UK.

Enquiries concerning reproduction outside the terms stated here should be sent to the publishers, The Steel Construction Institute, at the address given on the title page.

Although care has been taken to ensure, to the best of our knowledge, that all data and information contained herein are accurate to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time of publication, SCI, BRE, BCSA, ODPM, the authors and the reviewers assume no responsibility for any errors in or misinterpretations of such data and/or information or any loss or damage arising from or related to their use.

Publications supplied to the Members of the Institute at a discount are not for resale by them.

Publication Number: SCI P286

ISBN 1 85942 134 2

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A catalogue record for this book is available from the British Library.


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FOREWORD

The UK wind Code CP 3: Chapter V-2 ‘Code of basic data for the design of buildings. Loading. Wind Loads’ was published in 1970 and allowed designers to obtain estimates of wind loads on typical types of buildings. The Code was revised in 1972 and amended in 1986 and 1990. It was finally withdrawn in March 2002.

CP3-V-2 was replaced by BS 6399-2 ‘Loading for buildings. Code of practice for wind loads’ in 1995. The need to replace CP3-V-2 by BS 6399-2 was mainly due to the vast increase in knowledge of wind loading, compared to that used to draft CP3-V-2, and also the need to reduce the common misinterpretations in the practical application of CP3-V-2. A revised version of BS 6399-2 was issued in 1997 and Amendment 1 was issued in March 2002.

This publication provides guidance on the use of BS 6399-2:1997 (including Amendment 1) and was written by Professor C G Bailey of the University of Manchester of Science and Technology (formerly of the Building Research Establishment), under the guidance of a Steering Committee comprising:

Mr D G Brown The Steel Construction Institute

Mr P Williams British Constructional Steelwork Association Limited

Dr D B Moore The Building Research Establishment

This publication was prepared as part of a contract under the Partners in Innovation Initiative, managed by BRE and funded by the Department for Transport, Local Government and Regions (DTLR, now ODPM) and Corus plc. The project was supported by the British Constructional Steelwork Association Limited and The Steel Construction Institute.


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Contents Page No.

FOREWORD iii

SUMMARY vii

1 INTRODUCTION 1 1.1 Scope of this document 1

2 METHODS FOR DETERMINING WIND LOADS IN BS 6399-2 3 2.1 Basic principles 3 2.2 The codified methods 4 2.3 Directional Method 6 2.4 Standard Method 6 2.5 Hybrid Methods 8 2.6 Choice of method for hand calculation 8

3 PROCEDURES FOR HAND CALCULATION 13 3.1 Required data 13 3.2 Simplified Standard Method 14 3.3 Standard Method 15 3.4 Simplified Hybrid Method 17 3.5 Hybrid Method 19

4 CALCULATION OF DYNAMIC WIND PRESSURES 21 4.1 Dynamic classification 21 4.2 Basic wind speed 21 4.3 Site wind speed 21 4.4 Altitude factor 22 4.5 Topography 23 4.6 Directional factor 23 4.7 Seasonal factor 24 4.8 Probability factors 24 4.9 Effective wind speed 25 4.10 Terrain and building factor 25 4.11 Dynamic pressure 28 4.12 Building orientation 28

5 PRESSURE COEFFICIENTS 30 5.1 External pressure coefficients 30 5.2 Scaling lengths 31 5.3 External standard pressure coefficients for walls 32 5.4 External standard pressure coefficients for roofs 35 5.5 Size effect factor 40 5.6 Localised external pressures 43 5.7 Internal pressures 43

6 DOMINANT OPENINGS 47 6.1 Dominant openings that are likely to be open during a storm 47 6.2 Dominant openings that are likely to be shut during a storm 48


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7 OVERALL LOADS 49 7.1 Frictional drag 51

8 ASYMMETRIC LOADS 53

9 REFERENCES 55

APPENDIX A Worked examples 57

APPENDIX B Simplified uniform net pressure coefficients for portal frames 77 B.1 Localised loads and the design of purlins 77 B.2 Design of side rails 82 B.3 Design of main structural members in portal frames 83 B.4 Design of cladding and fixings 84 B.5 Summary of the application of the simplified net pressure

coefficients 84

APPENDIX C Balance of airflow 87 C.1 Basic principles 87 C.2 Example using balance of airflow 87


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SUMMARY

This publication presents guidance on the use of the design procedures given in BS 6399-2:1997, Loading for buildings. Code of practice for wind loads, to calculate the wind loads on buildings, using methods that are suitable for hand calculations. These procedures include the Standard Method and Hybrid Method, which are presented in the Code. Two new design methods are introduced in this publication, called the Simplified Standard Method and the Simplified Hybrid Method. As the names suggest these methods are simplified versions of the methods given in the Code. The purpose of introducing the additional methods is to present a ‘back-of-the-envelope’ method (the Simplified Standard Method) and a method that is considered to be a suitable balance between calculation effort and economical design (the Simplified Hybrid Method).

The publication presents a concise step-by-step procedure for all the design methods that are suitable for hand calculation. Detailed explanation is provided for each calculation step. Guidance is also presented on the use of pressure coefficients, and the treatment of dominant openings, overall loads and asymmetric loads. The guidance aims to produce economical design while ensuring the design effort is kept to a minimum. Design examples are presented to show the use of the Standard, Simplified Standard and Simplified Hybrid methods.

Throughout this publication, constant reference is made to the clauses and figures in the Code. This publication is not intended to be a ‘stand-alone document’ and designers will need to use it alongside the Code, BS 6399-2:1997.

Guide d’évaluation des charges de vent selon la BS 6399-2 :1997

Résumé

Cette publication est destinée à servir de guide d’utilisation concernant les procédures de dimensionnement données dans la norme BS 6399-2 :1997, charges pour les bâtiments - code de pratique pour les charges de vent, pour calculer les charges de vent sur les bâtiments, en utilisant des méthodes permettant un calcul manuel. Ces procédures comportent la méthode standard et la méthode hybride, qui sont données dans la norme. Deux nouvelles méthodes sont présentées dans la publication : la méthode standard simplifiée et la méthode hybride simplifiée. Comme leurs noms l’indiquent, ces méthodes sont des versions simplifiées de celles données dans la norme. L’objectif est de présenter une méthode très simple (la méthode standard simplifiée) et une autre qui peut être considérée comme présentant une bonne moyenne entre effort de calcul et économie (la méthode hybride simplifiée).

La publication présente, pour chaque méthode, une procédure pas-à-pas permettant un calcul manuel. Des explications détaillées sont fournies pour chaque étape. Il en est de même pour l’utilisation des coefficients de pression, pour la prise en compte des ouvertures significatives et pour les charges globales ou asymétriques. Ce guide permet un dimensionnement économique tout en assurant un effort de calcul limité. Des exemples sont présentés. Ils portent sur l’utilisation des méthodes simplifiées.

Tout au long de la publication, référence est faite aux articles et figures de la norme. Cette publication doit être utilisée en même temps que la norme BS 6399-2 :1997.


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Leitfaden zum Abschätzen von Windlasten nach BS 6399-2:1977

Zusammenfassung

Diese Publikation bietet eine Anleitung zur Anwendung von Berechnungsverfahren nach BS 6399-2:1997, Lastannahmen für Gebäude. Vorschrift für Windlasten, um Windlasten für Gebäude, mittels für die Handrechnung geeigneter Methoden, zu berechnen. Diese Verfahren schließen die Standard-Methode und die Hybrid-Methode ein, die in der Vorschrift vorgestellt werden. Zwei neue Berechnungsmethoden werden in dieser Publikation eingeführt, die sogenannte Vereinfachte Standard-Methode und die Vereinfachte Hybrid-Methode. Wie es die Namen schon andeuten, sind diese Methoden vereinfachte Versionen der Methoden aus der Vorschrift. Der Zweck, diese zusätzlichen Methoden einzuführen, liegt in der Vorstellung einer Kurzfassung (Vereinfachte Standard-Methode) und einer Methode, die als angemessener Ausgleich zwischen Rechenaufwand und wirtschaftlicher Berechnung betrachtet werden kann (Vereinfachte Hybrid-Methode).

Die Publikation stellt ein genaues Schritt-für-Schritt Verfahren für alle Berechnungsmethoden, die sich für eine Handrechnung eignen, vor. Es gibt für jeden Rechenschritt genaue Erklärungen. Eine Anleitung für die Anwendung von Druckbeiwerten, die Handhabung großer Öffnungen und der Ansatz von Gesamtlasten und asymmetrischen Lasten wird ebenso vorgestellt. Der Leitfaden bezweckt eine wirtschaftliche Berechnung und stellt sicher, daß der Rechenaufwand auf ein Minimum reduziert bleibt. Rechenbeispiele werden vorgestellt um die Anwendung der Standard-, der Vereinfachten Standard- und der Vereinfachten Hybrid-Methode aufzuzeigen.

Überall in der Publikation wird Bezug genommen auf die Sätze und Bilder der Vorschrift. Die Publikation soll kein selbständiges Dokument sein, sondern immer zusammen mit der Vorschrift, BS 6399-2:1997, verwendet werden.

Guía para la fijación de cargas de viento de proyecto según BS 6399-2:1997

RESUMEN

Esta publicación da consejos para el uso de los métodos de proyecto contenidos en BS 63992-2:1997, “Cargas en edificios. Norma práctica para acciones de viento”, para calcular las cargas de viento sobre edificaciones mediante métodos adecuados para cálculos manuales. Estos procedimientos incluyen el Método Tipificado y el Método Híbrido que están presentes en la Norma. En esta publicación se incluyen otros dos nuevos métodos de proyecto llamados el Método Tipificado Simplificado y el Método Híbrido Simplificado que, como su nombre indica, son versiones simplificadas de los métodos que incluye la Norma. La idea de introducirlos es suministrar un método fácilmente recordable (el Método Tipificado Simplificado) y otro en el que se considera equilibrado el esfuerzo calculado y el proyecto económico (el Método Híbrido Simplificado).

El Informe presenta un breve método paso a paso en todos y cada uno de los métodos de proyecto adecuados al cálculo normal y se suministran explicaciones detalladas para cada uno de los pasos.

También se presentan consejos sobre el uso de coeficientes de presión y sobre el tratamiento de las aberturas dominantes, cargas globales y cargas asimétricas. El intento es conseguir proyectos económicos con esfuerzo mínimo de cálculo. Se presentan ejemplos para ilustrar el empleo de los métodos precitados.


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A lo largo de toda la publicación se hace constante referencia a las cláusulas y figuras de la Norma, pues no se trata de un documento autónomo sino que los proyectistas deberán usarlo junto con la BS 6399-2:1997

Guida alla valutazione dei carichi di progetto a vento in accordo alla BS 6399-2: 1997

Sommario

Questa pubblicazione costituisce una guida all’utilizzo delle procedure progettuali riportate nella BS 6399-2:1997, Azioni sugli edifici: Regole pratiche per i carichi da vento, per il calcolo dell’azione del vento sugli edifici. Vengono proposti approcci adatti anche per i calcoli manuali.Queste procedure comprendono il Metodo Tradizionale ed il Metodo Ibrido, che sono presenti nella normativa. Vengono, in aggiunta, introdotti nella pubblicazione due nuovi metodi, denominati Metodo Tradizionale Semplificato e Metodo Ibrido Semplificato. Con già indicato dai nomi, questi metodi rappresentano una versione semplificata dei metodi contenuti nella normativa. La finalità di aggiungere ulteriori metodi è quella di presentare sia un metodo pratico di immediata applicazione (il metodo tradizionale semplificato) e un metodo che considera un adeguato compromesso tra complessità di calcolo ed economicità di progettazione (il metodo ibrido semplificato).

La pubblicazione presenta una sintetica procedura incrementale per tutti i metodi per i quali è possibile sviluppare calcoli manuali e viene proposta un’esaustiva spiegazione per ogni fase di calcolo. Viene anche presentata una guida all’utilizzo dei coefficienti di pressione considerando i dettagli pratici relativi alle aperture principali, i carichi globali e condizioni di carico non simmetrico.

La finalità di questa pubblicazione è di contribuire allo sviluppo di una progettazione economica pur mantenendo ridotta la complessità progettuale. Vengono inoltre proposte applicazioni progettuali sull’uso dei metodi Tradizionale ed Ibrido e delle relative versioni semplificate. All’interno della pubblicazione viene fatto costante riferimento alle notazioni ed alle figure della normativa. Questa pubblicazione non intende costituire un documento a sé stante ed i progettisti comunque necessitano della normativa BS 6399-2:1997.

Handledning för bedömning av vindlaster enligt BS 6399-2:1997

Sammanfattning

Denna publikation är en handledning för tillämpning av dimensioneringsmetoderna enligt BS 6399-2:1997, Loading for buildings. Code of practice for wind loads, för att beräkna vindlaster på byggnader genom att använda metoder som är lämpliga för handberäkningar. Procedurerna inkluderar Standardmetoden (the Standard Method) och Hybridmetoden (the Hybrid Method), vilka presenteras i standarden. Två nya dimensioneringsmetoder introduceras i denna publikation: den Förenklade Standardmetoden (the Simplified Standard Method) och den Förenklade Hybridmetoden (the Simplified Hybrid Method). Vilket namnen antyder är dessa metoder förenklade versioner av metoderna enligt standarden. Syftet med att introducera ytterligare metoder är att presentera en approximativ snabbmetod (den Förenklade Standardmetoden), och en metod som anses balansera beräkningsinsats och ekonomisk utformning (den Förenklade Hybridmetoden).


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Publikationen beskriver processen steg för steg för samtliga dimensioneringsmetoder som är lämpliga för handberäkning. Detaljerade förklaringar finns för varje beräkningssteg. Vägledning finns också för användningen av tryckkoefficienter, beaktande av betydande öppningar, övergripande laster och asymmetriska laster. Handledningen syftar till att generera en ekonomisk design samtidigt som beräkningsinsatsen minimeras. Beräkningsexempel presenteras för att visa användningen av Standardmetoden, den Förenklade Standardmetoden och den Förenklade Hybridmetoden.

Genom hela publikationen refereras till paragrafer och uppgifter i standarden. Denna publikation syftar inte till att vara ett fristående dokument, utan man behöver använda den tillsammans med standarden BS 6399-2:1997.


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1 INTRODUCTION

The behaviour of wind and the forces it generates is an extremely complex subject and one that is not easily incorporated into simplified design methods. To obtain an estimate of the likely wind loads on a building, the wind speed together with the resulting wind pressures need to be estimated. The wind speed is predominantly dependent on the direction of the wind and the type of surrounding terrain. The resulting wind pressures on the building are dependent on the wind speed and the geometry of the building.

The UK wind Code, BS 6399-2:1997, Loading for buildings. Code of practice for wind loads[1] presents a number of design methods that allow the designer to consider the variation of the parameters that affect the wind pressures on a proposed building. All the design methods are based on the same principles and only differ by the extent to which the designer wants to consider the variation of each parameter that affects the wind speed and pressure over the building’s envelope.

This Guide is intended to aid the designer when hand calculations are used to estimate wind pressures. Guidance is provided to simplify the procedures given in BS 6399-2.

1.1 Scope of this document This publication provides guidance that is to be used in conjunction with the published Standard; continual reference is made to clauses, tables and figures given in BS 6399-2. It is not intended to be a ‘stand-alone document’.

Section 2 of this publication discusses the basic principles of using the design procedures in BS 6399-2, identifying the Standard Method and Hybrid Method as procedures that can be used for hand calculation. Two new methods are introduced, called the Simplified Standard Method and Simplified Hybrid Method. As the names suggest these new methods are simplified versions of the design procedures given in the Code. The purpose of introducing two further methods is to present a ‘back-of-the-envelope’ calculation method (Simplified Standard Method) and a method that is considered to be a suitable balance between calculation effort and economical design (Simplified Hybrid Method). Some guidance is given in Section 2 on the choice of method used for hand calculation.

The data required to determine wind pressures is discussed in Section 3, together with recommendations for obtaining the data. The design procedures for the hand calculation methods, discussed in Section 2, are presented in concise step-by-step procedures. Cross-reference to other Sections in the publication is included, for more detailed guidance.

Section 4 provides a detailed explanation of the parameters used to estimate the dynamic wind pressures on a building. Detailed recommendations are presented on the evaluation of each parameter. Section 5 discusses the use of the pressure coefficients, which define the wind loads on the whole structure and individual elements.


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Dominant openings, overall loads and asymmetric loads are discussed in Sections 6, 7 and 8, respectively. Guidance is provided on how to deal with these aspects of the design to ensure economic designs are obtained from carrying out simple hand calculations.

Worked Examples are presented in Appendix A, which uses the Standard Method given in the Code. The same worked example is also carried out using the Simplified Standard Method and the Simplified Hybrid Method, which are introduced in this publication.

Simplified net pressure coefficients, for single storey buildings, are presented in Appendix B. Using these pressure coefficients will reduce the required calculation effort, but will result in more conservative designs compared to designs that use the coefficients in the Code.

The calculation of internal pressures, by using the basic principle of balance of airflow, is discussed in Appendix C. By using the balance of airflow, more economical designs can be achieved, but the design is significantly more complex than using the tabulated data in the Code.


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2 METHODS FOR DETERMINING WIND LOADS IN BS 6399-2

2.1 Basic principles The Code provides a means of determining the design wind loads for buildings based on the maximum wind speeds that are likely to be experienced at the site of the building. Rules are given for deriving wind pressures on the building and hence the loads for which the structure as a whole and its individual elements need to be designed.

The process of calculating wind loads involves a number of steps, determining the following values:

• Basic wind speed.

• Site wind speed.

• Effective wind speed.

• Dynamic pressure.

• Pressure coefficients.

The basic, site and effective wind speeds depend on many factors, including:

• Location of site in the UK.

• Altitude of the site.

• Direction of the wind.

• Upwind terrain.

• Upwind topography.

• Upwind distance from the sea.

• Spacing and height of upwind obstructions.

Apart from the location and altitude, the above factors will generally vary in different directions around the site.

The Code defines the site wind speed as a mean hourly wind speed in open country exposure at a height of 10 m above ground. The site wind speed is dependent on the location and altitude of the site and varies depending on its direction.

The effective wind speed adjusts the site wind speed to take into account the height of the proposed building and the effects of surrounding terrain, distance from the sea and any upwind obstructions. In effect, it is a gust wind speed that is relevant to the size and environment of the building.

Any effects on the wind speed from the surrounding topography are taken into account by adjusting either the site or effective wind speed, depending on the design procedure adopted.

The dynamic pressure that defines the wind loads on the building is based on the effective design wind speed.


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The wind pressures experienced by the building depend on the direction of the wind in relation to the building and on the geometry of the building. The pressures are determined by multiplying the dynamic pressure by pressure coefficients that are directionally dependent.

To obtain the most accurate estimate of the wind loads on the building, the wind speed, together with the above factors that determine its magnitude, needs to be considered for a significant number of directions around the site. This allows the designer to identify the variation of wind speed around the perimeter of the site. The more wind directions considered, the higher the accuracy in calculating the distribution of wind pressure over the building.

The Code presents a number of design methods that allow the designer to consider either a large number of different wind speeds and pressures over the building or, alternatively, to consider, by adopting conservative simplifications, a limited number of wind speeds and pressures. The choice of the design method will depend on:

• The willingness to conduct the calculations.

• The accuracy with which the designer knows the site-specific factors that affect the wind pressures.

• The design accuracy required in determining the wind pressures.

2.2 The codified methods BS 6399-2 presents two basic methods for determining wind loads for buildings: the Directional Method and the Standard Method. The Directional Method is considered to give the most accurate codified estimate of the wind pressures on a building, considering the variation of wind speed and pressure distributions for a significant number of directions around the building. The method requires repeated calculations, and provides a number of different pressure distributions. Finding the worst pressure distribution in terms of the structural response of the building may not be straightforward and generally each pressure distribution will need to be considered.

The Standard Method is derived from, and is a simpler form of, the Directional Method. It allows the designer to considerably reduce the number of load cases that need to be considered. The method allows the variation of the parameters that govern the wind speed to be considered in a simpler manner and the resulting wind speed is used with simplified and conservative pressure coefficients that represent the pressure distribution over the building. Compared to the Directional Method, the Standard Method significantly reduces the calculation effort but generally results in higher wind pressures.

The Code also allows combinations of some elements of the Directional and Standard Methods. These are called Hybrid Methods and can be used effectively to reduce some of the conservatism in the Standard Method and also to reduce the number of load cases that need to be considered when using the Directional Method.

The design procedures for each of the methods are shown diagrammatically in Figure 2.1. The clause numbers in the Figure relate to BS 6399-2 and the Section numbers relate to this publication.


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In addition to the methods that are explicitly described in the Code, there are further simplifications that can be made; these are described in the relevant Sections below.

Determine dynamic augmentation factor(Clause 1.6) Section 4.1

Obtain basic wind speed(Clause 2.2.1) Section 4.2

Define terrain categories and effective heights(Clause 1.7.2 and 1.7.3) Sections 4.10.1 & 4.10.3

Calculate site wind speed(Clause 2.2.2) Section 4.3

Standard Method(Clause 2.1) Section 2.4

Directional Method(Clause 3.1) Section 2.3

Obtain standard effective wind speed(Clause 2.2.3) Section 4.9

Obtain directional effectivewind speed (Clause 3.2.3)

Section 4.9

Calculate dynamic pressure(Clause 2.1.2) Section 4.11

Calculate dynamic pressure(Clause 3.1.2.1) Section 4.11

Calculate Standard pressurecoefficients

(Clause 2.3) Section 5.0

Calculate Directional pressurecoefficients(Clause 3.3)

Hybrid MethodNo. 1

(Clause 3.4.1)Section 2.5

Hybrid MethodNo. 2

(Clause 3.4.2)Section 2.5

Wind Loads Wind Loads

Hybrid No. 1

Hybrid No. 2

Hybrid No. 2

Hybrid No. 1

Hybrid No. 2 Hybrid No. 1

Determine influence of topography(Figure 7) Section 4.5Depending on the choice of

design method eachparameter needs to beconsidered over a number ofwind directions

Choose design method. Section 2.6

Figure 2.1 Flow chart showing design methods in BS 6399-2

(Clause numbers relate to BS 6399-2, Section numbers relate to this publication)


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2.3 Directional Method The Directional Method involves calculating the effective wind speed in a number of directions around the building. It is common practice to consider twelve directions at 30° intervals around the building, as shown in Figure 2.2. (The direction factor is tabulated in the Code at 30° intervals). For each wind direction, influences such as upwind terrain, topography, obstructions and distance from the sea need to be considered. The wind pressures on the building are directionally dependent, so a set of directional pressure coefficients, at the same intervals, also need to be determined. Because of the number of load cases and the complexity of determining each wind speed and associated pressure coefficients for different directions, the Directional Method is realistically only suitable for evaluation by computer.

2.4 Standard Method The Standard Method incorporates simplifications that reduce both the amount of calculation and the number of load cases to be considered. The method involves determining the wind speed in the orthogonal directions, normal to the faces of the building. The calculation of the wind speeds in these orthogonal directions (defined as covering a range ±45° either side of the normal to the face, as shown in Figure 2.3) are based on the most onerous values of the following factors within each 90° quadrant:

• Directional variation of wind speed (Sd factor).

• Upwind terrain.

• Upwind topography.


• Spacing and height of upwind obstructions.

30°

Wind speed consideredin 12 directions resultingin 12 differentdistributions of pressureover the structure

Wind speed varies indifferent directions

Figure 2.2 Wind speed calculated in twelve directions, resulting in

twelve load cases for the Directional Method


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The pressure distribution over the building is defined using the Standard Method pressure coefficients. These pressure coefficients have been derived from those used in the Directional Method, by considering the most onerous coefficients over a range ±45° either side of the direction normal to the face.

The simplifications introduced into the Standard Method typically result in higher design wind pressures compared to those calculated using the Directional Method.

It is possible to simplify the Standard Method further by calculating the wind speed for a given site exposure using the worst possible combination of parameters occurring in any direction. In this design approach, the most onerous values of each of the factors relating to direction, upwind terrain, upwind topography, upwind obstructions and upwind distance from the sea are combined to determine a single value of effective wind speed. This method will be referred to as the Simplified Standard Method in this publication. Although the calculation procedure in the Simplified Standard Method is kept to a minimum, higher wind loads will usually result, compared to the other design methods, since it is unlikely (although possible) that the most onerous case for the factors that influence the wind speed will all actually occur in the same direction, as is assumed in the Simplified Standard Method.

45° 45°

45°

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Wind speed calculated in eachorthogonal direction based on greatestwind speed over the range ±45° eitherside of the orthogonal direction.

Figure 2.3 Calculation of wind speeds using the Standard Method


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2.5 Hybrid Methods The Hybrid Methods allow combinations of the Standard Method and Directional Method to be used. These Hybrid Methods can be considered as reducing some of the conservatism inherent in the Standard Method without the penalty of carrying out the full range of calculations required by the Directional Method. The Hybrid Methods (Clause 3.4) involve using either:

• Effective wind speeds calculated from the Standard Method, combined with pressure coefficients from the Directional Method.

• Effective wind speeds calculated from the Directional Method combined with pressure coefficients from the Standard Method.

In the first Hybrid Method, the designer uses the Standard Method to calculate effective wind speeds and then uses the pressure coefficients given in the Directional Method. This results, typically, in twelve different pressure distributions for the building. This approach does not reduce the potential conservatism in the calculation of the effective wind speed from the Standard Method, but does reduce the conservatism in the representation of the pressure distribution. This design method may be useful for standard mass-produced buildings that are located throughout the UK. However, this method generally does not offer significant benefits to the designer and is not discussed further in this publication.

The second Hybrid Method is the most useful and allows most of the conservatism in the Standard Method to be reduced, without the need to conduct the full range of calculations required by the Directional Method. This method will be referred to in this publication as ‘the Hybrid Method’. The method requires the determination of the directional effective wind speed in twelve directions (at 30° intervals). The Code allows the twelve directions to be defined as either:

• Starting at North.

• Aligned with respect to the building axes.

• Starting from a line normal to the steepest slope of the significant topographic feature.

However, the need to determine directional effective wind speeds in twelve wind directions, results in repetitive and time consuming calculations.

A useful simplification of the Hybrid Method is to consider, within a 90° quadrant, the most onerous value of each of the directional factors relating to directional wind speed, upwind terrain, upwind topography, upwind obstructions and upwind distance from the sea and to determine an effective wind speed using these values. This method will be referred to, in this publication, as the Simplified Hybrid Method.

2.6 Choice of method for hand calculation The Standard and Hybrid methods are suitable for hand calculations. This publication describes two further methods, the Simplified Standard Method and the Simplified Hybrid Method. Both of these methods reduce the required calculation effort, but can result in higher estimates of wind speeds compared to the respective Standard and Hybrid methods. The four methods are summarised


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below, in order of increasing calculation effort and decreasing potential conservatism.

• Simplified Standard Method, where the most onerous values for all the factors that affect the effective wind speed are assumed irrespective of direction. One value of effective wind speed is calculated and is applied to all orthogonal directions of the building. This method will generally produce higher estimates of wind pressure compared to other design methods, but requires the least amount of calculation effort.

• Standard Method, where the effective wind speed in each orthogonal direction is calculated by considering the most onerous value for all the factors that affect the wind speed over a range ±45° either side of the orthogonal directions. If the orientation of the building is unknown, the most onerous value for the direction factor is assumed.

• Simplified Hybrid Method, where the most onerous values for the factors that affect the wind speed are used to determine the directional effective wind speed (using the approach in the Directional Method) over four 90° directional ranges (quadrants). The method increases the calculation effort slightly compared to the Standard Method, but can significantly reduce the conservative estimates of wind speed obtained when using the Standard Method.

• Hybrid Method, where the directional effective wind speed is calculated (using the approach in the Directional Method) in twelve directions at 30° intervals. The highest effective wind speed in the range ±45° either side of the orthogonal directions is used with the corresponding standard pressure coefficients.

Using one method for all design situations is not practicable. To obtain the optimum method for individual designs, consideration should be given to:

• Time-scale for completion of calculations.

• Capital cost of the project.

• Influence of the terrain, topography, obstructions and distance from the sea and how these vary with wind directional factors (Sd) in different directions around the site.

• Accuracy of the available information that defines the terrain, topography, obstructions and distance from the sea in each direction around the site.

• For ‘back-of-the-envelope’ calculations, the Simplified Standard Method can be used. It may also be argued that this level of calculation is all that is required for buildings with low capital costs.

If the availability or accuracy of information about terrain, topography, obstructions and distance from the sea in each direction is poor, and conservative estimates have to be taken, then the use of detailed calculations may not be justified.

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Although the choice of the optimum method is site specific, a suitable balance between calculation effort and determining economical estimates of the wind pressures can be obtained by using the Simplified Hybrid Method.

The Simplified Hybrid Method is discussed in more detail below.

2.6.1 Simplified Hybrid Method The Simplified Hybrid Method can be used irrespective of whether the orientation of the building is considered or ignored. If the orientation is considered, the directional effective wind speed is determined by considering the most onerous values for the following factors that affect the wind speed over the range ±45° in the orthogonal directions:

• Wind directional factor Sd (that adjusts the basic wind speed).

• Upwind terrain.

• Upwind topography effects.


• Spacing and height of any upwind obstructions.

The calculated effective wind speeds are used with the Standard Method pressure coefficients corresponding to the orthogonal directions, defining the wind pressure and thus the loads over the building.

If the orientation of the building is ignored or unknown, the highest effective wind speed from four 90° quadrants is considered. In each quadrant the most onerous value for all the factors that affect the wind speed is used in determining the effective wind speed for that quadrant. A number of examples for defining the orientation of the quadrants are shown in Figure 2.4. Since the highest effective wind speed from all quadrants is used to determine the pressure on the building, the procedure is not sensitive to the orientation of the quadrants with respect to North. It is difficult to provide guidance on the optimum orientation of the quadrants, since the variation of the factors that affect the wind speed in different directions are site dependent. However, the designer should be able to define the optimum orientation of the quadrants for each site, considering the upwind factors that define the extent and type of terrain, height and spacing of any obstructions, topography effects and the actual variation of the basic wind speed (Sd factor). The calculated maximum effective wind speed from all quadrants, for a given site, is used with the Standard Method pressure coefficients defining the pressure distribution in the orthogonal directions.

When using the Simplified Hybrid Method, lower effective wind speeds can be obtained, in some cases, if the designer chooses to ignore the orientation of the building. For example, considering Figure 2.5, it can be seen that placing the quadrants in relation to the building axes causes the detrimental effect of the inland water or sea to affect the calculation of the effective wind speed in two quadrants. Therefore the design assumption is that the detrimental effect of the inland water or sea, occurs over a 180° range and, when combined with other factors, will produce conservative estimates of the effective wind speed over this range. However, careful positioning of the quadrants (Figure 2.6), ignoring the orientation of the building, can reduce the effective wind speed by ensuring that the detrimental effect of the inland water or sea occurs over just one quadrant. The effect of the inland water or sea is therefore only combined with the other


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factors over this 90° quadrant. It should be emphasised that this approach of positioning the quadrants reduces the conservatism embodied in the Simplified Hybrid Method and will not result in lower design effective wind speeds compared to those calculated using the Directional Method.

Although lower effective wind speeds could be obtained by ignoring the orientation of the building there is a slight disadvantage in that the highest effective wind speed from the four quadrants must be used in both orthogonal directions when using the pressure coefficients to define the pressure distribution. If, however, the building’s orientation is considered, the effective wind speed in each quadrant can be used with the corresponding pressure coefficients in the relevant orthogonal directions. The effect of considering or ignoring the orientation of the building on the resulting pressure distribution on the structure is discussed further in Section 4.12.

The design data required to estimate the wind pressures is discussed in Section 3.1. The design procedures required for the Simplified Standard Method, the Standard Method, the Simplified Hybrid Method and the Hybrid Method are discussed in Sections 3.2, 3.3, 3.4, and 3.5 respectively. The use of the first three of these design methods is shown in the design examples presented in Appendix A.

Four 90° quadrantsaligned to North

Four 90° quadrantsaligned 45° to North

Four 90° quadrants aligned15° to North

Four 90° quadrants aligned30° to North

Figure 2.4 Examples of calculating wind speed over 90° quadrants


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Inland water or sea

Detrimental effect of inland water or sea influences the effective wind speed calculated in two of the four 90° quadrants

Figure 2.5 Positioning of quadrants in relation to the building’s axes

causes the detrimental effect of inland water or sea to affect two quadrants

Inland wateror sea

Careful positioning of the90° quadrants ensures thatthe detrimental effect ofthe inland water or seaaffects only one quadrant

Figure 2.6 Careful positioning of the quadrants ensures that the

detrimental effect of the inland water or sea will affect one quadrant only


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3 PROCEDURES FOR HAND CALCULATION

3.1 Required data The following data is required to obtain an estimate of the wind pressures, irrespective of the design method used.

Required Data Notes

Determine altitude of the site Estimate using 1:50,000 Landranger or 1:25,000 Explorer/Outdoor Leisure Ordnance Survey maps. The altitude is given in metres by contour lines.

Distance to sea For each upwind direction around the site, estimate using UK road atlas or 1:50,000 Landranger Ordnance Survey maps if site is close to sea.

Distance in town

For each upwind direction around the site, estimate using 1:50,000 Landranger or 1:25,000 Explorer/Outdoor Leisure Ordnance Survey maps. This map will also allow the density and extent of buildings to be estimated, to ensure that the criteria given in Clause 1.7.2 for town category are achieved.

Height and spacing of surrounding buildings

For each upwind direction around the site, estimate the height of the surrounding buildings assuming a typical story height of 3 m. If the spacing between the buildings is unknown, assume a value of 20 m for suburban and urban areas.

Topography

For each upwind direction around the site, estimate using altitude contours on a 1:50,000 Landranger or 1:25,000 Explorer/Outdoor Leisure Ordnance Survey maps. Estimate the upwind slope and position of the building in relation to the topographical feature.

Extent of load sharing from structural elements

If unknown, assume no load sharing occurs. Thearea of structure where load sharing occurs needs to be significant to allow a reduction in wind pressures.

Relative permeability of cladding to the building

Estimate the relative permeability between the four walls. If the permeabilities of all walls are within a factor of 3 of each other then assume all walls are equally permeable (Refer to Section 5.7)

Dominant openings

Openings need to be checked to determine whether they are dominant and whether these openings(such as windows and doors) are likely to be shut during a storm. (Refer to Section 6)


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3.2 Simplified Standard Method The Simplified Standard Method procedure uses the most onerous factors affecting the wind speed, irrespective of direction. The calculation effort is kept to a minimum and typically gives higher estimates of wind speed compared to other methods.

Task (Clause No.s relate to BS 6399-2, Section No.s relate to this Guide)

Notes

Determine dynamic augmentation factor


Check applicability of Code. The dynamic augmentation factor is used to increase overall loads on the structure (Refer to Section 7)

Obtain basic wind speed (Clause 2.2.1) Section 4.2

Use the UK ‘Wind Map’ given in Figure 6 of BS 6399-2 to find the basic wind speed for a particular location. Interpolate between the contours.

Determine significance of topography and calculate altitude

factor (Figure 7, Clause 2.2.2.2)

Sections 4.4 & 4.5

For topography to be significant, the upwind slope in any direction around the site must be greater than 5% and the building constructed within the shaded areas shown in Figure 7 of BS 6399-2. The calculation of the altitude factor (Sa) depends on whether topography is significant.

Calculate site wind speed (Clause 2.2.2) Section 4.3

Take values of 1.0 for the directional factor (Sd), seasonal factor (Ss) and probability factor (Sp). Hence Vs = Vb × Sa

Determine terrain categories and effective heights (Clause 1.7.2

and 1.7.3) Sections 4.10.1 & 4.10.3

Determine the most onerous terrain and effective height, considering all directions around the site. Calculate the terrain and building factor (Sb) using Table 4 of BS 6399-2. The use of Table 4 of BS 6399-2 generally results in conservative estimates of the wind pressure for sites in Town. Using the Simplified Hybrid or Hybrid Method can reduce this conservatism. .

Calculate effective wind speed (Clause 2.2.3) Section 4.9

Ve = Vs × Sb

Calculate dynamic pressure (Clause 2.1.2) Section 4.11

2es 613.0 Vq =

Determine standard pressure coefficients (Clause 2.3)

Section 5

Simplified uniform net pressure coefficients given in Appendix B could be used for portal frames and single storey buildings.

Calculate external, internal and net surface pressures (Clause 2.1.3.1, 2.1.3.2, and 2.1.3.3)

Section 5

Using pressure coefficients and size effect factors calculate wind loads on the structure or use the simplified uniform net pressure coefficients in Appendix B.


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3.3 Standard Method The Standard Method is the simplest procedure presented in the Code. If the orientation of the building is unknown it can lead to higher wind pressures compared to the Simplified Hybrid Method and the Hybrid Method.

Task (Clause No.s relate to

BS 6399-2, Section No.s relate to this Guide)

Notes






Define the orthogonal

directions (Clause 2.1.1.1) Section 2.4

The most onerous values for the factors (directional variation in wind speed, upwind terrain and topography effects together with upwind obstructions and distance from the sea) that affect the wind speed are defined over a range of ±45° either side of the orthogonal directions.

Determine significance of topography and calculate

altitude factor (Figure 7, Clause 2.2.2.2)

Section 4.4 & 4.5

For topography to be significant, the upwind slope must be greater than 5% and the building constructed within the shaded areas shown in Figure 7 of BS 6399-2. The calculation of the altitude factor (Sa) depends on whether topography is significant and can therefore vary in the four orthogonal directions considered.


Take values of 1.0 for the, seasonal factor (Ss) and probability factor (Sp). For the directional factor (Sd) take the highest value over the range ±45° either side of the orthogonal directions. If the building orientation is unknown, Sd should be taken as 1.0 (see Clause 2.2.2.3). Hence Vs = Vb × Sa × Sd

Determine terrain categories and effective heights (Clause

1.7.2 and 1.7.3) Sections 4.10 & 4.12

For each orthogonal direction, determine the most onerous case for the type and extent of the upwind terrain and calculate the effective height of the building, taking into account any upwind obstructions.

Calculate terrain and building

factor Sb

Calculate the terrain and building factor (Sb) for each orthogonal direction. This factor is based on the upwind type and extent of terrain and allows for the beneficial effect of upwind obstructions (Refer to Section 4.10). The use of Table 4 of BS 6399-2 generally results in conservative estimates of the wind pressure for sites in Town. Using the Simplified Hybrid or Hybrid Method can reduce this conservatism.


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Obtain effective wind speed in each orthogonal direction (Clause 2.2.3) Section 4.9

Ve = Vs × Sb

Calculate dynamic pressure in each orthogonal direction

(Clause 2.1.2) Section 4.11

2es 613.0 Vq =

Calculate Standard pressure coefficients (Clause 2.3)

Section 5



Section 5

Using pressure coefficients and size effect factors, calculate wind loads on the structure. Alternatively, use the simplified uniform net pressure coefficients in Appendix B.


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3.4 Simplified Hybrid Method For the Simplified Hybrid Method the orientation of building is either considered or ignored, and the wind speed is considered in four 90° directional quadrants.



Notes






Define the four 90° directional quadrants

Section 2.5

The most onerous values for the factors (directional variation in wind speed, upwind terrain and topography effects together with upwind obstructions and distance from the sea) that affect the wind speed are defined over each of the 90° directional quadrants. The directional quadrants can be positioned to coincide with the orthogonal directions of the building or at any rotation chosen by the designer. (Refer to Section 2.6.1)



Section 4.4 and 4.5

For topography to be significant, the upwind slope must be greater than 5% and the building constructed within the shaded areas shown in Figure 7 of BS 6399-2. The calculation of the altitude factor (Sa) is dependent on whether topography is significant and can therefore vary in the four quadrants considered.


Take values of 1.0 for the seasonal factor (Ss) and probability factor (Sp). For the directional factor, take the highest value over the chosen 90° quadrant. Hence Vs = Vb × Sa × Sd


1.7.2 and 1.7.3) Sections 4.10 and 4.12

For each directional quadrant, define the worst case for the type and extent of the upwind terrain and calculate the effective height of the building, taking into account any upwind obstructions.


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Calculate terrain and building factor Sb

Calculate the terrain and building factor (Sb) for each directional quadrant. This factor is based on the upwind type and extent of terrain and allows for the beneficial effect of upwind obstructions (Refer Section 4.10). The procedure given in the Directional Method requires more calculation effort compared to the Standard Method, but gives lower values if in town terrain.

Obtain effective wind speed in each quadrant (Clause 2.2.3) Section 4.9

Ve = Vs × Sb

Calculate dynamic pressure in each quadrant (Clause 2.1.2) Section 4.11

2

es 613.0 Vq =


Section 5



Section 5

Using pressure coefficients and size effect factors, calculate wind loads on the structure. Alternatively use the simplified uniform net pressure coefficients in Appendix B.


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3.5 Hybrid Method For the Hybrid Method, the orientation of building is either considered or ignored and the wind speed is considered in twelve directional ranges.



Notes






Define twelve 30° directional

ranges (Clause 3.4.2.2)

The directional variation in wind speed, upwind terrain and topography effects together with upwind obstructions and distance from the sea are defined in each of the 30° directional ranges. The twelve wind directions are defined following the guidance in Clause 3.4.2.2.1.



Section 4.4 & 4.5

For topography to be significant, the upwind slope must be greater than 5% and the building constructed within the shaded areas shown in Figure 7 of BS 6399-2. The calculation of the altitude factor (Sa) is dependent on whether topography is significant and can therefore vary in the twelve directions considered.


Take values of 1.0 for the, seasonal factor (Ss) and probability factor (Sp). For each of the twelve directions, calculate the site wind speed (Vs = Vb × Sa × Sd).


1.7.2 and 1.7.3) Sections 4.10 & 4.12

For each direction, define the type and extent of the upwind terrain and calculate the effective height of the building, which takes into account any upwind obstructions.

Calculate terrain and building factor Sb

Calculate the terrain and building factor (Sb) for each direction. This factor is based on the upwind type and extent of terrain and allows for the beneficial effect of upwind obstructions (Refer to Section 4.10). The procedure given in the Directional Method requires more calculation effort compared to the Standard Method, but gives lower values if in town terrain.

Obtain effective wind speed (Clause 2.2.3) Section 4.9

In the twelve directions, calculate the effective wind speed (Ve = Vs × Sb)


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For each orthogonal case determine the highest effective wind speed (Clause 3.4.2.2.2)

Determine the highest effective wind speed ±45° normal to the building faces. If the orientation of the building is unknown or ignored take the highest effective wind speed from the twelve considered directions.

Calculate dynamic pressure (Clause 2.1.2) Section 4.11

For each orthogonal direction, calculate the dynamic pressure ( 2

es 613.0 Vq = ).


Section 5



Section 5

Using pressure coefficients and size effect factors, calculate wind loads on the structure. Alternatively use the simplified uniform net pressure coefficients in Appendix B.


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4 CALCULATION OF DYNAMIC WIND PRESSURES

This Section provides detailed guidance on the calculation of the various parameters used to estimate the dynamic wind pressures.

4.1 Dynamic classification BS 6399-2 is valid for structures that are sufficiently rigid such that they are not overly susceptible to dynamic excitation due to wind pressures. The Code provides guidance on dynamic effects by defining a dynamic augmentation factor Cr. This is obtained from Figure 3 of BS 6399-2, based on the building-type factor Kb and the building height. The building-type factor is obtained from Table 1 of BS 6399-2, for typical types of buildings.

The dynamic augmentation factor is only used, within the design methods, to increase the overall loads on the building (Clause 2.1.3.6 and 3.1.3.3.2). The calculation of overall loads is discussed in Section 7.

If Cr cannot be determined from Figure 3 (i.e. the combination of height and Kb lie in the shaded part of the Figure) the Code cannot be used to determine wind loads.

4.2 Basic wind speed The basic wind speed Vb can be obtained from Figure 6 of BS 6399-2. This wind speed is defined as the mean hourly wind speed, for a given site location within the UK, that has an annual risk of exceedance of 0.02 (equivalent to ‘one-in-50-year’ risk). Figure 6 gives basic wind speed values that are maximum in any direction; in the calculation methods (Section 4.6) the direction of maximum wind speed is 240° from North.

Interpolation may be used between the isotachs shown in Figure 6 of BS 6399-2.

The basic wind speed has changed from a gust speed given in CP3-V-2 to a mean speed in BS 6399-2, to conform to the requirements of ISO and the forthcoming Eurocodes. Allowance for gusts are made in BS 6339 in the calculation of the effective wind speed by means of the terrain and building factor (Sb).

Guidance on how to use gust wind speeds with BS 6399-2 is provided in the Foreword of the Code and in Reference 7.

4.3 Site wind speed The site wind speed for any particular direction is given by:

Vs = Vb × Sa × Sd × Ss × Sp

where:

Vb is the basic wind speed


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Sa is an altitude factor

Sd is a directional factor

Ss is a seasonal factor

Sp is a probability factor.

The calculation of the site wind speed ignores the extent and type of upwind terrain and also the spacing and height of any upwind obstructions. Both these parameters significantly affect the wind speed and are considered in the terrain and building factor Sb, which is used, together with the site wind speed, to calculate the effective wind speed Ve.

Upwind topography significantly affects the wind speed and is considered in the calculation of the altitude factor in the Standard Method and in the calculation of the terrain and building factor in the Directional Method. Care is required when considering the effects of topography using the Hybrid Methods, since it is possible to consider the effects either when calculating the altitude factor (as in the Standard Method) or when calculating the terrain and building factor (as in the Directional Method).

When using the Hybrid Methods, it is recommended that the effects of topography are considered in the calculation of the altitude factor and ignored in the calculation of the terrain and building factor (i.e. Sh = 0). This is discussed further in Section 4.10.

The altitude, directional, seasonal and probability factors used to calculate the site wind speed (Vs) are discussed below.

4.4 Altitude factor The altitude of the site and knowledge of the site topography is required to calculate the altitude factor (Sa), when using the Standard or Hybrid Methods.

If the average slope of the ground is less than 5% within 1km of the site, or topography is not significant, as defined by the criteria in Figure 7 of BS 6399-2, then the site can be considered level and the effects of topography can be ignored.

If topography is not significant the altitude factor is calculated using:

S00101 ∆.S a +=

where ∆S is the site altitude in metres above sea level.

If topography is considered significant, the altitude factor is calculated using Clause 2.2.2.2.3 where Sa is taken as the greater of:

S00101 ∆.S a +=

and,

s..S eTa Ψ∆ 2100101 ++=

where ψe is the effective slope of the topographic feature.


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It should be noted that ∆S is the altitude of the proposed structure and ∆T is the altitude at the upwind base of the topographic feature.

4.5 Topography Topography is the features (hills, ridges, cliffs and escarpments) that can cause the wind speed to accelerate locally, for example over a summit or crest. BS 6399-2 presents simple criteria to check whether topography significantly affects the wind speed.

For topography to be significant, the upwind slope must be greater than 5% and the building must be constructed within the region indicated by the shaded area shown in Figure 7 of BS 6399-2. It should be noted that topography is only significant when the wind blows up the slope, as shown in Figure 4.1. Guidance to obtain values of parameters for more complex topography features, outside the scope of the Code, is given in Reference 2.

Upwind slopeψU > 0.05

Downwind slopeψD < 0.05

Topography increaseswind speed

Wind speed not affectedby topography

Location of building

Figure 4.1 Wind speed is only affected by topography when blowing up the slope

The effects of topography on the wind speed are governed by the effective upwind slope ψe and the location factor s. If the upwind slope is greater than 30% and the building is constructed on the crest or summit of the topographic feature, the wind speed increases by 85%, compared to the wind speed at the base of the feature. Generally the topographic feature and location of the building will not be as severe and lower values of the effective slope and location factor can be used.

4.6 Directional factor The basic wind speed map, given in Figure 6 of BS 6399-2, is based on measured wind speeds, irrespective of direction. The Code defines the wind direction corresponding to 240° from North as the direction from which the greatest wind speed occurs. To determine the speed with the same likelihood of occurrence for other wind directions, the value of the wind speed obtained from Figure 6 is reduced by applying a directional factor Sd. This is similar to the S4 factor given in CP3-V-2. The directional factors given by the Code are shown diagrammatically in Figure 4.2, at 30° intervals starting from North (0°). The values are assumed to be valid over a sector ±15° from the specified direction, and interpolation can be used.


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0° NorthSd = 0.78

30° Sd = 0.73

60° Sd = 0.73

90° East Sd = 0.74

120° Sd = 0.73

150° Sd = 0.80

180° SouthSd = 0.85

210° Sd = 0.93

240° Sd = 1.00

270° West Sd = 0.99

300° Sd = 0.91

330° Sd = 0.82

(Prevailing wind)

Figure 4.2 Variation of directional factors Clause 2.2.2.3, in the Standard Method, states that if the orientation of the building is unknown or ignored the value of the directional factor (Sd) should be taken as 1.0 for all wind directions. The conservatism inherent in this approach can be removed by using the Simplified Hybrid or Hybrid Method discussed in Sections 2 and 3, albeit with additional calculation effort.

4.7 Seasonal factor The seasonal factor Ss allows the wind speed to be reduced for structures that are exposed during a short period of time (i.e. less than 6 months). This allows account to be taken of the fact that the risk of a strong wind is lower in the summer months than in the winter months. Table D1 (Annex D) of BS 6399-2 gives values of seasonal factors for 1, 2 and 4 months exposure within a 12 month period and for the 6 month periods of October to March and April to September. If the building is permanent or exposed to wind for greater than 6 months the seasonal factor should be taken as 1.0.

4.8 Probability factors The probability factor Sp is used to change the basic wind speed for a different probability of being exceeded.

The wind speeds given in Figure 6 of BS 6399-2 have an annual design risk is 0.02, which is equal to a mean recurrence interval of 50 years (one-in-50-year wind). To change the wind speed for a different risk, the value of the probability factor (Sp) may be calculated using the following expression from Annex D of BS 6399-2:

( ){ }( )98.0lnln5

1lnln5p

−−

−−−=

QS

where Q is the annual probability required. For example, for a mean


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recurrence interval of 10000 years (i.e. one-in-10000-year wind) the value of Q is 10-4 resulting in a value of Sp = 1.263. This value is used in the calculation of the site wind speed, which is used to calculate the effective wind speed and then squared to obtain the dynamic pressure. Therefore a value of Sp = 1.263 will result in an increase of 60% in the wind pressures on the structure, compared to those for the one-in-50 year wind.

For permanent structures the annual design risk of the wind speed exceeding the design value is usually maintained at 0.02 and Sp is taken as 1.0. However, values of Sp less than unity can be calculated to allow for a greater risk that can be accepted; for example for checking the response of a structure should a dominant opening occur accidentally during a storm[7]. (This is discussed further in Section 6.2).

The probability factor may also be used in conjunction with the seasonal factor to vary the risk during sub-annual periods.

4.9 Effective wind speed The effective wind speed (Ve) is given by:

Ve = Vs × Sb

Vs is the site wind speed and takes account of a number of factors, as explained in Section 4.3, and so is the terrain and building factor. In the Standard Method, the effects of topography are included in the calculation of the altitude factor, which is used to determine the site wind speed. In the Directional Method, the effects of topography are included in the calculation of the terrain and building factor, which is used to determine the effective wind speed.

4.10 Terrain and building factor The terrain and building factor (Sb) primarily considers the extent and type of the upwind terrain and the spacing and height of any upwind obstructions. In the Directional Method the effects of topography, the influence of non simultaneous gusts and the time response for internal pressures are also included in the calculation of the terrain and building factor. In the Standard Method, these effects are included elsewhere in the calculation procedure. The difference between the two methods can cause confusion and lead to errors when adopting the Simplified Hybrid and Hybrid Methods. Recommendations are given below to alleviate confusion and reduce the possibility of design errors.

In the Simplified Standard Method and the Standard Method, the calculation of the terrain and building factor (Sb) involves simply using Table 4 of BS 6399-2. This Table provides values for Sb for various effective heights (He), distance from sea and whether the site is in country or town terrain. As well as fulfilling the criteria for town terrain (refer to Section 4.10.1), the upwind extent of the town terrain must be at least 2 km. Although the calculation of Sb is simple, when using the procedure in the Standard Method, it can be conservative for sites in town terrain. This conservatism can be reduced by using either the Simplified Hybrid Method or the Hybrid Method, where the terrain and building factor is calculated from the procedure given in the Directional Method.


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In the Simplified Hybrid Method or the Hybrid Method, the terrain and building factor (Sb) is calculated using Equation 28 in BS 6399-2, if the site is in country terrain, or Equation 29 in BS 6399-2, if the site is in town terrain. The following recommendations are given when calculating the terrain and building factor using the Simplified Hybrid or Hybrid Method.

• Ignore the effects of topography when calculating the terrain and building factor according to Clause 3.2.3.2 (i.e. let Sh = 0). If topography is significant, then take account of it in the calculation of the altitude factor (Sa) using the expressions given in the Standard Method (Clause 2.2.2.2.3).

• Set the gust peak factor (gt) (Clause 3.2.3.2.2 or 3.2.3.2.3) to 3.44 and consider non-simultaneous gusts on external pressures and the time response for internal pressures by using the size effect factor (Ca) given in the Standard Method.

In addition to the above parameters, the ground roughness categories, the ‘fetch’ (see Section 4.10.2) and effective height of the building need to be defined, irrespective of the method adopted. The ground roughness defines the terrain, the fetch defines the distance from the site to the upwind edge of each terrain category and the effective height of the building is based on the upwind spacing and height of any obstructions. These parameters are discussed below.

4.10.1 Ground roughness categories Three terrain categories are defined comprising sea, country and town.

a) Sea category. This applies to the sea and large areas of inland water, such as lakes, which are greater than 1 km in size and are within 1 km of the proposed building.

b) Country category. This applies to terrain that cannot be classed as Sea or Town. Typically it covers open areas where there is very little or no shelter.

c) Town category. This applies to terrain where the typical height to a building roof is 5.0 m or greater. Annex E of BS 6399-2 suggests that the density of the buildings should be greater than 8% of the total area considered and the buildings should extend at least 100m upwind of the site.

The sea category is only used to define the extent of upwind terrain. The Code is not suitable for defining wind pressures for offshore structures.

4.10.2 Fetch The fetch is the upwind distance from the site to the edge of each terrain (ground roughness) category. The fetch is considered in the calculation of the terrain and building factor by defining the upwind distance from the sea and the upwind distance to the boundary of the town terrain (Figure 4.3). Both these distances are required to estimate the wind speed.


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Wind

Distance of site in town

Distance of site to sea

country terrain town terrainsea

terrain

Figure 4.3 Defining extent and type of upwind terrain The upwind distance from the sea, or to an area of inland water that extends more than 1 km upwind, is required. The accuracy of this measurement should be defined to the nearest 0.1 km for distances less than 1.0 km from the sea, 1.0 km for distances less than 10 km from the sea and 10 km for distances less than 100 km from the sea. Once the distance is greater than 100 km from the sea the actual distance does not influence the wind speed.

The upwind distance in town should be defined to the nearest 0.1 km for distances less than 1.0 km in the town, 1.0 km for distances less than 10 km in the town and 5 km for distances less than 30 km in the town. Once the distance is greater than 30 km in town the actual distance does not influence the wind speed.

4.10.3 Effective height The height of the building affects the maximum wind speeds to which the structure is subjected. The reference height of the building (Hr) is defined according to the building form, but is conservatively taken as the maximum height of the building. The effective height (He) is less than (Hr) and allows wind speeds to be reduced due to shelter from any upwind buildings or permanent obstructions.

The effective height is used, together with ground roughness category, distance from the sea and (if applicable) distance in town, in the calculation of the building and terrain factor (Sb).

To calculate the effective height, the reference height (Hr) of the considered building, the height of the surrounding buildings (H0) and the distance (X) between the surrounding buildings are required. The reference height (Hr) is given by:

• Walls: For side walls the reference height is taken as the height from ground to eaves or to the top of any parapet. For gable walls the reference height is taken as the height from ground to the ridge.

• Roofs: The reference height is taken as the height from ground to the ridge.


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Conservatively, the reference height can be taken as the height to the ridge of the building, irrespective of whether the wall or roof is considered. If the orientation of the building is unknown or ignored, the reference height should be taken as the height to the ridge.

The effective height (He) needs to be determined for each considered wind direction.

Clause 1.7.3.3 allows the height of the upwind buildings (H0) to be estimated assuming a typical storey height of 3.0 m. Therefore if two storey buildings are in the upwind direction, the value of H0 is 6.0 m. If the distance X is not known then Reference 7 suggests that a value of 20 m can be assumed to be typical for suburban and urban areas.

When defining H0 and X, the density of the upwind buildings must exceed 8% plan density or 12 houses per hectare. In addition, the buildings must extend 100 m upwind. If this criteria is not met then H0 = 0.

4.11 Dynamic pressure The dynamic pressure is used, together with pressure coefficients, to define the pressure distribution over a surface. The dynamic pressure is given by:

2V6130q e.=

where q is the dynamic pressure (Pa) and Ve is the effective wind speed (m/s).

(Note: 1 Pa = 1 N/mm2)

For the Standard and Hybrid methods, the dynamic pressure is denoted qs. For the Directional Method, the reference dynamic pressures for deriving pressures on the external and internal surfaces are denoted qe and qi respectively. This distinction is necessary since the Directional Method accounts for non-simultaneous actions of gusts on the external surface and the response time to establish internal pressures, in the calculation of the terrain and building factor (Sb), resulting in different effective wind speeds and dynamic pressure for external and internal surfaces.

4.12 Building orientation If the orientation of the building is unknown or ignored, the Standard Method requires the directional factor (Sd) to be taken as 1.0 for all directions. This will generally result in conservative wind pressures, as it is unlikely (although possible) that the most onerous value for all parameters, that affect the wind speed, will occur in the same wind direction.

The Simplified Hybrid Method and the Hybrid Method can be used irrespective of whether the orientation of the building is known or unknown. If the orientation is unknown or ignored then the highest dynamic pressure, irrespective of wind direction, must be used to define all pressure coefficients. If the orientation is considered, the pressure coefficients relating to the considered wind direction can be used. The possible conservatism introduced by ignoring the building’s orientation is highlighted in Figure 4.4.


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With the simplifications introduced into the Simplified Hybrid Method, ignoring the orientation of the building could reduce the calculated design value of the effective wind speed. The method involves calculating the effective wind speed over four 90° quadrants, using the worst case for each parameter within the quadrant. Careful positioning of the quadrants may allow the worst case for each parameter that affects the wind speed to occur in one quadrant and thus reduce the effective wind speed calculated using four quadrants positioned in relation to the building’s axes. An example of ‘efficient’ positioning of the quadrants was previously discussed in Section 2.6.1. It should be remembered that the efficient use of the Simplified Hybrid Method will not reduced the effective wind speed below values calculated using either the Hybrid or Directional Method, it only reduces the conservatism embodied in the Simplified Hybrid Method caused by the method’s balance between calculation effort and accuracy.

500 Pa

453 Pa

650 Pa 550 Pa

If orientation is known the building will be designed for a longitudinal pressure of 650 Pa and a transverse pressure of 500 Pa.

If orientation is unknown or ignored the greatest wind pressure from all directions must be used as shown below.

650 Pa

650 Pa

650 Pa

650 Pa 650 Pa

650 Pa

650 Pa

650 Pa

If the orientation of the building is ignored in the design but is finally positioned as shown then the transverse pressures will be conservative.

If the orientation of the building is ignored in the design but is finally positioned as shown then the longitudinal pressures will be conservative.

Figure 4.4 Example of how conservative pressure estimates are

obtained if the orientation of the building is ignored


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5 PRESSURE COEFFICIENTS

The structural response of a building to wind is assessed assuming an equivalent static load normal to the face of a building and equivalent shear force (frictional drag) parallel to face of a building. The equivalent static load acting normal to the building face is estimated using the calculated dynamic pressure in various wind directions and the corresponding external and internal pressure coefficients. The external pressure coefficients define the distribution of wind pressure around the building and the internal pressure coefficients define the balance of airflow in and out of the building, through openings in its envelope. The equivalent static load on the face of a building is given by the difference between the internal and external pressures. The use of external and internal pressure coefficients for various buildings is discussed below. The calculation of frictional drag is discussed in Section 7.1.

5.1 External pressure coefficients External pressure coefficients provide a simple representation of the variation of wind pressure over the external surface of the building (Figure 5.1). This simplification defines discrete zones where the wind pressure is assumed to be constant. The width of these zones is fairly small near the edges of walls and roofs of the building to account for high pressures that occur in these areas.

Code representation of externalpressure coefficients

Actualpressurecoefficientdistribution

Suction on roof

Pressure onwindward wall

Suction onleeward wall

Wind

Figure 5.1 Code representation of pressure coefficient distributions

The Directional Method provides external pressure coefficients that are dependent on the angle between the face of the building and the wind direction considered. This creates unique external pressure distributions for each of the wind directions (usually twelve) that are considered. Generally it will not be obvious which of the wind directions governs the design of the building and the structural response for all directions will need to be considered. Therefore, the use of the Directional Method and associated pressure coefficients is only suitable for evaluation using computer software. Typically, designs will be carried out using the simpler design methods discussed in Section 3, which use the pressure coefficients given in the Standard Method.


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Compared to the Directional Method, the Standard Method pressure coefficients are presented in a simpler form, with less discrete zones of pressure. The Standard Method coefficients are derived from the Directional Method coefficients, taking the most onerous value for each zone over a range of wind directions ±45° from the orthogonal directions. There were some inconsistencies between the Standard and Directional Method pressure coefficients in the original (1997) issue of BS 6399-2. These were addressed in the 2002 amendment.

The discrete zones for the pressure coefficients are defined in terms of scaling lengths (refer Section 5.2), with different sizes of zones for walls and roofs. A typical variation of external pressure coefficients for the side walls and roof, on a duopitch portal frame, is shown in Figure 5.2, where it can be seen that the pressure zones for the walls and roof do not coincide.

For simple portal frames, Appendix B shows how the pressure coefficients can be simplified, and presented as net pressure coefficients, reducing the required calculation effort.

Discrete zones ofpressure in Code

Wind

Note: Length of pressurezones is different forroofs and walls

Figure 5.2 Discrete zones of external pressures

5.2 Scaling lengths The scaling length ‘b’ defines the sizes of the external pressure coefficient zones. The scaling length is based on the building’s geometry and the building’s resistance to the flow of wind. The value of the scaling length is given by the smaller of:

b = 2H and b = B

where, H is the height of the wall or roof above ground and B is crosswind breadth of the building, as defined in Figure 5.3.

In the case where the scaling length is given by 2H, the least resistance to wind flow is over the roof, as shown in Figure 5.3. Therefore the pressure zones are governed by the height of the building. As the building height increases, the resistance to wind flow over the roof increases and the pressure zones change in size.


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In the case where the scaling length is controlled by the dimension B (i.e. 2H>B) the least resistance to wind flow is around the building, as shown in Figure 5.3. For this building geometry, the crosswind breadth of the building governs the size of the pressure zones.

B

H

B > 2H therefore wind finds it easier to go over the roof and size of the pressure zones are governed by the building’s height

B

H

B < 2H therefore wind finds it easier to go around the building and size of the pressure zones are governed by the building’s cross wind breadth.

Figure 5.3 Basis of scaling length used to define sizes of pressure

zones BS 6399-2 and Reference 2 provides further guidance on defining the scaling length for various and more complex building shapes, based on the basic principles discussed above.

5.3 External standard pressure coefficients for walls

Table 5 of BS 6399-2 gives the pressure coefficients for walls to be used in the Standard Method and the Hybrid Method. It should be noted that Table 5 was significantly revised in the 2002 amendment and designers should ensure that the latest version of the Code is used.

The coefficients in Table 5 can be used for rectangular plan buildings with walls that are within ±15° of the vertical.

Table 5 gives pressure coefficients for windward, side and leeward walls, governed by the ratio D/H, the scaling factor b, and whether funnelling needs to be considered. D is the inward depth of the building and H is the height to the eaves for the side walls or to the ridge for the gable walls. The definition of dimensions is summarised in Figure 5.4.


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B=L

Wind

Wind

B=W

D=

L

Side wall Gable

H H

D=W

Figure 5.4 Calculation of D/H The external pressure coefficients are constant across the windward and leeward walls. Table 5 gives values of coefficients for D/H ≤ 1 and D/H ≥ 4; interpolation may be used between these ratios.

For the side walls, the pressure varies across the length of the wall and values for up to 3 zones are given in Table 5. The number of zones depends on the ratio of D to b, where b is the scaling length. A key to the wall zones is given in Figure 5.5, which is based on Figure 12 of BS 6399-2.

D

H

b

0.2b

A B C

0.2b

b

H

D > b

A B

0.2b

D

H

0.2b

H

D

D ≤ b

WindWind

Wind

A B C

A B

Wind

Figure 5.5 Key to wall pressure zones on side walls


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The side wall pressure coefficients are increased if funnelling occurs. Research into the behaviour of wind on structures has shown that if similar structures are constructed close together, an increase in suctions on the walls due to funnelling will occur.

Funnelling is significant when the walls of two buildings face each other and the gap between them is less than the scaling length b. Separate coefficients for side face walls in the ‘isolated’ and ‘funnelling’ cases are given in Table 5. These coefficients should be used as follows:

• If the gap between the two walls facing each other is greater than b or less than b/4, then the isolated coefficients should be used.

• If the gap is greater than b/4 and less than b, interpolated values may be used. Interpolated values are shown in 5.6.

When assessing the wind loads for a new building, where funnelling does not occur, it would be prudent to assess the possibility of the construction of any future buildings that may cause funnelling.

For walls outside the range of ±15° from the vertical, the Directional Method coefficients given in Table 29 of BS 6399-2 should be used. Guidance is also provided in BS 6399-2 for:

• Buildings with corner angles other than 90°.

• Buildings with re-entrant corners, recessed bays or internal wells.

• Buildings with irregular or inset faces.

-1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

Gap / b

Exte

rnal

wal

l pre

ssur

e co

effic

ient

s

Zone C

Zone B

Zone A

Geometry over which funnelling occurs

Figure 5.6 Interpolation of external wall pressure coefficients for funnelling


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5.4 External standard pressure coefficients for roofs

5.4.1 Flat roofs Rules are given in Clause 2.5.1 for roofs with pitch angles that are less than 5° (defined as flat roofs in the Code). The external standard pressure coefficients are limited to flat roofs that are rectangular on plan. Cut outs from recessed bays or wells are acceptable, provided that these are also rectangular. Coefficients for different shaped eaves details and roofs with parapets are given in the Standard Method. If the plan shape is irregular, the coefficients given in the Directional Method should be used.

The pressure zones in the Standard Method are shown in Figure 5.7 for two orthogonal directions. The scaling length b is governed by either the height or the crosswind breath of the building as discussed in Section 5.2. The values of the pressure coefficients are obtained from Table 8 of BS 6399-2.

A B A

Wind

b / 4b / 4

b / 10b / 2

C

D

WindA

A

B C D

b/10b / 2

b / 4

b /4

Figure 5.7 Zones of pressure coefficients for flat roofs 5.4.2 Mono and duopitch roofs Rules are given in Clause 2.5.2 for monopitch and duopitch roofs of buildings with a pitch greater than or equal to 5°.

For monopitch roofs, the pitch angle is always considered positive. When the low eave is on the windward side, the angle of wind direction is taken as 0° (i.e. θ = 0°); when the high eave is windward the angle of wind direction is taken as 180° (i.e. θ = 180°).


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For a monopitch roof, the pressure zones are shown in Figure 5.8, which is based on Figure 19 of BS 6399-2. The scaling lengths bw and bL are given by:

bw = W or bw = 2H, whichever is the smaller value

and

bL = L or bL = 2H, whichever is the smaller value.

Windθ = 180°

αWindθ = 0°

AA

B

Hig

h ea

ve

b L /2

L

b L /2

Wind

θ = 0°

AB

A

Wind

θ = 180°

b L /2

b L /2

L

AL

Hig

h ea

ve

AUB

b w /1

0

bw /4 bw /4

Windθ = 90°

Hig

h ea

ve

C

C

b w /2

C

D

W

W

Figure 5.8 Pressure zones for monopitch roofs Rules are given in Clause 2.5.2 for duopitch roofs where the pitch angle of the two slopes is equal or nearly equal (difference not more than 5°). A duopitch roof with a ridge is considered to have a positive pitch; a duopitch roof with a trough (valley) is considered to have a negative pitch.

The pressure zones for duopitch roofs are shown in Figure 5.9, which is based on Figure 20 of BS 6399-2. For duopitch roofs where the difference in the upwind and downwind slopes is greater than 5°, BS 6399-2 refers to the original source data given in Reference 6.

For positive pitch angles between 5° and 7° where the width of the building ‘W’ is greater than the scaling length bL, then Clause 2.5.2.4.2 applies. This clause states that for a wind direction θ = 0° the pressure zone C should extend over the ridge a distance bL/2 from the eaves, replacing ridge zones E, F and possibly part of zone G, as shown in Figure 5.10.


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Figure 5.9 Pressure zones for duopitch roofs

b L/1

0

Wind direction θ =0°

bL/2bL/2

A B A

C

G

b L /

2

Figure 5.10 Pressure zones for a duopitch roof when

5°<α < 7° and W < bL 5.4.3 Multi-bay roofs Clause 2.5.5 states that multi bay roofs can be considered as a series of single bays (using the same pressure zones and coefficients for each bay). The approach is, however, conservative, so the clause provides alternative rules for three cases:

b L/1

0 b L

/10


bL/2 bL/2

A B A

C

E F E

G

A

B

B

A

C

C

D

D

bw/10

bw/2

= =

= =

Wind

Wind direction θ = 90°

α ≥ 5° α ≤ -5°

b L/1

0 b L

/10


bL/2 bL/2

A B A

C

E F E

G

Wind

A

B

B

A

C

C

D

D

bw/10

bw/2

= =

= =

Wind direction θ = 270°


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(a) Multi-bay monopitch.

(b) Equal pitch duopitch multi-bay.

(c) Equal pitch duopitch (pitches of the two slopes within 10°).

For multi-bay roofs of enclosed buildings, the following guidance is given to calculate the external pressure coefficients in the orthogonal directions.

External pressure coefficients in wind direction 0° and 180°

Multi-bay monopitch roof

Windθ = 0°

Windθ = 180°

Figure 5.11 Multi-bay monopitch roof Each roof bay should be treated as if it were an individual monopitch roof. The pressure coefficients for each roof are obtained from Table 9 of BS 6399-2 for wind direction θ = 0° and θ = 180°. If any positive coefficients occur in the second and subsequent downwind bays, they should be replaced with a value of –0.4. In addition, Table 12 of BS 6399-2 allows a reduction factor of 0.8 to be applied to all coefficients in the second downwind bay, and a reduction factor of 0.6 to be applied to all further downwind bays.

Multi-bay unequal duopitch roof

For multi-bay unequal duopitch roofs (Figure 5.12) the first and last slope is treated as a monopitch roof. The rest of the structure is divided into duopitch roofs with negative pitch angles. The pressure coefficients are obtained from Table 10 of BS 6399-2 using different negative pitch angles for each slope of the duopitch roof. A reduction, as given in Table 12 of BS 6399-2, can be applied to the area of roof, which is treated as individual troughed duopitch roofs.

Treat asmonopitch

Treat as individual troughed duopitch roofs

Treat asmonopitch

Windθ = 0°

Windθ = 180°

Upwind pitch

Downwind pitch

Wind

Assumed individual troughedduopitch roof.

Figure 5.12 Multi-bay unequal duopitch roof


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Multi-bay equal ridged duopitch roof

Treat as upwind(Positive pitch)

Treat as ridged downwind(Positive pitch)

Treat as troughed downwind(Negative pitch)

Wind

Figure 5.13 Multi-bay equal ridged duopitch roof The first slope (Figure 5.13) is treated as an upwind positive pitch with zones A, B and C (Figure 20 of BS 6399-2). The slope behind each ridge is treated as a downwind positive pitch with zones E, F and G (Figure 20 of BS 6399-2). The slope behind a trough is treated as a negative pitch with zones E, F and G. An example for a 30° pitch multi-bay roof is shown in Figure 5.14.

A reduction factor (Table 12 of BS 6399-2) can be applied to the second and subsequent downwind bays.

A

B

A

C

E

F E

G

E

F E

G

E

F E

G

E

F E

G

E

F E

G

E

F

E

F

E

G

G

E

+30° +30° -30° +30° -30° +30° -30° +30°

Pitch angle used to obtain pressure coefficients from Table 10 of BS 6399 (Wind direction θ = 0°)

30°Wind

Wind

Figure 5.14 Pressure zones for multi-bay equal ridged duopitch roof


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Multi-bay equal troughed duopitch

Treat as ridged downwind(Positive pitch)

Treat as troughed downwind(Negative pitch)

Treat as upwind(Negative pitch)

Wind

Figure 5.15 Multi-bay equal toughed duopitch roof A similar procedure to the multi-bay equal ridged duopitch roof is used, together with the reduction factors given in Table 12 of BS 6399-2. An example, for a 30° roof pitch is shown in Figure 5.16.

AB

A

C

E

F E

G

E

F E

G

E

F E

G

E

F E

G

E

F E

G

E

F

E

F

E

G

G

E

-30° -30° +30° -30° +30° -30° +30° -30°

Pitch angle used to obtain pressure coefficients from Table 10 of BS 6399 (Wind direction θ = 0°)

-30° +30°

Figure 5.16 Pressure zones for multi-bay equal troughed duopitch roof External pressure coefficients in wind direction 90°

For all the examples of multi-bay roofs shown above, the external pressure coefficients for the wind direction 90° should be obtained by treating each bay as if it were a single bay roof. No reduction (i.e. Table 12 of BS 6399-2) should be used.

5.5 Size effect factor In the Standard Method, the size effect factor Ca given in Figure 4 of BS 6399-2, allows for the non-simultaneous action of wind gusts on the building’s envelope and the time response to establish internal pressures.


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The size effect (similar to the gust peak factor in the Directional Method) is defined by the dimension a. For external pressures, dimension a is defined in Clause 2.1.3.4 as the diagonal dimension of an area over which load sharing takes place. BS 6399-2 states that dimension a should be taken as 5 m for all individual structural components, cladding and associated fixings, unless there is adequate load sharing capacity to justify the use of a higher dimension. Some guidance on defining the dimension a for external pressures is given in Section 5.5.1. If dimension a is assumed to be 5 m, the size effect factor Ca (Table 4) is 1.0 and no reduction of external pressure, to allow for the non-simultaneous action of gusts, is assumed.

For internal pressures, dimension ‘a’ is based on the geometry of the building, as discussed in Section 5.5.2. Dimension a and the corresponding size effect factor should be calculated for internal pressures, since assuming a value of 1.0 for the size effect factor could result in unconservative net pressures, depending on the relative sign (direction of force) of the external and internal pressure coefficients.

Care is required when using the Hybrid methods, discussed in Sections 2 and 3, and the recommendations given in Section 4.10 should be followed.

5.5.1 Calculation of dimension a for external pressures For external pressures/suctions, dimension a is defined as the largest diagonal dimension over which load sharing takes place. The code provides no direct guidance on the required level of load sharing.

The following general guidelines are provided. However, it is the responsibility of the designer to ensure that adequate load sharing occurs for the type of structure considered. Alternately, a conservative value of 5m can be assumed.

a) Cladding and fixings; a=5.0 m.

b) Purlins and sheeting rails; a = largest diagonal over loaded area (tributary area) of member, or conservatively, the length of the span. Figure 5.17 shows some typical examples. For continuous purlins, limit dimension a to a loaded area comprising a maximum of two spans.

c) Portal frames (ignoring stress-skin action); a = largest diagonal over loaded area of member, or conservatively, the length of the span. Figure 5.18 show some typical examples.

d) Portal frames (including stress-skin action); a = largest diagonal over which stress-skin action can be guaranteed (Figure 5.19). Guidance is provided in BS 5950-9[12] on how to include stress-skin action into the design and construction of the building. However, the following points need careful consideration.

i) Some well known systems have zero shear stiffness and cannot accommodate seam fixings. (Consult manufacturers for more information).

ii) The fixing of the roofing system will require strict supervision during construction.

iii) The frame and cladding systems are typically designed by different organisations, so the problem of responsibility may occur.

iv) In future, the cladding may be replaced causing the original design assumptions for the frame to become invalid.


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Main frames

Spacing ofrails/purlins

Dimension a for singlespan rail/purlin

Dimension a fordouble span purlins

Figure 5.17 Diagonal dimensions for rails and purlins

Dimension a forend frame

Dimension a forinternal frame

Figure 5.18 Diagonal dimension ‘a’ for main portal frames ignoring stress-skin action of the cladding

Dimension a basedon stress skin actionof the cladding

Figure 5.19 Diagonal dimension ‘a’ for main portal frames by including stress-skin action of the cladding

5.5.2 Calculation of dimension ‘a’ for internal pressures The size effect factor in BS 6399-2 is also used to determine the response time to establish internal pressures. The size effect factor allows for the different speed of airflow into the building between small non-dominant openings and dominant openings. BS 6399-2 gives values for dimension a, which are used to determine the size effect factor. If the building is subdivided into rooms,


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dimension a depends on the permeability of internal doors compared to external doors, and whether there are any dominant openings in the building’s envelope.

Depending on dimension a, the size effect factor will vary between the values of 1.0 and 0.52. Assuming a value of 1.0 (or 0.52) may lead to unconservative net pressures, depending on the relative values of the external and internal pressures.

5.6 Localised external pressures For the distribution of wind loads over enclosed buildings, both CP3-V-2 and BS 6399-2 give external localised pressures/suctions at the edges of the faces, to allow for the high pressures/suctions that occur in these areas. There are significant differences between the codes in the magnitude and area over which these external localised pressures occur. The differences are due to the increase in knowledge and experimental data on wind loading used when preparing BS 6399-2. There are also differences in the codes as to whether the localised loads need to be considered or ignored when designing different parts of the building.

The steel codes BS 5950-5[5] and the superseded Code BS 5950-1:1990[4] state that the localised pressures could be ignored when designing purlins and side rails to CP3-V-2, and this was the typical approached adopted by designers. However, full-scale tests,[9,10,11] conducted on typical portal frames, which involved measuring the external and internal pressures, together with the response of the structure, have shown that localised pressures cannot readily be disregarded, as suggested in the steel design codes. Therefore, the steel codes, at present, do not allow localised pressures to be ignored when designing purlins and side rails to withstand wind loads calculated using BS 6399-2.

5.7 Internal pressures The internal pressure within a building is set by the balance of airflow, driven by the external pressures, in and out of the building. The flow of air passes through the external envelope of the building through gaps and openings, such as gaps around closed doors and windows or permanent openings such as ventilation grills and flues. In addition the cladding envelope of the building has small pores, which allow a degree of airflow in and out of the building.

Compared to the methods for estimating the external pressure coefficients, BS 6399-2 provides sparse and very simplified guidance on the calculation of the internal pressure coefficients. The effect of negative or positive internal pressures on the net pressure is shown in Figure 5.20.


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Wind

Negative internal pressureswill reduce external suctionsand increase external pressures

Wind

Positive internal pressures willincrease external suctions anddecrease external pressures

Figure 5.20 Effect of internal pressures For enclosed buildings where the external doors and windows are generally kept closed, and internal doors are generally kept open or are at least three times more permeable than the external doors and windows, the internal coefficients can be assumed to be uniform as shown in Table 5.1 (taken from Table 16 of BS 6399-2)

Table 5.1 Internal pressure coefficients (Table 16 of BS 6399-2)

Type of walls Cpi

Two opposite walls equally permeable; other faces impermeable:

Wind normal to permeable face +0.2

Wind normal to impermeable face −0.3

Four walls equally permeable; roof impermeable −0.3

A positive internal pressure coefficient (Cpi = +0.2) can only occur in an enclosed building when two opposite walls are equally permeable, the other faces are impermeable and the wind direction is normal to the permeable face. In all other cases the internal pressure is negative (Cpi = −0.3). For single storey clad structures BRE Digest 436[7] advises that the internal pressure coefficient should be taken as −0.3, provided that there are no dominant openings (or that these are closed during a storm) and the building faces have a reasonably equal permeability.

Dominant openings together with the likely risk of them being open or closed during a storm are considered in Clauses 2.6.1.3 and 2.6.2. This approach is discussed further in Section 6.


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The internal pressure coefficients in BS 6399-2 (Table 16), are defined based on the permeability of the building’s envelope. Permeability is defined as the open area in a face. Therefore, for equally permeable walls, each wall must have the same area of openings, as shown in Figure 5.21. If openings are typically shut, the permeability of the external wall is governed by the porosity of the wall sometimes referred to as the ‘background’ porosity. For equal permeability, based only on the background porosity, the area of the walls must be equal if similar cladding is used for the walls of the building (Figure 5.21).

The total area of openingsin each face is equal

Equallypermeable

The total area of openingsin each face is equal

Equally permeable

All faces equally porous

Equallypermeable

All faces equally porous

Not equally permeable

Figure 5.21 Definition of permeability considering constant openings or

porosity BRE Digest 436[7] provides some guidance on defining permeable and impermeable faces, by distinguishing between the two cases by a factor of 2.0. For example if the first face is more than twice as permeable as the second face, the first should be considered as permeable and the second as impermeable. This guidance should only be considered ‘as-a-rule-of-thumb’ and assumes that the difference in permeability between the walls will not significantly alter the internal pressure coefficient of −0.3. Reference 6 provides slightly different guidance and suggests that the difference between a permeable and impermeable face should be taken as a factor of 3.0. Both these ‘rules-of-thumb’ are equally valid considering the assumptions and simplifications for internal pressure coefficients given in BS 6399-2. If the two opposite walls are greater than three times the permeability (using the guidance given in Reference 6) of the other walls, the internal coefficients of +0.2 and −0.3, given in Table 16 BS 6339-2, should be used. However, it should be noted that the positive value of +0.2 represents an extreme bound and if used, conservative net suctions on the roof, rear and side walls will be calculated (Figure 5.20), resulting in increased size or reduced spacing of structural members in these areas. As an alternative the balance of airflow in and out of the building could be considered, as shown in Appendix C. Designers can use


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the approach to obtain a better estimate of the internal pressure coefficient and typically obtain more economical designs.

The forthcoming Eurocode on wind loading , derived using the principles of the balance of airflow discussed in Appendix C, provides a more accurate design method for determining the internal pressure coefficients.

For enclosed buildings that are subdivided into rooms where the internal doors are more than three times as permeable as the external doors, BS 6399-2 provides some guidance in Clause 2.6.1.2 on the calculation of internal pressure coefficients. Either conservative values given in this clause could be used, or the balance of airflow (as discussed in Appendix C) between the rooms and external envelope can be considered.

BS 6399-2 provides no guidance on:

• Permeable roofs.

• Buildings with either one or three permeable walls.

• Non-uniformly distributed permeability.

• Small non-dominant openings such as windows, vents, chimneys etc.

For portal frames the lack of guidance on the effect of the permeability of the roof on internal pressures is a significant omission. For further guidance designers could refer to the forthcoming Eurocode or use the balance of flows method discussed in Appendix C, which can result in more economical designs compared to the simple guidance given in the Code.


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6 DOMINANT OPENINGS

BS 6399-2 states ‘An opening will be dominant, and control the internal pressure coefficients when its area is equal to, or greater than, twice the sum of the openings in other faces which contribute porosity to the internal volume containing the opening’.

For a single storey building, if an opening is considered in one wall, ‘other faces’ refers to the other three walls and the roof. ‘Openings in other faces’ refers to physical openings and the ‘leakage’ through the face defined by the porosity of the cladding.

When there are two openings in one face they should be treated as a single opening with an area equal to the combined area of the two openings. This opening will be dominant if its area is at least twice that of all the other openings, including areas calculated from distributed porosity, from the other faces added together.

The areas of distributed porosity take into account the ‘leakage’ through the envelope of the building. Table 6.1 summarises guidelines given in References 2 and 7.

Table 6.1 Values of porosity for typical construction forms

Form of construction Porosity (φ)

Office curtain walling Open Area/total area = 3.5×10-4

Office internal partition walling Open Area/total area = 7.0×10-4

Typical housing in UK Open Area/total area = 10.5×10-4

Energy-efficient housing Open Area/total area = 4.0×10-4

Single leaf door Calculated using gap width = 1.5 mm when closed

The designer needs to define whether a dominant opening is likely to be open or closed during a severe storm. The guidance provided in BS 6399-2 for these two cases is discussed below.

6.1 Dominant openings that are likely to be open during a storm

For a dominant opening that is likely to be open during a storm, BS 6399-2 gives the following guidance for defining the internal pressure coefficient.

• When the area of the opening is at least twice as large as the sum of the other openings and porosity of the rest of the building, then Cpi = 0.75Cpe, where the external pressure coefficient Cpe is taken as the average external pressure coefficient at the dominant opening.

• When the area of the opening is three or more times than the sum of the other openings and porosity of the rest of the building, then Cpi = 0.9Cpe, where the external pressure coefficient Cpe is taken as the average external pressure coefficient at the dominant opening.

Alternatively the internal pressure coefficients could be calculated using the balance of airflow into and out of the building, as discussed in Appendix C.


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The response time to establish the internal pressure, increases with the size of the dominant openings and this is accounted for in BS 6399-2 via the calculation of dimension a, which governs the size effect factor (Ca). Dimension a is given by the greater of:

a = diagonal dimension of the dominant opening or,

3 internal2.0 volumea ×=

where the internal volume is the volume of the storey or room containing the dominant opening.

6.2 Dominant openings that are likely to be shut during a storm

For dominant openings such as windows and doors which are assumed to be shut during a storm, Clause 2.6.1.3 suggests that the response of the structure should be considered with the door or window open at the serviceability limit state (SLS) using appropriate partial load factors. This clause presents some confusion since SLS checks are primarily concerned with deflection, vibration, wind induced oscillation and durability, as stated in BS 5950-1.

A reasonable interpretation of the Clause 2.6.1.3 recommendation is to consider the situation as an Accidental limit state, with partial load factors of 1.0. In addition, BRE Digest 436[7] and Reference 2 suggests using a reduced probability factor Sp = 0.8 for this check. This reduced probability factor changes the risk from a mean recurrence interval of 50 years to a mean recurrence interval of approximately 1.8 years. If this guidance is followed the designer should be aware of the reduced levels of safety introduced by changing both the level of design risk and using partial safety factors of 1.0. Both BRE Digest 436 and Reference 2 state that if Sp is assumed to be 0.8 and serviceability load factors are used, then in practice this would require that doors and windows remain closed during periods when winter storms are forecast by the UK Meteorological Office as ‘likely to cause structural damage’. This will require some sort of active or passive safety management.


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7 OVERALL LOADS

Equation 7 in BS 6399-2 states that the overall horizontal loads are given by:

P = 0.85 ( ∑Pfront − ∑Prear) (1 + Cr)

where:

∑Pfront is the horizontal component of surface load summed over the windward-facing walls and roofs.

∑Prear is the horizontal component of surface load summed over the leeward-facing walls and roofs.

Cr is the dynamic augmentation factor.

This equation makes allowance, by introducing a 0.85 factor, for the non-simultaneous action of gusts on the windward and leeward faces.

The 0.85 factor, to reduce the wind load, can also be taken to apply when frame behaviour is considered, since it is unlikely that the full codified wind pressures will act over the entire frame at the same time. For example, the applied load on a simple portal frame (Figure 7.1) from wind direction θ = 0° (i.e. in the same plane as the frame) can be reduced by multiplying the net pressures by 0.85.

The 0.85 factor to reduce the wind loads cannot be used when:

(i) Considering the wind in the direction perpendicular to the frame.

(ii) Designing purlins and sheeting rails.

(iii) Designing cladding and associated fixings.

Wind parallel to frame

All net pressures are multiplied by0.85 when designing the frame forwind parallel to the frame.

Figure 7.1 Reduction in net wind pressures when designing frames The 2002 amendment resulted in an increase in external pressure coefficients for the walls (Table 5 of BS 6399-2) in the Standard Method. The amendment also introduced a new Table (Table 5a of BS 6399-2) giving net pressure coefficients for overall loads, when considering the load on the walls. The amendment for external wall pressures in Table 5 is based on the highest absolute values given by the Direction Method pressure coefficients over a range of wind directions. By adopting this approach, the Standard Method pressure coefficients become very conservative when considering overall loads. Therefore Table 5a was introduced, specifying the net pressure coefficients for overall loads on walls. The reduction in net pressure coefficients is additional to the 0.85 factor discussed above, as suggested in the note to Clause 2.1.3.6.


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Table 7.1 Net pressure coefficients for walls when calculating overall load (Table 5a of BS 6399-2)

B/D D/H ≤ 1.0 D/H ≥ 4.0

≤ 0.5 1.2 1.0

1.0 1.2 0.8

2.0 1.2 0.8

≥ 4.0 1.0 0.8

The 0.85 reduction factor can be applied to all net pressures for frame design, reducing the internal forces within the frame, which remain proportional to each other. The 0.85 factor also reduces the horizontal displacement of the frame.

Table 7.1 shows the reduction to the net pressures of both walls and therefore it is difficult to apply these reductions when designing the frame. For example, consider a building with D/H ≥4.0 and B/D=1. From Table 5 of BS 6399-2 the front coefficient is 0.6 and the rear coefficient is –0.5, resulting in an overall coefficient of 1.1, as shown in Table 7.1 (internal coefficients for the walls are ignored since they cancel out). Table 5a allows this overall coefficient to be reduced to 0.8 (see Table 7.1). However, for the design of the frame the actual net pressures are required on the front and rear of the building to allow the internal forces in the frame to be defined. Without guidance on how the reductions in Table 5a are applied to the structure, they should not be used for the design of the frame and connections.

The reductions in Table 5a can, however, be used for calculating the horizontal displacement, since the internal pressures are constant and the horizontal displacement of the frame is not governed by whether the reduction is applied to the front or rear walls. Therefore the frame should be designed using net pressures on each face, which are reduced by 0.85, as noted above and shown in Figure 7.1, and the resulting horizontal displacements are reduced by the factors given in Table 7.2 to account for the reductions in net pressures between the front and rear walls given in Table 5a of BS 6399-2.

+0.6

-0.3 -0.3

-0.5

Considering walls only the net pressure coefficient between front and rear walls is 1.1. Table 5a of BS 6399-2 allows this net coefficient to be reduced to 0.8. However, this is an overall reduction and no guidance is provided on how to reduce each net face pressures, which is necessary to determine the internal forces on the frame.

Figure 7.2 Pressure coefficients for horizontal loading on a typical

portal frame building


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Table 7.2 Reduction factors applied to horizontal displacements

B/D D/H ≤ 1.0 D/H ≥ 4.0

≤ 0.5 0.89 0.9

1.0 0.89 0.73

2.0 0.89 0.73

≥ 4.0 0.81 0.73

7.1 Frictional drag Frictional drag needs to be considered for long walls (wind parallel to the wall) and for roofs that are flat or where the roof is considered to be long.

The calculation of the frictional forces is given in Clause 2.1.3.8, with specific guidance given in Clause 2.4.5 for walls and Clause 2.1.3.6 for roofs. For portal frames with the wind parallel to the frames the frictional drag component is small and typically ignored. However, with the wind perpendicular to the frames (i.e. parallel to ridge and eaves) the component can be significant and should be considered when designing bracing in this direction.

In the Standard Method, the frictional drag component is calculated using Equation 7a in BS 6399-2. For walls, the area considered is zone C (refer to Clause 2.4.5) and for roofs the area considered is zone D (refer to Clause 2.1.3.6), as shown in Figure 7.3.

Wind

ZonesA&B

Zone C

Zone D

Zone AZone B

Zone C

Frictional loads

Figure 7.3 Area of structure where frictional loads are calculated when using the pressure coefficients given in the Standard Method

When deriving the overall forces on the building, the frictional loads should be added to the overall horizontal loads calculated using Equation (7). Therefore, the overall force parallel to the ridge is given by:

Overall force = [ 0.85( ∑qs Cp Ca A) (1 + Cr) ] + (qs Cf As Ca )roof + (qs Cf As Ca ) walls

(Note: The 0.85 factor only applies to the overall loads and not the frictional drag component; Cp is the net pressure coefficient given in Table 5a.)


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As is defined as the area of the building that contributes to the frictional drag. Ca is the size effect factor for this area. For flat, monopitch and duopitch roof, the frictional drag should be calculated for the area in zone D either side of the ridge and then added together to obtain the frictional load from the whole roof. The diagonal dimension a, used to determine Ca, is shown in Figure 7.4.

For a hipped roof, zone J (Figure 21 of BS 6399-2) should be used.

Zone D

Zone D

Zone C

Zone C

Zone A

Zone B

Dimension a

Dimension a

Zone B

Zone A

Figure 7.4 Definition of dimension ‘a’ for frictional drag on the roof A similar procedure is used to calculate the area that contributes to the frictional drag and size effect factor for the walls, except zone C is used.

The frictional drag coefficient Cf is obtained from Table 6 of BS 6399-2. An indication of the type of surfaces corresponding to the various values of Cf are illustrated in Table 7.3.

Table 7.3 Frictional drag coefficients

Type of surface Cf

Smooth surfaces without corrugations or ribs across the wind direction

0.01

Surface with corrugations across the wind direction

0.02

Surfaces with ribs across the wind direction

0.04


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8 ASYMMETRIC LOADS

Clause 2.1.3.7 of BS 6399-2 was introduced to allow for possible asymmetry of wind pressures, when using the symmetrical pressure coefficients given in the Standard Method. A similar clause is not included in the Directional Method, since asymmetry is considered by defining external pressure coefficients over the envelope of the building, from a number of different wind directions.

The use of symmetrical pressures over the building envelope (such as those defined using the Standard Method) can underestimate the structural response of the building. For example, the typical twelve load cases considered by using the Directional Method coefficients will result in a torque being placed on the footprint of the building due to the asymmetry of the pressure distribution. Whereas, using the Standard Method, symmetrical pressure coefficients are obtained and no torque is calculated. In addition, the pressure distributions from the Direction Method may result in one or more load cases where the minimum pressure/suction acts on one part of the structure and the maximum pressure/suction acts on another part of the structure. The Standard Method coefficients do not consider this variation in pressures and assume the maximum suctions/pressures occur at all points over the building. This simplification may not result in the worst pressure distribution for the structural response of the building.

Clause 2.1.3.7 was significantly revised in the 2002 amendment, and the onerous requirements in the 1997 version of the Code, have been removed. For pitched roofs, asymmetric loading is taken into account by considering the different load cases suggested by Table 10, which is reproduced for +5°, 15° and 30° pitch in Table 8.1.

Table 8.1 External pressure coefficients for duopitch roofs of buildings (taken from Table 10 of BS 6399-2)

Zone for θ = 0° Zone for θ = 90° Pitch angle α A B C E F G A B C D

–1.8 –1.2 –0.6 –0.9 –0.3 –0.4 –2.0 –1.1 –0.6 –0.5 +5°

0.0 0.0 0.0 –0.9 –0.3 –0.4

–1.1 –0.8 –0.4 –1.3 –0.9 –0.5 –1.6 –1.5 –0.6 –0.4 +15°

+0.2 +0.2 +0.2 –1.3 –0.9 –0.5

–0.5 –0.5 –0.2 –0.9 –0.5 –0.5 –1.2 –1.1 –0.6 –0.5 +30°

+0.8 +0.5 +0.4 –0.9 –0.5 –0.5

Notes.

1. At θ = 0° the pressure changes rapidly between positive and negative values in the range of pitch angles +5° < α < + 45°. Two sets of values are given at these pitch angles and they should be treated as separate load cases.

2. Interpolation for intermediate pitch angles may be used between values with the same sign.

Note 1 in the above Table states that two load cases should be considered for pitch angles between 5° and 45° when the wind direction is across the roof. The two load cases for a typical portal frame with a 10° pitch, considering pressure zones B, C, F and G, (Figure 20 of BS 6399-2) for a wind direction θ = 0°, are shown in Figure 8.1.


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-1.0 (zone B)

-0.5 (zone C)

-0.6 (zone F)

-0.45 (zone G)

10° pitch

Load Case 1

Wind

+0.1 (zones B and C)

-0.6 (zone F)

-0.45 (zone G)

Load Case 2

Figure 8.1 Load cases to be considered to account for fluctuations in

wind pressure Both load cases shown in Figure 8.1 will need to be considered. Load case 1 will generally govern the maximum reversal of moment at the eaves connection and Load Case 2 will govern the maximum horizontal displacement at eaves level and the length of the rafter where the bottom flange is in compression. The external pressure coefficients for the walls are the same in both load cases. Reductions can be applied to the pressure coefficients as discussed in Section 7.


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9 REFERENCES

1. BS 6399 Loading for buildings BS 6399-2: 1997 Code of practice for wind loads British Standards Institution, 1997

2. COOK, N. J. Wind loading a practical guide to BS 6399-2 wind loads on buildings Thomas Telford, 1999

3. COOK, N. J. Parametric calibration of BS 6399-2:1997 – Wind loads on rails and purlins The Structural Engineer, 2002. Vol 80 No. 4

4. BS 5950 Structural use of steelwork in building BS 5950-1: 2000 Code of practice for design. Rolled and welded sections British Standards Institution, 2001

5. BS 5950 Structural use of steelwork in building BS 5950-5: 1998 Code of practice for design of cold formed thin gauge sections British Standards Institution, 1998

6. COOK, N. J. The designer’s guide to wind loading of building structures: Part 2 Static structures Butterworths, 1990

7. BRE Digest 436: Wind Loading on Buildings Part 1: Brief Guidance for Using BS 6399-2:1997 Part 2: BS 6399-2:1997 worked examples – effective wind speeds for a site, and load on a two-storey house Part 3: BS 6399-2:1997 worked examples – loads on a portal frame building and on an office tower on a podium CRC Ltd, London, 1999. (Publishers for Building Research Establishment)

8. BRE Report. Wind loading handbook Building Research Establishment, 1974

9. ROBERTSON, A. P. Codification of local pressure for low-rise structures Journal of Wind Engineering and Industrial Aerodynamics, 1988. Vol 30

10. HOXEY, R., ROBERTSON A. P., and SHORT, L. The role of corner vortices in the design of structures Structural Engineering International, 1998. Vol. 8, No. 1

11. HOXEY, R. P. Structural response of a portal framed building under wind load Journal of Wind Engineering and Industrial Aerodynamics, 1991. Vol 38

12. BS 5950: Structural use of steelwork in building BS5 950-9: 1994 Code of practice for stressed skin design British Standards Institution, 1994


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APPENDIX A Worked examples

Page

Introduction 59

Using the Simplified Standard Method 60

Using the Standard Method 63

Using the Simplified Hybrid Method 69

Pressure coefficients 72


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P:\PUB\PUB800\SIGN_OFF\P286\P286V01d09.doc 59

Job No. BCB 733 Sheet 1 of 18 Rev

Job Title Design wind loads to BS 63992-2:1997

Subject Calculation of wind loads on a portal frames

Made by CJB Date April 2002

CALCULATION SHEET

Client

Checked by DGB Date May 2002

BS 6399-2 Ref

INTRODUCTION

This example demonstrates the calculation of wind loads on a typical portal frame building in Edinburgh using the following methods:

• Simplified Standard Method

• Standard Method

• Simplified Hybrid Method.

The site and location of the building are shown below.

Proposed building

Existing buildings

N

30°

Site Plan : Location Edinburgh. The site is 6 km from the sea

56.5m 33.9m

3.6m120mSlope = 0.064

Section through site

7m 9.64m

60m

30m

10°

Geometry of the building


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Sheet 2 of 18 Rev

BS 6399-2 Ref

USING THE SIMPLIFIED STANDARD METHOD (Refer to design procedure given in Section 3.2)

DYNAMIC CLASSIFICATION 1.6

H = 9.64 m (Note that Reference 7, Part 3 suggests a height to eaves may be assumed. Adopting the ridge height is conservative.)

Kb = 2 Table 1

Cr = 0.04 Fig. 3

Cr < 0.25, therefore BS 6399-2 can be used. 1.6.2

Basic wind speed Vb = 23.5 m/s 2.2.1 Fig. 6

Check topography

From inspection of the site plan the most onerous case for topography is in the upwind direction 90° to the longer side of the building (a bearing of 300°).

84.8m

30m33.9m56.5m

Wind

120m 3.6m

If building within shaded areatopography is significant

Section through significant topography

Fig 7

Upwind slope = 3.6 / 56.5 = 0.064 > 0.05

0.064 < 0.3 therefore extent of significant topography downwind = 1.5 × 56.5 = 84.8

Building is within shaded area ∴ Topography significant

Significant topography is included in the calculation of the altitude factor Sa

Sa is taken as the greater of: 2.2.2.2.3

Sa = 1 + 0.001∆s Eq. 10

or,

Sa = 1 + 0.001∆T + 1.2ψes Eq. 11

∆s = 123.6 m (site altitude) Fig. 8


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Sheet 3 of 18 Rev

BS 6399-2 Ref

∆T = 120 m (altitude of the upwind base of significant topography)

ψe = effective slope of the topographic feature

Upwind slope (ψu) is shallow 0.05 < ψu < 0.3 2.2.2.2.4

∴ ψe = ψu = 0.064

and effective slope length Le = Lu = 56.5 m

X/Le = X/Lu = 33.9 / 56.5 = 0.6 2.2.2.2.5

H/Le = 9.64 / 56.5 = 0.17

s = 0.6 (topography location factor) Fig. 10a

∴ Sa = 1 + 0.001(123.6) = 1.12

or,

Sa = 1 + 0.001(120) + 1.2(0.064)(0.6) = 1.17

∴ Sa = 1.17

SITE WIND SPEED

Vs = Vb × Sa × Sd × Ss × Sp 2.2.2.1

Vb = 23.5 m/s 2.2.1

Sa = 1.17 2.2.2.2.3

Sd = 1.00 (Note that if the orientation of the building is unknown or ignored, Sd may be taken as 1.0)

2.2.2.3

Ss = 1.00 2.2.2.4

Sp = 1.00 2.2.2.5

Vs = 23.5 × 1.17 × 1.0 × 1.0 × 1.0 = 27.5 m/s

TYPE AND EXTENT OF TERRAIN CATEGORIES AND EFFECTIVE HEIGHTS

Most onerous terrain, irrespective of direction (from approximately 300°) is Country Terrain

1.7.2

Closest distance from the sea, irrespective of direction = 6.0 km

Effective height = 9.64 m (Reference height to ridge and no upwind obstructions) 1.7.3.2

EFFECTIVE WIND SPEED

Ve = Vs × Sb 2.2.3

Sb = 1.74 Table 4

Ve = 27.5 × 1.74 = 47.9 m/s


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Sheet 4 of 18 Rev

BS 6399-2 Ref

DYNAMIC PRESSURE

qs = 0.613Ve2 2.1.2

qs = 0.613 (47.9)2 = 1406 N/m2 = 1.41 kN/m2

The above Simplified Standard Method is the design method that requires the least amount of calculation effort. If topography were not significant then the design procedure would be simplified further. The dynamic pressure could be used with the Standard Method pressure coefficients or the simplified uniform coefficients given in Appendix B.


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Sheet 5 of 18 Rev

BS 6399-2 Ref

USING THE STANDARD METHOD

The same design example is carried out using the Standard Method (Refer to design procedure given in Section 3.3)

DYNAMIC CLASSIFICATION

(As previous calculation)

1.6

Cr = 0.04

Cr < 0.25 therefore, BS 6399-2 can be used.

Basic wind speed Vb = 23.5 m/s (see previous calculations)

Orthogonal directions

Since the orientation of the building is known, the orthogonal directions can be defined. If the orientation is unknown, or ignored, then either the Simplified Standard Method or the Simplified Hybrid Method should be used. The Hybrid Methods may reduce any conservatism in the estimate of dynamic pressures.

The four orthogonal directions are shown below. The upwind effective height, terrain, distance from the sea and in town, topography, directional factors and the terrain and building factors need to be defined for each 90° quadrants.

N

30°

75°

75°

120°

165°

165°210°

255°

255°

300°

345°

345°

Orthogonal directions


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Sheet 6 of 18 Rev

BS 6399-2 Ref

CHECK TOPOGRAPHY

Topography is considered in the calculation of the altitude factor. Therefore, if topography is significant and is dependent on wind direction, the altitude factor will also be dependent on the wind direction.

From inspection of the site plan, the most onerous case for topography is from the upwind direction of 300°. However, topography is significant in the directional ranges: 255° to 345°, 345° to 75° and 165° to 255°C. The altitude factor could be calculated based on the worst topography in each directional range or the worst case assumed for all directional ranges. In this case, the worst topography is assumed for all ranges where topography is significant.

The altitude factor, Sa is taken as the greater of:

Sa = 1 + 0.001∆s 2.2.2.2.3

or,

Sa = 1 + 0.001∆T + 1.2ψes

(as previous calculation)

Sa = 1 + 0.001(123.6) = 1.12

or,

Sa = 1 + 0.001(120) + 1.2(0.064)(0.6) = 1.17

Therefore for directional ranges 255° to 345°, 345° to 75° and 165° to 255°, Sa is 1.17.

For the directional range 75° to 165° topography is not significant and the altitude factor is given by:

Sa = 1 + 0.001(123.6) = 1.12

SITE WIND SPEED 2.2.2

Each of the four directional ranges needs to be considered.

Vs = Vb × Sa × Sd × Ss × Sp

Direction 345° to 75°

Vb = 23.5 m/s

Sa = 1.17

Sd = 0.8 (most onerous at 345°) (Interpolation from Table 3) 2.2.2.3

Ss = 1.0 2.2.2.4

Sp = 1.0 2.2.2.5

Vs = 23.5 × 1.17 × 0.8 × 1.0 × 1.0 = 22.0 m/s


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Sheet 7 of 18 Rev

BS 6399-2 Ref


Vb = 23.5 m/s

Sa = 1.12

Sd = 0.83 (most onerous at 165°) (Interpolation from Table 3) 2.2.2.3

Ss = 1.0 2.2.2.4

Sp = 1.0 2.2.2.5

Vs = 23.5 × 1.12 × 0.83 × 1.0 × 1.0 = 21.9 m/s


Vb = 23.5 m/s

Sa = 1.17

Sd = 1.0 (most onerous at 240°) (Interpolation from Table 3)

Ss = 1.0 2.2.2.4

Sp = 1.0 2.2.2.5

Vs = 23.5 × 1.17 × 1.0 × 1.0 × 1.0 = 27.5 m/s


Vb = 23.5 m/s

Sa = 1.17

Sd = 0.995 (most onerous at 255°) (Interpolation from Table 3)

Ss = 1.0 2.2.2.4

Sp = 1.0 2.2.2.5

Vs = 23.5 × 1.17 × 0.995 × 1.0 × 1.0 = 27.4 m/s

TYPE AND EXTENT OF TERRAIN CATEGORIES

Each of the four directional ranges needs to be considered (take the most onerous condition within the directional range).

Direction 345° to 75° ⇒ Country

Direction 75° to 165° ⇒ Town 1.7.2




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Sheet 8 of 18 Rev

BS 6399-2 Ref

Distance from sea and in town: (taken from survey data)

Direction 345° to 75° : Nearest distance from the sea = 6.0 km Distance-in-town (not applicable – country terrain)

Direction 75° to 165° : Nearest distance from the sea = 60 km Nearest distance-in-town = 2.0 km

Direction 165° to 255° : Nearest distance from the sea = 141 km Distance-in-town (not applicable – country terrain)

Direction 255° to 345° : Nearest distance from the sea = 154 km Distance-in-town (not applicable – country terrain)

EFFECTIVE HEIGHTS 1.7.3

In all directions, assume reference height (Hr) = 9.64 m (height to ridge). A reduced effective height may be calculated based on upwind shelter.


Reference height = 9.64 m

Consider worst case of no shelter in directional range: ∴ Hd = 0.0

∴He = Hr = 9.64 m



Assume X0 is typical for suburban areas = 20 m (Refer to Reference 7)

Assume surrounding buildings are the same height as the proposed building and extend for at least 100 m (see Note 2 of Clause 1.7.3.3).

H0 = 9.64m

Since 2H0< X0 <6 H0 then Hd = 1.2 H0 – 0.2 X0

= 1.2 × 9.64 − 0.20 = 7.75 m

The reference height is given by the greater of

He = Hr – Hd or He = 0.4Hr

He = 9.64 – 7.57 = 2.07 m

or,

He = 0.4(9.64) = 3.86 m

∴He = 3.86 m


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Sheet 9 of 18 Rev

BS 6399-2 Ref



Consider most onerous condition of no shelter in directional range: ∴Hd = 0.0

∴He = Hr = 9.64 m



Consider most onerous condition of no shelter in directional range: ∴Hd = 0.0

∴He = Hr = 9.64 m

EFFECTIVE WIND SPEED


Ve = Vs × Sb 2.2.3.1


Vs = 22.0 m/s

Sb = 1.74 2.2.3.3 (Table 4)

Ve = 22.0 × 1.75 = 38.3 m/s


Vs = 21.9m/s

Sb = 1.29 2.2.3.3 (Table 4)

Ve = 21.9 × 1.29 = 28.3 m/s


Vs = 27.5 m/s

Sb = 1.61 2.2.3.3 (Table 4)

Ve = 27.5 × 1.61 = 44.3 m/s


Vs = 27.4 m/s

Sb = 1.61 2.2.3.3 (Table 4)

Ve = 27.4 × 1.61 = 44.1 m/s


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Sheet 10 of 18 Rev

BS 6399-2 Ref

DYNAMIC PRESSURE


qs = 0.613Ve2 2.1.2.1

Direction 345° to 75° ⇒ qs = 899 N/m2 = 0.90 kN/m2




0.90 kN/m2

0.49 kN/m2

1.20 kN/m2

1.19 kN/m2

Dynamic wind pressures

The dynamic wind pressures are lower than 1.41 kN/m2 value calculated using the Simplified Standard Method. The conservatism in the Simplified Standard Method arises from assuming the most onerous Sd factor (Sd = 1.0) coincides with the closest distance from the sea, in the most onerous terrain category and the most onerous topography.


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Sheet 11 of 18 Rev

BS 6399-2 Ref

USING THE SIMPLIFIED HYBRID METHOD

The same design example is carried out using the Hybrid Method (Refer design procedure given in Section 3.4).

Since the orientation is known, the only difference between the Standard and Hybrid Method, for this example, is in the calculation of the terrain and building factor.

TERRAIN AND BUILDING FACTORS (Using the procedure given in the Directional Method)


He = 9.64 m

Country terrain, distance from the sea = 6.0 km

Sb = Sc{1 + (gt × St) + Sh} 3.2.3.2.2

Sc = 1.078 Table 22

St = 0.177

gt = 3.44 (Assume a = 5 m and use size effect factor to account for load sharing with external pressures and time response for internal pressures)

3.2.3.3.3

Sh = 0.0 (Topography is significant, but has been taken into account in the calculation of the altitude factor; refer to guidance in Section 4.10)

3.2.3.4

Sb = 1.078{1 + (3.44 × 0.177) + 0

Sb = 1.73


He = 3.86 m

Town terrain

Distance from sea = 60.0 km, distance in town = 2 km

Sb = ScTc{1 + (gt × St × Tt) + Sh} 3.2.3.2.3

Sc = 0.85 Table 22

St = 0.20

Tc = 0.69 Table 23

Tt = 1.74

gt = 3.44 3.2.3.3.3

Sh = 0.00

Sb = 0.85 × 0.69{1 + (3.44 × 0.20 × 1.74) + 0}

Sb = 1.29


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Sheet 12 of 18 Rev

BS 6399-2 Ref


He = 9.64 m

Country terrain

Distance from sea = 141 km

Sb = Sc{1 + (gt × St) + Sh} 3.2.3.2.2

Sc = 0.99 Table 22

St = 0.18

gt = 3.44 3.2.3.3.3

Sh = 0.0

Sb = 0.99{1 + (3.44 × 0.18) + 0}

Sb = 1.60


He = 9.64 m

Country terrain

Distance from sea = 154 km

Sb = Sc{1 + (gt × St) + Sh} 3.2.3.2.2

Sc = 0.99 Table 22

St = 0.18

gt = 3.44 3.2.3.3.3

Sh = 0.00

Sb = 0.99{1 + (3.44 × 0.18) + 0}

Sb = 1.60

EFFECTIVE WIND SPEED 2.2.3.1


Ve = Vs × Sb


Vs = 22.0 m/s (from Standard Method)

Sb = 1.73

Ve = 22.0 × 1.73 = 38.1 m/s


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Sheet 13 of 18 Rev

BS 6399-2 Ref


Vs = 21.9 m/s

Sb = 1.29

Ve = 21.9 × 1.29 = 28.3 m/s


Vs = 27.5 m/s

Sb = 1.6

Ve = 27.5 × 1.6 = 44.0 m/s


Vs = 27.4 m/s

Sb = 1.6

Ve = 27.4 × 1.6 = 43.8 m/s

DYNAMIC PRESSURE

Each of the four directional ranges needs to be considered 2.1.2.1

qs = 0.613Ve2





The dynamic pressures calculated from the Simplified Hybrid Method and the Standard Method are almost identical. This is due to the proposed building being exactly 2 km in town in one directional range and classed as country terrain in the other directional ranges. At exactly 2 km into towns, Sb calculated from the Directional Method is the same as that calculated from the Standard Method. At other distances into town, use of the Directional Method, reduces Sb and thus the dynamic pressure. The reduction is most significant when close to the sea, and at a small effective height.


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Sheet 14 of 18 Rev

BS 6399-2 Ref

PRESSURE COEFFICIENTS

The dynamic pressure calculated using the above methods could be used with the Standard Method pressure coefficients in BS 6399-2 or the simple net coefficients presented in Appendix B. Both procedures are shown below.

Assume a dynamic pressure of 1.2 kN/m2 acting in all directions.

STANDARD METHOD PRESSURE COEFFICIENTS

External pressure coefficients for walls 2.4

a) Wind parallel to ridge

H = 9.64 m 2.4.1.2

B = 30 m 2.4.1.3

D = 60 m

Span ratio D/H = 6.22

Scaling length b = 19.3 m (Smaller of B or 2H)

External pressure coefficients Table 5

+0.6 -0.5 -0.5-0.8

-1.3

3.9m

19.3m

Windward Leeward Side face

b) Wind normal to ridge

H = 7.00 m 2.4.1.2

B = 60 m

D = 30 m

Span ratio D/H = 4.3

Scaling length b = 14 m (Smaller of B or 2H)

Wind

Wind


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Sheet 15 of 18 Rev

BS 6399-2 Ref

External pressure coefficients

+0.6 -0.5

Windward Leeward

14m

2.8m

-0.5-0.8

-1.3Wind

Side face

Table 5

External pressure coefficients for roofs 2.5

a) Wind parallel to ridge 2.5.2

H = 9.64 m

bw = 19.3 m (Smaller of W or 2H) 2.5.2.2

Wind

-1.8

-1.3

-1.3

-1.8

-0.6

-0.6 -0.45

-0.45

9.65m1.9m

(Coefficients interpolated from Table 10 for 10° slope)

Table 10

b) Wind normal to the ridge

H = 9.64 m

bL = 19.3 m

-0.45

-0.5 / +0.1

-1.1 -0.6 -1.1

-1.45 / +0.1 -1.45 / +0.1-1.0 / +0.1

1.9m

1.9m

9.65m 9.65mWind (Coefficients interpolated from Table 10 for 10° slope)

Table 10


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Sheet 16 of 18 Rev

BS 6399-2 Ref

Calculate internal pressure coefficient

The permeability of the walls are within a factor of 3.0 of each other (refer to Section 5.7).

∴ Four walls are equally permeable Cpi = -0.3 Table 16

Calculate internal diagonal dimension ‘a’

It is assumed that there are no dominant openings

∴ 310 storeyofvolumeernalinta ×= 2.6.1.1

( ) m2462/64.27306010 3 =+×××=a

Note that the size effect factor, Ca differs for external and internal surface pressures. In this example, the size effect factor for internal pressures is identified as Cai and that for external pressure as Cae

Calculate internal size effect factor Cai 2.1.3.4 Figure 4

Height = 9.64 m

Most onerous terrain = Country

Closest distance from the sea = 6.0 km

∴ use line A in Figure 4

Cai = 0.76

Calculate internal surface pressure 2.1.3.2

pi = qs Cpi Cai

where: qs = 1.2 kN/m2

pi = 1.2 × (-0.3) × 0.76 = -0.27 kN/m2

Calculate external size effect factor 2.1.3.4

Let a = 5.0 m (Conservative)

∴ Cae = 1.0

Calculate net surface pressure 2.1.3.3.

p = pe – pi

where:

pe = qs Cpe Cae 2.1.3.1

qs = 1.2 kN/m2

Cae = 1.0

and the external coefficients were calculated previously


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Sheet 17 of 18 Rev

BS 6399-2 Ref

NET SURFACE PRESSURES

a) Walls – Wind parallel to ridge

(All loads in kN/m2)

b) Walls – Wind normal to ridge

+0.99 -0.33

Windward Leeward

14m

2.8m

-0.33 -0.69

-1.2

9 Wind

Side face

c) Roof – Wind parallel to ridge

Wind

-1.8

9-1

.29

-1.2

9-1

.89

-0.45

-0.45 -0.27

-0.27

9.65m1.9m

d) Roof – Wind normal to ridge

-0.27

-0.33 / +0.39

-1.05 -0.45 -1.05

-1.47/+0.39 -1.47/+0.39-0.93 / +0.39

1.9m

1.

9m

9.65m 9.65mWind

+0.99 -0.33 -0.33-0.69

-1.2

9

3.9m

19.3m

Windward Leeward Side face


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Sheet 18 of 18 Rev

BS 6399-2 Ref

SIMPLIFIED NET PRESSURE COEFFICIENTS

(Refer to Appendix B for derivation and guidance on using simplified net pressure coefficients)

Roof purlins

Assume a span of 6.0

Bw = 1.9 m (Wind normal to ridge)

Bw ≤ span/2, therefore coefficients can be used.

∴ Net pressure coefficients = –1.04 /+0.25 (Zone X; Appendix B, Table B.2)

∴ Purlin loads = -1.25/+0.3

Double purlins in zone Y (9.64 m × 1.9 m)

1.9m

Y

Y

Y

Y

Zone X

Zone X

1.9m

9.64m 9.64m

Reduce spacing of purlins by ½ in zone Y

(Conservative)

Side rails

Span = 6.0 m

0.2b = 3.9 m

0.2b ≤/ span/2, therefore coefficients cannot be used

Pragmatic solution : Use the coefficients given in Table B.3. Specify rails using net pressure coefficients and reducing the spacing by half in the end bays.

Cladding and fixings

Use net coefficients in Tables B.4 and B.5.


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APPENDIX B Simplified uniform net pressure coefficients for portal frames

The derivation is presented for simplified uniform net pressure coefficients, based on the principles of BS 6399-2. Although these coefficients have been derived for use in single storey buildings with duopitch roofs, the basic principles can easily be applied to other types of buildings to determine similar net pressure coefficients.

B.1 Localised loads and the design of purlins It is possible to represent the external localised pressures on a simply supported beam as an equivalent uniform pressure that would give the same maximum displacement and the same resistance to lateral torsional buckling (according to BS 5950-1), assuming the tension flange is unrestrained.

The equivalence is shown diagrammatically in Figure B.1.

βwp

wp

x

L

(=)(β -1)wp

(+)wp

αwp

(≡)

Figure B.1 Representation of an equivalent uniform pressure (load)


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The value of α, representing the equivalent pressure, is determined from Figure B.2 and is based on β (which defines the relative magnitude of the localised pressure at one end) and X (which defines the distance along the span of the localised pressure).

Inspection of Figure B.2 shows that at X = L/2 the value of α = (β + 1)/2, that is, the equivalent pressure is the mean of the stepped pressures. For simple evaluation, this value can be used conservatively for all cases where X ≤ L/2.

For a wind direction of 90° (parallel to the ridge and eaves), Figure 20c) of BS 6399-2 indicates that the width of the area where the localised pressure occurs is given by bW/10, where bW is the scaling length. The value bW is the smaller of the width of the building or twice the height of the building (see Section 5.2). Provided that the value of bW/10 is less than or equal to half the span of the purlin, the equivalent pressure coefficients presented in this Appendix can be used. That is, the mean of coefficients A and C for one half and the mean of B and C for the other half (A, B and C are given in Table 10 of BS 6399-2). Since the magnitude of A is always equal to or greater than that of B, the mean of A and C can be used conservatively for all the purlins adjacent to the gable and also, since A is always greater than C, for all the purlins. Note that if the building’s geometry is such that the extent of the localised zones (bW/10) is more than half the span of the purlins, equivalent pressure coefficients cannot be used and an equivalent uniform load should be determined by applying Figures B.1 and B.2 directly.

For a wind direction of 0° (perpendicular to the ridge) the extent of the localised zones are as shown in Figure 20(b) of BS 6399-2 as extending a distance bL/2 transversely and a distance bL/10 up (or down) the slope of the roof. Since the distance bL/2 is in all practical cases greater than half the span of the purlins, an equivalent pressure coefficient cannot be used. Therefore, it should be assumed that the purlins under the localised zone attract the full wind load and should be designed accordingly. The purlins concerned are those within a distance bL/10 from the eaves, as shown in Figure B.3.

Equivalent UDL for a single span member

0.5

1

1.5

2

2.5

3

3.5

4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

x / length of beam

α

β =2.5

β =2.0

β =3.0

β =3.5

Figure B.2 Values of α to define an equivalent UDL


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It should also be noted from Table 10 of BS 6399-2 that the equivalent negative coefficients for wind at 90° (the mean of coefficients A and C) are always greater than the coefficient for Zone C with wind at 0°. Consequently, it is possible to give a simple conservative representation of the external negative pressure coefficients by using the equivalent pressure coefficient (the mean of A and C, as described above) for the majority of the roof and, where greater, the localised coefficient for wind at 0°. This can be represented as Zones X and Y, as shown in Figure B.4. The greatest positive pressures occurring in Zones X and Y are obtained from considering Table 10 and Figures 20(b) and 20(c).

The equivalent external pressure coefficients applicable to these zones are given in Table B1.

Purlins need to bedesigned for localisedpressures

Wind

Figure B.3 Location of purlins that must be designed for localised

pressures

Y

X

Y

X

Y Y

bL / 2 bL / 2

bL / 10

bL / 2 bL / 2

bL / 10

bL / 10

bL / 10

90° 270°

180°

0°

NOTE: The use of the Zones shown and equivalent pressure coefficients are applicable only when bW / 10 ≤ 0.5 times the purlin span

Figure B.4 Simple conservative representation of zones for external and net pressure coefficients for all wind directions


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Table B.1 Simple equivalent external pressure coefficients

Equivalent external pressure coefficients Cpe (Conservative values) Pitch angle α

Zone X Zone Y

–45° –1.25 –1.25

–30° –1.35 –1.70

–15° –1.70 –2.60

–5° –1.45 –2.40

+5° –1.30

+0.00 –1.80

+0.00

+15° –1.10

+0.20 –1.30

+0.20

+30° –0.90

+0.50 –0.90

+0.80

+45° –0.90

+0.70 –0.90

+0.80

+60° –0.95

+0.80 –0.95

+0.80

+75° –1.20

+0.80 –1.20

+0.80

In Figure B.4, Zone Y has an area of (bL /10 × bL /2), where bL is the smaller of the length of the building or twice the height of the building. For portal frames, twice the height of the building typically governs and therefore Zone Y is generally small. For example, for a portal frame 30 m wide, 100 m long and 12 m to the height of the ridge, the area of Zone Y would be 2.4 m by 12 m. The design of the majority of the purlins will generally be based on the coefficients in Zone X; for Zone Y, either stronger or additional purlins would be required. A simple pragmatic approach would be to design the purlins based on Zone X and reduce the spacing by half (while retaining the same purlin size) in the area covered by Zone Y.

To obtain net pressure coefficients, load sharing and internal pressure coefficients have to be considered.

Load sharing is taken into account in the Standard Method by the use of the size effect factor Ca, such that the external pressure is given by:

pe = qsCpeCa

where:

qs is the dynamic pressure

Cpe is the external pressure coefficient

Ca is the size effect factor for external pressures.

The size effect factor is obtained from Figure 4 of BS 6399-2 and is dependent on dimension a and the site location. The dimension a is defined as the diagonal dimension of an area over which load sharing occurs (refer to Section 5.5). For purlins, dimension a should, conservatively, be taken as the diagonal dimension of its loaded (tributary) area. Typically, the value of Ca will be near 1.0 for single span purlins and this value is conservatively assumed in the derivation of the simple net coefficients, given below.


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BS 6399-2 gives an internal pressure coefficient of –0.3 if the four walls are equally permeable. The use of –0.3 for an internal pressure coefficient is also recommended in Reference 8.

The internal pressure calculated by BS 6399-2 is given by:

pi = qsCpiCa

where:


Cpi is the internal pressure coefficient

Ca is the size effect factor for internal pressures.

The size effect factor is governed by dimension a given by:

310 storeyofvolumeinternala ×=

It is difficult to provide definitive values for CpiCa, as the size effect factor changes for a given building geometry However, consider the net pressure, which is given by:

pe = qs (CpeCae – CpiCai)

where:


Cpe is the external pressure coefficient

Cae is the size effect factor for external pressures (conservatively assumed to be 1.0)

Cpi is the internal pressure coefficient (assumed to be –0.3, refer to Section 5.7)

Cai is the size effect factor for internal pressures.

If the limits given in Figure 4 of BS 6399-2 are taken, very conservative (and unrealistic) values of Cai can be obtained. If the worst site location is considered, the limits of the size effect factor (Figure 4 of BS 6399-2) are 1.0 and 0.52. Depending on the sign of the external coefficient a conservative value of 1.0 or 0.52 should be applied to the internal coefficient of –0.3 to obtain the highest possible absolute value of net pressure coefficient. For example, if the external pressure is negative (suctions) then a conservative value for Cai of 0.52 should be assumed, giving a value of –0.156 for CpiCai. If the external pressures are positive then a conservative value for Cai of 1.0 should be assumed, giving a value of –0.3 for CpiCai. By using these values with the equivalent external coefficients shown in Table B.1, conservative net pressure coefficients, as shown in Table B.2, can be calculated and used for all single story buildings with duopitch roofs, provided that:

• bW/10 ≤ half the span of the purlins,

• Cpi = –0.3 (Table 16 of BS 6399-2; see Section 5.7).

It is worth emphasising that this simplified representation of the BS 6399-2 pressure coefficients is extremely conservative, since unrealistic (but safe) values are assumed for the size effect factors for the internal pressures. More


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economical designs can be obtained by determining actual values for the size effect factors for the building considered.

Table B.2 Simplified net pressure coefficients for purlins in a duopitch roof

Simplified net pressure coefficients Cnet (conservative values) Pitch angle

α Zone X Zone Y

–45° –1.09 –1.09

–30° –1.19 –1.54

–15° –1.54 –2.44

–5° –1.29 –2.24

+5° –1.14 +0.30

–1.64 +0.30

+15° –0.94 +0.50

–1.14 +0.5

+30° –0.74 +0.80

–0.74 +1.10

+45° –0.74 +1.00

–0.74 +1.10

+60° –0.79 +1.10

–0.79 +1.10

+75° –1.04 +1.10

–1.04 +1.10

Note: Values can be reduced significantly by considering actual value of the size effect factor for the internal pressures.

For simple conservative design, the purlins can be designed using the net pressures for Zone X given in Table B.2, i.e. the wind load on the purlins is given by:

p = qsCnet

where:

qs is the dynamic pressure,

Cnet is the net pressure coefficient for Zone X, from Table B.2.

In Zone Y, the same purlins may be used, at a spacing half that in Zone X.

B.2 Design of side rails Table 5 of BS 6399-2 gives the external pressure coefficients for vertical walls, when using the Standard Method.

A similar procedure for calculating net pressures on purlins can be used for side rails provided that:

• 0.2b ≤ half the span of the side rails,

• Cpi = −0.3 (Table 16 of BS 6399-2, refer to Section 5.7 and Reference 8).


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It should be noted that the value of 0.2b (refer to Figure 12 of BS 6399-2) is greater than BW/10 (refer Section 5.1) and it may be found that the simple uniform net coefficients could be used for the roof but not for the walls. However, for the vast majority of portal frames the limits, which ensure that the localised pressures act over less than half the span of the purlins/side rails, will not be exceeded.

The net pressure coefficients are given in Table B 3.

Table B.3 Simplified net pressure coefficients for the design of rails within a wall

Simplified net pressure coefficients (conservative values)

Span ratio of building Exposed case Vertical wall face D/H ≤ 1 D/H ≥ 4

Vertical wall face Isolated Funnelling

Windward face

+1.15 +0.9

Leeward face

–0.34 –0.34

Side face –0.9 –1.1

B.3 Design of main structural members in portal frames

Unless the building is unusually high, wind loads do not generally govern the sizing of the structural members of the portal frame. However, wind loads will generally govern the design of connections for reversal of moment, the stability of rafters and the deflection of the frame. Appropriate load combinations and partial load factors are needed for these design checks. The critical wind condition will generally be perpendicular to the ridge of the building, and the simplified net pressure coefficients presented in this Appendix can be used. For the roof, the coefficients corresponding to Zone Y in Table B.2 can be ignored, because of the conservatism of the Zone X coefficients. Since the portal frame is continuous, Clause 2.1.3.6 of BS 6399-2 can be used which allows the coefficients to be reduced by applying a factor of 0.85. In addition, a reduction factor can be applied to the calculated horizontal displacements, as discussed in Section 7.

Where two coefficients are given in Table B.2 for Zone X, two load cases should be considered when designing the frame. The first consists of the full wind load on the frame, which generally governs the maximum reversal of moment in the eaves connection. The second consists of placing the positive net pressure coefficient on the windward slope and the negative net pressure coefficient on the leeward rafter. This will result in a load case that produces the maximum horizontal displacement of the frame and the length of the rafter where the bottom flange is in compression. These wind loads should be part of the loading combination considered (i.e. dead + wind) and the associated partial safety factors for ultimate and serviceability limit states must be used.

This simplified approach of checking the main frame for asymmetric loads is more onerous compared to designs using the true variation of coefficients given in BS 6399-2, together with the actual internal and external size effect factors (instead of the conservative values of 1.0 and 0.52 used in this Appendix). However, the approach is not expected to increase the member size of the


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frame, which is generally governed by gravity load. It is possible however, that more stays to the rafters will be required and an extra row of bolts needed to account for the reversal of moment in the connections. If this is unacceptable to the designer then the actual coefficients in BS 6399-2 could be used. This will involve checking a number of frames. For example, the penultimate frame will have a different wind load compared to a frame in the centre of the building, due to the variation of external pressure coefficients across the structure. This will result in an increase in calculation effort and the possible use of different frames, or spacing of frames, throughout the building.

B.4 Design of cladding and fixings Following the simplified procedure presented in this Appendix for purlins and side rails, a conservative set of net maximum pressure coefficients (derived from BS 6399-2) can be calculated for the design of the cladding and associated fixings. These coefficients are based on an internal pressure coefficient of –0.3 and the conservative limits of the size effect factor discussed in Section B.1. These net coefficients are given in Table B.4 for duo-pitch roofs and Table B.5 for walls.

B.5 Summary of the application of the simplified net pressure coefficients

The uniform net pressure coefficients, presented in this Appendix, are conservative when compared with the full range of BS 6399-2 coefficients over the building. If the actual coefficients in BS 6399-2 are used, the number of purlins/rails, particularly in the central region of the building, could be reduced, provided that the spacing is not governed by the required restraint to the main frames. The penalty, however, is an increase in calculation effort and complicated site layouts. Similarly, the simplified net pressure coefficients for the design of the cladding and its fixings could be significantly reduced,

Load case will generally define maximumreversal of moments in connections.

Negative pressures are removedfrom windward rafter (if positivepressures are possible theyshould be applied)

Load case will define maximum horizontaldisplacement and typically defines maximumlength of bottom flange compression in rafter

Figure B.5 Load cases to be considered for the design of portal

frames


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particularly in the central region of the building, if the actual coefficients in BS 6399-2 are used. Again, increase in calculation effort and complicated (and impractical) site layouts will be the penalty.

There are some limitations on the use of the uniform net pressure coefficients given in this Appendix for the design of purlins and side rails. These are required to ensure that the localised pressure coefficients act over less than half the span of the purlins and side rails. Considering the size of the localised pressure coefficients given in BS 6399-2, these limitations will not be exceeded for most typical sizes of buildings.

A further limitation on the use of the simplified net pressure coefficients is the requirement that the internal pressure coefficient must be taken as –0.3. Table 16 of BS 6399-2, states that a uniform value of –0.3 can be used if the four walls are equally permeable in an enclosed building containing non-dominant external doors and windows that are closed during a storm. The four walls are assumed to be equally permeable if the permeability of each wall does not differ from that of the other walls by more than a factor of 3.0, as suggested in Reference 7 and discussed in Section 5.7. The positive value of +0.2 given in BS 6399-2 is not possible unless it is assumed that no airflow occurs through the impermeable walls. Therefore for typical portal frames, with non-dominant windows and doors that remain closed during a storm, the internal pressure coefficient should be taken as –0.3. Permanent non-dominant openings in the walls, such as ventilators, will not affect the internal pressure coefficient provided they are distributed approximately equally around the perimeter of the building so that the assumption of the four walls being equally permeable is still valid. For permanent non-dominant openings that are present in only some of the walls, resulting in the walls not being equally permeable, the worst case of +0.2 and –0.3 for the internal coefficient (Table 16 of BS 6399-2) should be adopted. Alternatively, the balance of airflow in and out of the building could be considered, as discussed in Appendix C, and the simple net coefficients given in this Appendix revised to suit. If any openings are considered to be dominant then the recommendations given in Section 6 should be followed.


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Table B.4 Simplified net pressure coefficients for cladding and fixings forming a duopitch roof

Pitch angle α Simplified net pressure

coefficients Cnet (conservative values)

–45° –1.34

–30° –1.54

–15° –2.44

–5° –2.24

+5° –1.84

+0.30

+15° –1.44

+0.50

+30° –1.04

+1.10

+45° –1.04

+1.10

+60° –1.04

+1.10

+75° –1.04

+1.10

Note: Values can be reduced significantly by considering actual value of size effect factor on the internal pressures

Table B.5 Simplified net pressure coefficients for the design of cladding and fixings within a wall

Simplified net pressure coefficients (conservative values)

Span ratio of building Exposed case Vertical wall face D/H ≤ 1 D/H ≥ 4

Vertical wall face Isolated Funnelling

Windward face

+1.15 +0.9

Leeward face

–0.34 –0.34

Side face –1.14 –1.44


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APPENDIX C Balance of airflow

C.1 Basic principles The internal pressures in a building with or without dominant openings can be determined by considering the balance of airflow in and out of the building. The following guidance is offered for determining the internal pressures in this way.

The net airflow (Q) through an opening may be expressed as:

Q = kApn

where k is a coefficient, A is the area of the opening and p is the pressure difference across the opening. The power n is the flow exponent and is dependent on the type of airflow. For the calculation of the internal pressures given in BS 6399-2 the value of n was taken as 0.5, which is generally considered sufficient for most buildings.

The total flow in and out of the building must balance (i.e. inflow = outflow), therefore:

∑∑ −=−outflowwithsWalls/roof

eiinflowwithsWalls/roof

ie ppkAppkA

C.2 Example using balance of airflow In the above expression, the pressure difference in each term for inflow or outflow is always positive, resulting in a valid square root. The use of the above expression is shown by considering the example of four walls forming a typical single storey building and assuming that the coefficient k is the same for all openings.

The external pressure coefficients together with the discrete zones over which each coefficient occurs are shown in Figure C.1. These external coefficients are taken from Table 5 of BS 6399-2. The height of the building, for single storey industrial buildings typically governs the scaling factor (refer Section 5.2) used to define the width of the zones on the side walls. For this example, the height is assumed to be 7.0 m.


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Using the external coefficients given in Figure C.1, and dividing through by the coefficient k (assumed the same for all openings), the balance of airflow equation is:

)5.0(

)5.0()8.0()3.1(

)5.0()8.0()3.1(6.0

piRear

piSideC2piSideB2piSideA2

piSideC1piSideB1piSideA1piFront

−−

+−−+−−+−−

+−−+−−+−−=−

CA

CACACA

CACACACA

If the pressure difference is negative in any of the square root terms in the above equation, the order of the external and internal coefficient is reversed to achieve a positive number and the term moved to the other side of the equation. (i.e. outflow becomes inflow or inflow becomes outflow).

The solution of Cpi from the above equation requires an iterative process. This can be carried out using any spreadsheet program, such as Microsoft Excel, which iterates using ‘circular references’. The procedure for developing the spreadsheet is given in Reference 3.

The variation of the internal pressure coefficient can be investigated by considering the external pressure coefficients on the four walls shown in Figure C.1. It is assumed that all four walls have the same porosity. Therefore, by increasing the cross-wind breadth of the building (B) the permeability of the front and rear walls will increase in relation to the side walls. The value of the internal pressure coefficient for various aspect ratios of D:B is shown in Figure C.2. Also shown in this Figure is the internal pressure coefficient for buildings with different heights, which controls the size of the zones of external pressure on the side walls.

B (vary to increase permeability)

D =

30m

2.8m

11

.2m

16

m

-0.5

-0.8

-1.3

+0.6

-1.3

-0.8

-0.5

Side A1

Side B1

Side C1

Side A2

Side B2

Side C2Rear

Front

Wind

-0.5

Figure C.1 External pressure coefficients used for the calculation of

balance of airflow


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Figure C.2 shows that when the four walls are equally permeable (aspect ratio = 1) the internal pressure coefficient is –0.46 for a building height of 7.0 m (BS 6399-2 gives a value of –0.3 if the four walls equally permeable). As the internal pressures are driven by the external pressures, the use of the balance-of-flow theory is valid and the coefficients given in Figure C.2 could be used. This approach is endorsed in References 3 and 7 and is the method referenced in Clause 2.6.1.2 of BS 6399-2. The use of a value of −0.46 will increase the net coefficients when the external coefficients are positive (windward) and decrease the net coefficients when the external coefficients are negative (side-walls, rear-walls and roof).

The balance-of-flow, using the coefficients in Figure C.1, shows that positive internal pressures do not occur even though the permeability of the front and rear walls are 10 times more permeable than the side walls, with the wind normal to the permeable face (Figure C.2). The internal pressure coefficient can only approach the value of +0.2, given in Table 16 in BS 6399-2, when the side-walls are infinitely impermeable. This assumes that there is no airflow across the side-walls. Therefore, it can be argued that the +0.2 internal coefficient cannot be obtained and should be considered as an upper bound.

The balance of airflow can be used to determine the internal pressures for any building and more accurate results can be obtained, compared to the Code, because the effect of defined openings around the envelope and the permeability of the roof can be considered. The forthcoming Eurocode (EN 1991-1-4) will give a more accurate method for determining the internal pressures based on the principle of balance of airflow.

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

00 1 2 3 4 5 6 7 8 9 10

Permeability aspect ratio

Inte

rnal

pre

ssur

e co

effic

ient

H = 12m

H = 5.0m

H = 7.0m

Figure C.2 Internal pressure coefficients for different permeability

between walls (Wind normal to the permeable face)


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Documents

Guide to Evaluating Wind Loads