Wind Loading From BS6399 to Cp3

WIND LOADING ON TRADITIONAL DWELLINGS: AMENDMENT OF SIMPLIFIED DESIGN GUIDANCE P A Blackmore Building Research Establishment 1999 EXECUTIVE SUMMARY The British Standard code of practice for wind loading, CP3: Chapter V: Part 2, is soon to be withdrawn and replaced by BS6399: Part 2. The methodology used in this new code is quite different in many respects from that used in CP3. The simplified design approach for wind loading currently contained in the Scottish Office ‘Small Buildings Guide’, which is currently based on CP3, therefore needs to be modified to reflect the new methodology of BS6399. A simplified design approach for wind loads on traditional buildings has been previously developed for the Department of Environment, Transport and the Regions by BRE under the guidance of an industry Steering Group. This previous method was developed specifically for inclusion in Approved Document A of the England and Wales Building Regulations. This report extends the methodology to the wind climate in Scotland. This proposed simplified design method has a simple five-step approach and includes factors that take account of altitude and topography. There are some quite large differences in allowable building heights between the current and proposed methods, but in general, these differences are largely beneficial. In many town and country areas of Scotland, the allowable building height has increased, although at high altitudes or in the northernmost Islands the allowable building heights are reduced. The proposed method includes a more accurate treatment of topography, which allows significant increases in building height for all sites on hilly or moderately hilly terrain.

Building Research Establishment Centre for Structural Performance

CONTENTS 1 Introduction....................................................................................................................1 2 Background to Previous Work.......................................................................................1 3 Principle Differences Between CP3: ChV: Part 2 & BS6399: Part 2...........................2 4 Description of the Proposed Simplified Method ...........................................................2 5 Implications of the Proposed Changes for Traditional Building Design.......................8 6 Conclusions..................................................................................................................11 7 References....................................................................................................................11 Annex A List of Steering committee members Annex B Sensitivity analysis of factors affecting design wind load in BS6399 Annex C Assumptions made in the development of the simplified method


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1 INTRODUCTION The latest revision to the British Standard Code of Practice for wind loads, BS6399: Part 2 [1] was published in August 1997. The methodology used in this new Code is different from that used in its predecessor, CP3: Chapter V: Part 2 [2]. Some of the major changes include: ! Basic wind speed - now given as hourly mean values rather than gust values ! Terrain categories - reduced to two from four ! Site altitude - now included as a separate factor ! Distance of site from coast - now included Results from calibrations between these two Codes show that in general that the two methods give similar results, although significant differences in design wind loads can arise in some circumstances, particularly for sites at high altitudes. The Scottish Office Small Building Guide and the equivalent guidance for England & Wales and Northern Ireland, together with a large number of British Standards, including BS 8103: Parts 2 & 3 "Structural design of low rise housing" and BS 5268 Part 3 "Code of practice for trussed rafter roofs", all currently include design guidance based on the former wind Code. These authoritative documents therefore need to be amended to take account of the provisions in the latest wind Code. 2 BACKGROUND TO PREVIOUS WORK A previous study [3], funded by DETR, was undertaken by BRE to develop simplified design guidance for wind loading based on BS 6399: Part 2, for inclusion in Approved Document A, BS8103: Parts 2 & 3 and BS5268, to replace current guidance based on CP3: ChV: Part 2. This work was carried out under the guidance of an industry steering group, membership of which is given in Annex A. As these foregoing authoritative documents referred to primarily concern the construction of traditional dwellings, the objective of this study was to produce an amended means of allowing for wind loading without generally resulting in changes to current construction. However, it was accepted that, exceptionally, some changes in construction might be warranted. Existing simplified methods based on BS6399: Part 2, such as the method from the Centre for Window and Cladding Technology (CWCT) and the draft revision to BS6262: Code of Practice for Glazing for Buildings, Part 3: Fire, Security and Wind Loading, were considered by the steering committee. However, it was concluded that these were not appropriate because they were developed specifically for cladding design, were too detailed or too material dependant.


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The steering group considered a number of different simplified methods, which are described in detail in [3], before eventually settling on a simple five step approach which is described in Section 4. 3 PRINCIPLE DIFFERENCES BETWEEN CP3: CHV: PART 2 & BS6399: PART 2 The principle reasons for the revision of CP3: Chapter V: Part 2 are: ! To take account of the considerable advances in knowledge and experience of wind

engineering gained in the last 30 years. ! To move from a 3 second gust windspeed to an hourly mean value in order to allow more

accurate treatment of wind flow over topography and to provide a starting point for fatigue and dynamic response calculations.

! To move towards harmonisation of methodologies with the Eurocode the ISO wind code

and other National Codes. A full summary of the differences between CP3: ChV: Part 2 and BS6399: Part 2 is given in Annex B The factors in BS6399: Part 2 which most affect the design wind loads are the topography factor (similar to the S1 factor in CP3: ChV: Part 2) and the altitude factor. Topography can increase wind loads by 21% for every 100m increase in altitude. The effects from shelter, terrain, distance from the sea or edge of town and size of loaded element, are smaller, although still significant. Annex B contains a detailed sensitivity analysis of the parameters that affect the design wind load. 4.0 DESCRIPTION OF THE PROPOSED SIMPLIFIED METHOD This method has been implemented in the form of a flow chart based on a simple five-step approach. The major differences from the current method are: • Basic wind speed map now given as mean wind speeds instead of gust wind speeds. • Current conservative topography rules replaced with a simple topography model that has an

additional moderate hill slope category and up to three location zones. • Site altitude now given as a separate factor The assumptions used in developing this simplified method are set out in Annex C.


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The flow chart and accompanying figures (Figures 1 and 2) and tables (Tables 1, 2 and 3) for the proposed simplified method are shown below, and summarised in Figure 3.

Calculatevalue of factor S

from:S = V x T x A

Obtain maximum allowablebuilding height

from Table 3

Read Mapwind speed Vfrom Figure 1

Find the topographic zone for the site from Figure 2 and

obtain factor T from Table 1

Obtain value of factor Afrom Table 2

Step 1: Read windspeed V from Figure 1 Step 2: Use Figure 2 to determine which topographic zone the site is situated in and use Table 1 to determine the value of the Factor T for the appropriate topographic zone and average ground slope around the site. Step 3: Use Table 2 to determine the value of Factor A. Step 4: Calculate the value of Factor S from

S = V x T x A Step 5: Read the maximum allowable building height from Table 3


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Figure 1 Map of wind speed V


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zone 2 zone 1 zone 2

Lu Ld

zone 2 zone 1 zone 2 zone 3

Lu

0.4Lu0.4Lu 0.4Ld 0.4Ld

0.25Lu0.25Lu 0.4Lu 0.4Lu 1.2Lu

Hills and Ridges

Cliffs and Escarpments

Figure 2 Topographic zones for Factor T


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Table 1 Factor T Factor T Topographic category and average slope of whole

hillside, ridge, cliff or escarpment Zone 1 Zone 2 Zone 3 Category 1: Nominally flat terrain, average slope < 1/20 Category 2: Moderately steep terrain, average slope < 1/5 Category 3: Steep terrain, average slope > 1/5

1.0 1.24 1.36

1.0 1.13 1.20

1.0 1.10 1.15

Note: Outside of these zones factor T = 1.0 Table 2 Factor A

Site Altitude (m)

Factor A

0 50 100 150 200 300 400

1.00 1.05 1.10 1.15 1.20 1.30 1.40

Table 3 Maximum allowable building height (m)

Country Sites

Town Sites* Distance to the coast

Distance to the coast

Factor S

<10km 10 to 50km

>50km

<10km

10 to 50km

>50km

23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

15 15 11 8 6

4.5 3.5 3

15 15

14.5 10.5 8.5 6.5 5 4

3.5 3

15 15 15 13 10 8 6 5 4

3.5 3

15 15 15 15 15

13.5 11 9 8 7 6 5 4 3

15 15 15 15 15 15 13 11 9.5 8.5 7.5 7 6

5.5 4.5 4 3

15 15 15 15 15 15

14.5 12.5 10.5 9.5 8.5 8 7 6

5.5 5 4 3

*for sites on the outskirts of towns not sheltered by other buildings use the values for Country sites


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Table 1 Factor T

Factor T Topographic category and average slope of whole hillside, ridge, cliff or escarpment

Zone 1 Zone 2 Zone 3

Category 1: Nominally flat terrain, average slope < 1/20 Category 2: Moderately steep terrain, average slope < 1/5 Category 3: Steep terrain,

average slope > 1/5

1.0

1.24

1.36

1.0

1.13

1.20

1.0

1.10

1.15

Note: Outside of these zones factor T = 1.0

Figure 1 Map of basic windspeed

Table 2 Factor A Site Altitude (m)

Factor A

0 50 100 150 200 300 400

1.00 1.05 1.10 1.15 1.20 1.30 1.40

Table 3 Maximum allowable building height (m) Country Sites Town Sites*

Distance to the coast Distance to the coast Factor

S <10km 10 to 50km >50km <10km 10 to 50km >50km

23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

15 15

11.5 8 6

4.5 3.5 3

15 15

14.5 10.5 8.5 6.5 5 4

3.5 3

15 15 15 13 10 8 6 5 4

3.5 3

15 15 15 15 15

13.5 11 9 8 7 6 5 4 3

15 15 15 15 15 15 13 11 9.5 8.5 7.5 7 6

5.5 4.5 4 3

15 15 15 15 15 15

14.5 12.5 10.5 9.5 8.5 8 7 6

5.5 5 4 3

* for sites on the outskirts of towns not sheltered by other buildings use the values for Country sites

Figure 2 Topographic zones for Factor T

Calculate

va f Slue of actorfrom:

V x T x AS =

Obtain maximum allowablebuilding height

from Table 3

Read Mapwind speed Vfrom Figure 1

Find the topographic zone forthe site from igure nd

obtain factor T om ableF 2 afr T 1

Obtain value of factor Afrom Table 2

zone 2 zone 1 zone 2

Lu Ld

zone 2 zone 1 zone 2 zone 3

Lu

0.4Lu0.4Lu 0.4Ld 0.4Ld

0.25Lu0.25Lu 0.4Lu 0.4Lu 1.2Lu

Hills and Ridges

Cliffs and Escarpments


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Figure 3 Summary of the Proposed Simplified Method


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In BS6399: Part 2, for cases where topography is significant the calculation of altitude factor is based on the altitude at the base of the topographic feature. For simplicity in this proposed method, the calculation of altitude factor is based on the site altitude even when topography is significant. This can lead to an overestimation of the wind speed for sites on large hills and ridges, with the consequence that the maximum allowable building height could be lower than necessary. Therefore, it is suggested that the following note be added, either in the text or attached to Table 2: >Note: For sites in topographic categories 2 and 3, a more accurate assessment of Factor A can be obtained by using the altitude at the base of the topographic feature instead of the altitude at the site=. This approach is fully compatible with BS6399: Part 2, and would allow the user to take advantage of this more accurate approach if required. For some hilly sites the user might not have prior information on the dimensions of the hill and so will not be able to determine which zone his site lies in. It is suggested that the following note be added, either in the text or attached to Table 1 to overcome this difficulty; >Note: If the exact location of the site is not known then assume Zone 1=. 5.0 IMPLICATIONS OF THE PROPOSED CHANGES FOR TRADITIONAL

BUILDING DESIGN Eight sites in Scotland were chosen for a comparison between the proposed simplified method and the current simplified method in the Scottish Office Small Building Guide. These sites were chosen to give a geographical spread and range of altitudes and distance from the coast. Site details, including the basic hourly mean wind speed from the proposed method and the basic gust wind speed from the current Scottish Office Small Building Guide are shown below for the eight sites selected.

Site Altitude (m)

Distance to coast (km)

V (m/s) (current method)

V (m/s) (proposed method)

Edinburgh Glasgow Inverness Aberdeen

Oban Lerwick

Fort William Kingussie

60 30 20 20 80 10 100 270

1 20 1 0

0.5 0

0.5 52

50 51 51 49

51.7 54

51.5 50.5

23.5 24 24 24

25.3 29.5 24.5 23.5


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Each site has been assumed to be in both Country terrain and Town terrain on flat level ground and on moderate and steeply sloping ground; for hilly terrain the site has also been assumed to be at the hill crest and at a point some distance upwind from the crest. This gave a total of about 160 notional site locations for which comparisons between the current Small Buildings Guide method and this proposed simplified method were made. The maximum allowable building heights using the current and proposed methods are shown in Table 4, (heights are rounded down to the next 0.1m) A comparison such as this is somewhat subjective because the current Small Buildings Guide method is based on four terrain categories whereas the proposed method is based on two categories. The Country terrain category in BS6399 includes terrain >... from the flat open level, or nearly level country with no shelter, such as fens, airfields, moorland or farmland with no hedges or walls, to undulating countryside with obstructions such as occasional buildings and windbreaks of trees, hedges and walls=. This category therefore encompasses categories 1 and 2 in the Small Buildings Guide. Similarly, the Town category in BS6399 encompasses categories 3 and 4 in the Small Buildings Guide. For the purposes of this exercise, Country terrain in the proposed method has been compared with category 1 in the current method and Town with category 3. The results in Table 4 show that there are some quite marked differences between the two methods. In flat terrain, i.e. the no topography case, the proposed method results in significantly greater allowable building heights at four sites (Edinburgh, Glasgow, Inverness and Aberdeen), where the allowable heights have increased from about 6m to 8m to 12m to 15m. At Fort William there is little change, at Lerwick and Oban there is a small reduction of about 1m, whilst at Kingussie there is a 50% reduction - about 3.5m, due to the altitude of this site. The largest and most striking differences occur for the cases where topography is significant. The Small Buildings Guide uses a very conservative treatment of topography, which assumes that the site is always on the crest of a very steep hill. This method does not allow any buildings to be constructed on steeply sloping sites at any of the eight selected locations. Whereas the proposed method allows, for example, in Glasgow, buildings of up to 7.3m in height on the crest of a steep hill. Where the building is assumed to be in Town terrain and away from the crest (as the majority of buildings are) or on a moderate slope then the proposed method allows building heights of up to 15m in Glasgow, over 13m in Edinburgh, Inverness and Aberdeen, over 8m in Oban and Fort William and over 5m in Lerwick and Kingussie. Compare this with the current method, which does not allow any building in these situations. Much of the variation between the current and proposed methods is due to changes in BS6399, such as the general reduction of the basic wind speeds in Scotland and the separating out of the altitude factor, rather than differences between the approaches used in the two simplified methods. In general, the proposed method gives greater allowable building heights except where the site is at high altitude. It should be noted that the altitude within a town vary quite considerably, i.e. at Oban the altitude varies from sea level up to about 100m. For every 100m increase in altitude, the wind pressure increases by 20%. Therefore, the allowable building heights presented in this report should be considered as indicative only and not assumed to apply to all locations within the towns.

Building Research Establishm

ent Centre for Structural Performance

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6.0 CONCLUSIONS This proposed simplified design method uses a simple five-step approach, which includes separate factors to take account of altitude and topography. There are some quite large differences in allowable building heights, but in general, these differences are largely beneficial. The main conclusions that can be drawn from this study are: Χ For flat, level sites in open country or town terrain the proposed simplified method will

generally give greater maximum allowable building heights that the current Small Buildings Guide method, the main exception appears to be sites in the northernmost islands where the allowable heights are slightly lower. For sites at altitudes greater than about 100m, the maximum allowable building heights could be significantly lower than those currently allowed by the Small Buildings Guide.

Χ The current simplified method in the Small Buildings Guide is overly conservative for

sites on hilly terrain and does not allow any buildings on hilly sites at any of the eight selected locations. The proposed method treats topography in a more realistic and accurate manner that allows buildings in all of these previously excluded areas.

Χ The proposed simplified methodology is more complex than that currently included in

the Small Buildings Guide due to the inclusion of altitude and topographic factors. However, the steering committee feel that this new method is still well within the capabilities of the users of the Small Buildings Guide.

7.0 REFERENCES 1. BS6399: Part 2: British Standard Code of Practice for Wind Loads, BSI, 1997. 2. CP3: Chapter V: Part 2: Code of Practice for Wind Loads, BSI, 1972 3. Wind Loading on Traditional Dwellings, Amendment of Simplified Design

Approaches, DETR Report Number 2651/96, 1997.


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Table 4 Comparison of Maximum Allowable Building Heights in Metres Using the Current and Proposed Simplified Methods

No Topography With Topography

Current Proposed Current method1 Proposed – Steep slope >1/5 Proposed – Moderate slope <1/5 Country Town Country Town Country Town Zone 1 Zone 2 Zone 1 Zone 2

Site

Country Town Country Town Country Town Country TownEdinburgh Glasgow Inverness Aberdeen

Oban Lerwick

Fort William Kingussie

7.1 6.4 7.1 7.9 6.0 4.6 6.1 6.7

15 15 15 15 15 15 15 15

11.8 14.6 13.2 13.2 4.9 3.1 6

3.1

15 15 15 15

13.9 9.4 15 9.4

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

5.1 7.3 5.7 5.7 0 0 0 0

3 4.3 3.3 3.3 0 0 0 0

9.2 11.6 9.6 9.6 6.2 3.2 6.7 3.2

0 3.6 0 0 0 0 0 0

8.1 9.9 8.6 8.6 5.1 0

5.6 0

4.4 6.7 4.9 4.9 0 0 0 0

13.6 15

13.9 13.9 8.1 5.3 8.5 5.3

1 The current Small Buildings Guide method assumes maximum speed-up at all locations on >steeply sloping sites=

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Wind Loading From BS6399 to Cp3