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UNIVERSITY OF NAIROBI
STRUCTURAL DESIGN OF A PROPOSED OFFICE BLOCK
ALONG KINDARUMA LANE OFF NGONG ROAD –
NAIROBI COUNTY.
ANTHONY CHOMBA NDWIGA. F16/40265/2011
A project submitted as a partial fulfillment for the requirements for the award of
the degree of
BACHELOR OF SCIENCE IN CIVIL ENGINEERING
2016
1
DECLARATION :
I, the undersigned, declare that the work contained herein is the original and has
not been submitted for examination or degree at any other university.
Signed…………………………. Date………………………….
SUPERVISOR’S APPROVAL
This project report has been submitted with my approval as the supervisor
ENG. SAMUEL.S MIRING`U
Lecturer
School of Engineering
Department of Civil and Construction Engineering
Signed………………………… Date -----------------------------------------
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DEDICATION :
This project is humbly dedicated to my family and friends for their revered love and
support throughout my life and the endless sacrifices for my success.
To Mrs. Charity Ndwiga, mum, your prayers always remind me that I have no option but
to succeed in all what I focus on. You instilled the necessary discipline required in this
life and made me strongly believe that I can be whoever I wanted, your dream has now
come true. Thank you.
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ACKNOWLEDGEMENT :
My greatest gratitude goes to the Almighty God for the strength, good health, safety and
the wisdom that have enabled me to complete this study.
My sincere acknowledgement to Eng. Miring`u my project supervisor, a mentor and
structural design lecturer my favorite course. His guidance, patience and positive
criticism made me realize and sometimes recite clauses in BS8110,part 1. Kudos Sir.
To my family and friends for encouragement and prayers. You made a great impact on
my studies.
4
ABSTRACT :
This project presents the analysis and design of a five storey building located in the
residential outskirts of Nairobi central business district. It was designed to meet both
strength and serviceability requirements when subjected to both gravity loads and lateral
loads. The project was designed by manual method and by use of computer aided
software (PROKON) and the results obtained were compared in terms of moments and
bending schedule.
The project report has adopted the following model: an introduction giving the
basic needs and requirements of any civil engineering project as well as project needs.
Literature review that gives important properties of materials used in construction.
Methodology which gives an overview on how the design of the project will be done and
various codes in practice to be used in the design process. Calculation and analysis shows
the application of the codes and knowledge learnt. References that show various sources
of knowledge that is applied to the research study.
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Table of ContentsDECLARATION..............................................................................................................................................2
DEDICATION..................................................................................................................................................3
ACKNOWLEDGEMENT................................................................................................................................4
ABSTRACT.....................................................................................................................................................5
1.0 INTRODUCTION......................................................................................................................................8
1.1 OVERVIEW OF THE CONSTRUCTION INDUSTRY.......................................................................8
1.2 The Design Process................................................................................................................................8
Definition of the client’s needs and priorities...........................................................................................8
Development of project concept...............................................................................................................9
Design of individual systems....................................................................................................................9
1.3 Problem statement................................................................................................................................10
1.4 OBJECTIVES.......................................................................................................................................11
2. LITERATURE REVIEW.........................................................................................................................12
2.1. Reinforced concrete.............................................................................................................................12
2.1.1 Concrete.........................................................................................................................................12
2.1.2 Cement...........................................................................................................................................13
2.1.3 Aggregates.....................................................................................................................................13
2.1.4 Admixtures....................................................................................................................................14
2.1.5 concrete mix design.......................................................................................................................14
2.1.6. properties of concrete...................................................................................................................15
2.1.7 Reinforcing steel............................................................................................................................18
2.2 Pre-stressed concrete............................................................................................................................20
2.2.1 Pre-stressing methods....................................................................................................................20
2.3 Structural steel......................................................................................................................................21
2.3.1 Properties of structural steel..........................................................................................................21
2.3.2 Steel sections.................................................................................................................................24
2.4 Composite construction........................................................................................................................25
2.5 Structural Timber..................................................................................................................................25
2.6 Masonry................................................................................................................................................26
2.6.1 Mortar..........................................................................................................................................26
2.6.2 Wall bonds.....................................................................................................................................27
2.6.3 Material properties.........................................................................................................................27
2.7 Structural Elements..............................................................................................................................30
2.7.1. SLABS..........................................................................................................................................30
2.7.2. Beams...........................................................................................................................................31
2.7.3 Columns.........................................................................................................................................31
2.7.4 Walls..............................................................................................................................................32
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3.1.4 Shear wall......................................................................................................................................32
2.7.5 Foundations/ Footings...................................................................................................................33
2.8 Loads....................................................................................................................................................36
2.8.1Dead Loads.....................................................................................................................................36
2.8.2Imposed Loads................................................................................................................................36
2.8.3Environmental Loads......................................................................................................................36
3.0 METHODOLOGY...................................................................................................................................38
3.1 STURUCTURAL ANALYSIS.............................................................................................................38
3.1.2. Limit State Design........................................................................................................................39
3.2 Reinforced concrete beams...................................................................................................................41
3.3 Slab Design...........................................................................................................................................43
3.3.1 Depth of slab (clause 3.5.7, BS 8110)...........................................................................................43
3.3.2 Steel areas (clause 3.5.4, BS 8110)...............................................................................................43
3.3.3 Reinforcement details (clause 3.12,BS 8110)...............................................................................44
3.4 Columns................................................................................................................................................45
3.4.1 Requirements for links..................................................................................................................45
3.5 Foundation design................................................................................................................................46
3.6 Seismic loading....................................................................................................................................47
3.7 wind loading design..............................................................................................................................48
3.8 Design Codes and Specifications.........................................................................................................48
3.9 DRAWINGS AND BAR BENDING SCHEDULE.............................................................................52
3.9.1 Drawings........................................................................................................................................52
3.9.2 Detailing........................................................................................................................................52
3.9.3 Requirements to reinforcement construction drawings...........................................................53
3.9.4 Bar Bending Schedule...................................................................................................................54
4. REFERENCES.........................................................................................................................................55
5. SAMLE CALCULATIONS………………………………………………………………………..……56 5.1. Solid Floor Slab 5.2. Ribbed Slab 5.3. Floor Beams 5.4. Ramp 5.5. Staircase 5.6. Columns and Bases 5.7. Bending Schedules 5.8. Architectural Drawings 5.9. Structural Drawings
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1.0 INTRODUCTION
1.1 OVERVIEW OF THE CONSTRUCTION
INDUSTRY
Construction is an act and science that involve bringing together resources, skills and
materials to bring up a structure. The construction sector is an intertwined web of various
players including the developers, the contractors, the government and affiliated bodies,
the public stakeholders and the various professionals within the sector.
For every structure to be constructed, it would have to undergo one the very most
important process, which is the design process
1.2 The Design Process
The design process is a sequential and iterative decision-making process and gives the
bases of instruction, guidelines and checks during construction. This process involves
three major stages
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Definition of the client’s needs and priorities.
All buildings or other structures are built to satisfy a need. It is important that the owner
or user be involved in determining the attributes of the proposed building. These include
functional requirements, aesthetic requirements and budgetary requirements. The latter is
the initial cost, premium for rapid construction to allow early occupancy, maintenance,
and other life-cycle costs.
Development of project concept.
Based on the client’s needs and priorities, a number of possible layouts are developed.
Preliminary cost estimates are made with systems that are conceptually simple and have
standardized geometries and details that allow construction to proceed as a series of
identical cycles are the most cost effective.
During this stage, the overall structural concept is selected. From approximate analyses of
the moments, shears, and axial forces, preliminary member sizes are selected for each
potential scheme. After the analysis, it is possible to estimate costs and select the most
desirable structural system. The overall thrust in this stage of the structural design is to
satisfy the design criteria dealing with appropriateness, economy and maintainability.
Design of individual systems.
Once the overall layout and general structural concept have been selected, the structural
system can be designed. Structural design involves three main steps. Based on the
preliminary design selected in phase 2, a structural analysis is carried out to determine the
moments, shears, torques, and axial forces in the structure. The individual members are
then proportioned to resist these load effects. The proportioning, sometimes referred to as
member design, must also consider overall aesthetics, the constructability of the design,
coordination with mechanical and electrical systems, and the sustainability of the final
structure. The final stage in the design process is to prepare construction drawings and
specifications.[1]
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1.3 Problem statement
The research study is an office block along Kindaruma lane off Ngong road in Nairobi
County.
The structure consists of two basement levels and upper ground floor to be used as
parking. First floor to sixth floor levels will be utilised as offices and the terrace floor
level will be the member’s club and conference room.
The building is to be designed as a braced structure where lateral loads are carried by
shear wall around the staircase and the lift core.
The structural analysis will be done manually method and compare the results with
analysis from computer aided software (PROKON).
Over the years engineers used manual calculations and analyzing and design for
structures but though computer development and advancement, engineers have created
software from the knowledge in computers and various principles in structural design and
analysis that have made design work less tedious, time saving and produce clean
presentable work
But still not all the analysis is done with the computer aided software, they still require
some manual calculations. The softwares are expensive to acquire and require high skill
standard to operate.
Software manufactures keep on updating and changing various aspects of the software
and hence engineers have to keep updating their knowledge in accordance to these
changes.
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Errors that could occur, ignorance some aspect of structural that may be not important in
the design and analysis could lead to large differences in results between manual method
of design and computer aided design software such as PROKON and MASTER SERIES
.
1.4 OBJECTIVES
1. The purpose of the project is to analyze and design all the structural members to attin
the most stable and economical structure.
2. To compare results from analysis of manual method with those of computer aided
software (PROKON)
3. To show an understanding of structural design analysis through provision of adequate
information on assumptions, codes, symbols and formulas.
11
2. LITERATURE REVIE W
2.1. Reinforced concrete
Reinforced concrete is composite material of steel reinforcements embedded in concrete.
it utilizes the concrete in resisting compression forces and some other material, usually
steel bars to resist tension forces.
Reinforced concrete is widely used since it has various advantages such as concrete is
cheap, versatile, and high compressive strength and the weakness to carry tension forces
is taken care of by the reinforcing steel.
2.1.1Concrete
Concrete is a composite material composed of aggregate bonded together with a fluid
cement which hardens over time. when the aggregate is mixed together with the dry
cement and water, they form a fluid mass that is easily molded into shape. The cement
reacts chemically with the water and other ingredients to form a hard matrix which binds
all the materials together into a durable stone-like material that has many uses. Often,
additives such as pozzolans or super plasticizers are included in the mixture to improve
the physical properties of the wet mix or the finished material. The bulk of solid volume
of concrete is composed of the aggregates, coarse and fine. The water cementitious
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material matrix binds the aggregates together through hydration of cement to become a
solid.
2.1.2 Cement
It is a binder, a substance that sets and hardens and can bind other materials together.
When used plain or reinforced concrete it has the ability to form a paste which hardens
over time holding all larger aggregates together
There are different types of cement, these may include.
a. General purpose Portland cement
I. Normal- used in most construction for general purposes.
ii. Moderate sulfate resistant -used when structure will be exposed to soils or water
having a moderate alkali concentration.
iii. High early strength - for use when very high strength is required at an early stage.
iv. High sulfate resistance - where the structure would be exposed to Soils with high
alkali concentration
v. Low heat of hydration - used in large masses such as dams. It is desirable for reducing
cracking and shrinkage [2]
2.1.3Aggregates
The fine aggregates consist of sand or other fine grained inert marterial usually less than
6.4mm
Coarse aggregates consist of gravel or crushed rock usually larger than 6.4 mm and less
than 76mm.
13
2.1.4 Admixtures
Admixtures are those ingredients in concrete other than Portland cement, water, and
aggregates that are added to the mixture immediately before or during mixing that help to
improve certain properties by chemical or physical effects. Admixtures can be classified
by function as follows:
1. Air-entraining admixtures
2. Water-reducing admixtures
3. Plasticizers
4. Accelerating admixtures
5. Retarding admixtures
6. Hydration-control admixtures
7. Corrosion inhibitors
8. Shrinkage reducers
9. Alkali-silica reactivity inhibitors
10. Coloring admixtures
Concrete should be workable, finishable, strong, durable, watertight, and wear resistant.
The major reasons for using admixtures are:
1. To reduce the cost of concrete construction
2. To achieve certain properties in concrete more effectively than by other means
3. To maintain the quality of concrete during the stages of mixing, transporting, placing,
and curing in adverse weather conditions
4. To overcome certain emergencies during concreting operations [2]
2.1.5 Concrete mix design
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Concrete mix design is the process of choosing suitable ingredient of concrete and
determining their relative quantities with the object of producing as economically as
possible concrete of certain minimum properties, notable workability, strength and
durability. Only a mix made and used on the site can guarantee that all properties of the
concrete are satisfactory in every detail for the particular job in hand., The selection of
mix proportions is an art as much as a science. It is not enough to select a suitable
concrete mix; it is also necessary to ensure a proper execution of all the operation
involved in concreting. It cannot be stated too strongly that, competently used, concrete is
a very successful construction material but, in the literal service of the word, concrete is
not fool proof. The mix proportions once chosen, cannot expected to remain entirely
immutable because the properties of the ingredients (cement, sand, aggregate, water and
admixture) may vary from time to time.
2.1.6. Properties of concrete
The tensile strength is considered when calculating resistance to shearing force and in
design of cylindrical liquid containing structures.
The tensile strength of concrete in flexure is quite important when considering beam
cracks and deflections. For these considerations, the tensile strengths obtained with the
modulus of rupture test have long been used. The modulus of rupture which is defined as
the flexural tensile strength of concrete and is usually measured by loading a rectangular
beam to failure with equally concentrated loading.
a) Compressive Strength
The compressive strength of concrete, fc is determined by testing to failure 28dayold
concrete at a specified rate of loading. For the 28-day period, the cylinders are usually
kept under water or in a room with constant temperature and 100% humidity. Although
concrete is available with 28day ultimate strength. Comprehensive strengths vary from
15
10N/mm2to 55N/mm2 for special concretes. The minimum characteristics strength of
concrete made with dense aggregate according to BS8110, is 25N/mm2 and for concrete
made of lightweight aggregate is 20 N/mm2. With given proportions the comprehensive
strength is dependent on age, cement content and cement water ratio, where an increase
in any of the factors result to an increase in strength [2]
b) Shrinkage
When the materials for concrete are mixed, the paste consisting of cement and water fills
the voids between the aggregate and bonds the aggregate together. This mixture needs to
be sufficiently workable or fluid so that it can be made to flow in between the reinforcing
bars and all through the forms. To achieve this desired workability, considerably more
water is used than is necessary for the cement and water to react through hydration. After
the concrete has been cured and begins to dry, the extra mixing water that was used
begins to work its way out of the concrete to the surface, where it evaporates. As a result,
the concrete shrinks and cracks. The resulting cracks may reduce the shear strength of the
members and be detrimental to the appearance of the structure. In addition, the cracks
may permit the reinforcing to be exposed to the atmosphere or chemicals, such as deicers,
thereby increasing the possibility of corrosion. The amount of moisture that is lost varies
with the distance from the surface. Furthermore, the larger the surface area of a member
over its volume, the larger the rate of shrinkage; that is, members with small cross
sections shrink more proportionately than do those with large cross sections. The amount
of shrinkage is heavily dependent on the type of exposure. For instance, if concrete is
subjected to a considerable amount of wind during curing, its shrinkage will be greater. In
a related fashion, a humid atmosphere means less shrinkage, whereas a dry one means
more. It should also be realized that it is desirable to use low-absorptiveaggregates such
as those from granite and many limestones. When certain absorptive slates and sandstone
aggregates are used, the result may be one and a half or even two times the shrinkage
To minimize shrinkage, it is desirable to:
(1) Keep the amount of mixing water to a minimum;
(2) Cure the concrete well;
16
(3) Place the concrete for walls, floors, and other large items in small sections thus
allowing some of the shrinkage to take place before the next section is placed
(4) Use of construction joints to control the position of cracks;
(5) Design for shrinkage reinforcement;
(6) Use appropriate dense and nonporous aggregates. [2]
c) Creep
Creep is long term sustained loading. Under sustained compressive loads, concrete will
continue to deform for long periods of time. After the initial deformation occurs, the
additional deformation is called creep, or plastic flow. If a compressive load is applied to
a concrete member, an immediate or instantaneous elastic shortening occurs. If the load is
left in place for a long time, the member will continue to shorten over a period of several
years, and the final deformation will usually be two to three times the initial deformation.
Should the long-term load be removed, the member will recover most of its elastic strain
and a little of its creep strain. If the load is replaced, both the elastic and creep strains will
again develop. The amount of creep is largely dependent on the amount of stress. Several
other items affecting the amount of creep are
• The longer the concrete cures before loads are applied, the less will be the creep. Steam
curing which causes quicker strengthening, will also reduce creep.
• Higher-strength concretes have less creep than do lower-strength concretes stressed at
the same values. However, applied stresses for higher-strength concretes are, in all
probability, higher than those for lower-strength concretes, and this fact tends to cause
increasing creep
• Creep increases with Increase in temperatures.
• The higher the humidity, the smaller will be the free pore water that can escape from the
concrete. Creep is almost twice as large at 50% humidity than at 100% humidity. It is
obviously quite difficult to distinguish between shrinkage and creep.
• Concrete with the highest percentage of cement–water paste has the highest creep
because the paste, not the aggregate, does the creeping. This is particularly true if a
limestone aggregate is used.
17
• Obviously, the addition of reinforcing to the compression areas of concrete will greatly
reduce creep because steel exhibits very little creep at ordinary stresses. As creep tendsto
occur in the concrete, the reinforcing will block it and pick up more of the load.
• Large concrete members (i.e., those with large volume-to-surface area ratios) will creep
proportionately less than smaller thin members where the free water has smaller distances
to travel to escape. [2]
d) Poisson’s ratio.
Its value varies from about 0.11 for the higher-strength concretes to as high as 0.21 for
the weaker-grade concretes, with average values of about 0.16. There does not seem to be
any direct relationship between the value of the ratio and the values of items such as the
water–cement ratio, amount of curing, aggregate size, and so on.For most reinforced
concrete designs, no consideration is given to the so-called Poisson effect. It may very
well have to be considered, however, in the analysis and design of arch dams, tunnels,
and some other statically indeterminate structures
2.1.7 Reinforcing steel.
Reinforcing Steel consist of bars, wires, welded wire fabrics
BS4499(specification for carbon steel bars for reinforcement of concrete) which is used
in Kenya specifies requirements for wieldable steel bars for reinforcement in concrete.
There are two types of reinforcing bars.
(a) Hot rolled high yield steel with yield strength of 250N/mm2
(b) Cold worked high yield steel with yield strength of 460N/mm2
Reinforcing bars are manufactured as plain or deformed bars. Deformed bars have ribbed
projections which grip the concrete to provide better bond between the two materials.
18
The most important properties of reinforcement steel are:
a. Modulus of elasticity Es,
b. Tensile strength
c. Yielding stress
d. Steel grade designation (yield strength)
e. Size or diameter of the bar or wire
The stress strain diagram is idealized by assuming that stress is constant in plastic region
and equal to fy
The stress strain curve of reinforcing bar is shown on the figure below. The hot rolled
bars have a definite yield point. The value of Young's modulus E is 200kN/mm2. The
idealized design stress strain curve for all Reinforcing bars is shown in BS8110: part 1.
The behavior in compression and tension is taken the same. [2]
19
2.2 Pre-stressed concrete
Pre-stressed concrete consists of concrete resisting compression and reinforcing steel
resisting tension. Pre-stressing is the deliberate creation of permanent internal stresses in
structure or system in order to improve its performance. Such stresses are designed to
counteract those induced by external loadings. The advantage of having the concrete in
pre-stressed state is that tensile cracking is prevented therefore increasing the resistance
of steel to corrosion. Pre-stressing also increases the overall stiffness of the members.[4],
[5]
2.2.1 Pre-stressing methods
There are several methods and techniques of pre-stressing available. However, except for
chemical pre-stressing, most can be classified within two major groups.
a) Pre-tensioning
In this method the pretesting tendons (wire, strands) are stretched to a predetermined
tension and anchored fixed bulkheads or molds. The concrete is cast around the tendons
and cured, and upon hardening the tendons are released. As the bond between the tendons
and concrete resists shortening of tendons the concrete gets compressed
20
b) Post-tensioning
The tendons are stressed and anchored at the ends of concrete members after the member
has been cast and has attained sufficient strength.
Mostly a mortar-tight metal tube or duct or sheath is placed along the member before
casting of concrete. The tendons may be preplaced loose inside the sheath prior to casting
or could be placed after hardening of the concrete. After stressing and anchoring, the void
between each tendon and its duct is filled with a mortar grout, which subsequently
hardens. Grouting ensures bonding of the tendon to the surrounding concrete improves
the resistance of the members to cracking and reduces the risk of corrosion for the steel
tendons. [5]
2.3 Structural steel
Steel can be used by itself as a structural material for most type of members. Structural
steel is available in many shapes which are efficient in resisting bending and buckling.
Structural steel has the property of exhibiting approximately the same stress – strain
relationship in tension and compression, hence a steel section which can carry their loads
in bending will generally be symmetrical about the neutral axis. However local buckling
often restricts on the allowable compressive stresses in such member.[4]
2.3.1 Properties of structural steel
a)Fatigue
21
Fatigue failure can occur in members or structures subjected to fluctuating loads such as
crane girders, bridges and offshore structures. Failure occurs through initiation and
propagation of a crack that starts at a fault or structural discontinuity and the failure load
may be well below its static value.
Welded connections have the greatest effect on the fatigue strength of steel structures.
Tests show that butt welds give the best performance in service while continuous fillet
welds are much superior to intermittent fillet welds.
Bolted connections do not reduce the strength under fatigue loading. To help avoid
fatigue failure, detail should be such that stress concentrations and abrupt changes of
section are avoided in regions of tensile stress.[8]
b) Brittle fracture
Structural steel is ductile at temperatures above 10◦C but it becomes more brittle as the
temperature falls, and fracture can occur at low stresses below0◦C.
In design, brittle fracture should be avoided by using steel quality grade with adequate
impact toughness. Quality steels are designated JR, J0, J2, K2and so forth in order of
increasing resistance to brittle fracture. The Charpyimpact fracture toughness is specified
for the various steel quality grades: for example, Grade S275 J0 steel is to have a
minimum fracture toughness of 27 Jat a test temperature of 0◦C.
In addition to taking care in the selection of steel grade to be used, it is also necessary to
pay special attention to the design details to reduce the likelihood\of brittle fracture. Thin
plates are more resistant than thick ones. Abrupt changes of section and stress
concentration should be avoided. Fillets welds should not be laid down across tension
flanges and intermittent welding should not be used. [8]
a) Fire protection
22
Structural steelwork performs badly in fires, with the strength decreasing with increase in
temperature. At 550◦C, the yield stress has fallen to approximately
0.7 of its value at normal temperatures; that is, it has reached its working stress and
failure occurs under working loads. The statutory requirements for fire protection are
usually set out clearly in the approved documents from the local Building Regulations or
Fire Safety Authority. These lay down the fire-resistance period that any load-bearing
element in a given building must have, and also give the fire-resistance periods for
different types of fire protection. Fire protection can be provided by encasing the member
in concrete, fire board. More recently, in tumescent paint is being used especially for
exposed steelwork. [8]
d) Corrosion protection
Exposed steelwork can be severely affected by corrosion in the atmosphere, particularly
if pollutants are present, and it is necessary to provide surface protection in all cases. The
type of protection depends on the surface conditions and length of life required.
The main types of protective coatings are:
(a) Metallic coatings: Either a sprayed-on in line coating of aluminium or zinc is used or
the member is coated by hot-dipping it in a bath of molten zinc in the galvanizing
process.
(b) Painting: where various systems are used. One common system consists of using a
primer of zinc chromate followed by finishing coats of micaceousiron oxide. Plastic and
bituminous paints are used in special cases.
The single most important factor in achieving a sound corrosion-protection coating is
surface preparation. Steel is covered with mill scale when it cools after rolling, and this
must be removed before the protection is applied, otherwise the scale can subsequently
loosen and break the film. Blast cleaning makes the best preparation prior to painting.
Acid pickling is used in the galvanizing process.
Careful attention to design detail is also required and access for future maintenance
should also be provided. [8]
23
2.3.2 Steel sections
a) Rolled and formed sections
Rolled and formed sections are produced in steel mills from steel blooms, beam blanks or
coils by passing them through a series of rollers. The main sections and their principal
properties and uses are briefly discussed below:
(a)Universal beams: These are very efficient sections for resisting bending moment about
the major axis.
(b) Universal columns: These are sections produced primarily to resist axial load with a
high radius of gyration about the minor axis to prevent buckling in that plane.
(c) Channels: These are used for beams, bracing members, truss members and in
compound members.
(d) Equal and unequal angles: These are used for bracing members, truss members and
for purlins, side and sheeting rails.
(e) Structural tees: The sections shown are produced by cutting a universal beam or
column into two parts. Tees are used for truss members, ties and light beams.
(f) Circular, square and rectangular hollow sections: These are mostly produced from hot-
rolled coils, and may be hot-finished or cold-formed.
A welded mother tube is first formed and then it is rolled to its final squarer rectangular
shape. In the hot process, the final shaping is done at the steel normalising temperature
whereas in the cold process, it is done at ambient room temperature. Both types of hollow
sections are now permitted in BS 5950. These sections make very efficient compression
24
members, and are used in a wide range of applications as members in roof trusses, lattice
girders, in building frames, for purlins, sheeting rails, etc [8]
2.4 Composite construction
The advantages of reinforced concrete and structural steel can be combined to form a
composite structure. The cheaper reinforced concrete slab is used to span large areas to
create floor space while a combination of the structural steel beams and the concrete are
used to support the slab and applied loadings.
2.5 Structural Timber
Timber is one of the earliest construction materials used by man. The strength of timber is
directly related to the variety of tree from which it is derived. The strength is also
dependent on its density, moisture content, grain structure and number of inherent defects
such as cracks, knots and insect infections.
Typical permissible stresses for softwoods loaded parallel to the grain orientation is less
than 6 N/mm2 for members in compression, tension and bending. While for members
loaded normally to the grain, the permissible stress is even smaller. However with use of
laminating techniques, where thin strips of timber are glued together to form large
sections, with permissible stresses of up to 20 N/mm2 can be achieved.[4]
25
2.6 Masonry
Masonry is construction which use bricks or block units which are fixed together with
mortar. Unreinforced masonry walls and columns are treated as integral elements by
virtue of alignment and pattern in which the units are laid on each other, the wall bond
and more crucially by the attachment or bond between the units.
The properties or quality of masonry wall will depend on type of mortar and masonry
unit used and the quality of workmanship. Masonry units are usually pre-wetted or laid
on wet mortar so as the suction of water from the mortar by the masonry unit does not
weaken the bond strength.[6]
2.6.1 Mortar
Mortar is the medium which binds together the individual structural units to create a
continuous structural form e.g. brickwork, stonework etc. Mortar serves a number of
functions in masonry construction, such as bind together the individual units, distribute
the pressures evenly throughout the individual units, infill the joints between the units
and hence increase the resistance to moisture penetration, maintain the thermal
characteristics of a wall.
26
In most cases they are mixtures of sand, cement and water. The workability is often
improved by the inclusion of lime or a mortar plasticiser. Lime is used in mortar for
several reasons:
♦To create a consistency which enables the mortar to ‘cling and spread’,
♦To help retain the moisture and prevent the mortar from setting too quickly,
♦To improve the ability of the mortar to accommodate local movement.
Modern mortars containing lime should not be confused with lime mortars. True lime
mortars are mortars in which lime is used instead of Portland cement as the primary
binder material. There are two types, hydrated (non-hydraulic) lime and hydraulic lime
mortars.
The set and strength characteristics of each type are different. The physical properties of
lime mortars have not been quantified as comprehensively as those of widely used
Portland cement mortars should only be specified after careful consideration of their
intended use and suitability. They should not be used as direct substitutes for Portland
cement mortars.
Plasticisers can be used with mortars which have a low cement sand ratio to improve the
workability. Their use introduces air bubbles into the mixture which fill the voids in the
sand and increase the volume of the binder paste. The introduction of plasticisers into
admix must be carefully controlled since the short-term gain in improved workability can
be offset in the longer term by creating an excessively porous mortar resulting in reduced
durability, strength and bond. [6] [7]
2.6.2 Wall bonds
Masonry units are bonded together in a wall to become an integral element, by using
mortar which gives a good bond and by choosing a suitable staking arrangement of the
masonry units. The staking arrangement is the wall bond a bond ina wall is said to exist if
the cross joints in the adjacent layers of the wall are staggered by not less than a quarter
of the length of the masonry unit. However, in a single leaf wall with English or Flemish
bond, the cross joints are staggered by 50% of the length of the masonry units. [6]
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2.6.3 Material properties
Masonry is a non-homogeneous, non-isotropic composite material which exists in many
forms comprising units of varying shape, size and physical characteristics. The
parameters which are most significant when considering structural design relate to
strength and elastic properties, e.g. compressive, flexural and shear strengths, modulus of
elasticity, coefficient of friction, creep, moisture movement and thermal expansion.
Tensile strength is generally ignored in masonry design.
The workmanship involved in constructing masonry is more variable than is normally
found when using most other structural materials and consideration must be given to this
at the design stage.
a) Compressive Strength
The compressive strength of masonry is dependent on numerous factors such as, the
mortar strength, unit strength, relative values of unit and mortar strength, aspect ratio of
the units i.e. Ratio of height to least horizontal dimension, orientation of the units in
relation to the direction of the applied load, bed-joint thickness.
This list gives an indication of the complexity of making an accurate assessment of
masonry strength. Unit strengths and masonry strengths are given in BS 5628: Part
1:1992
The failure mode of masonry in compression is usually one in which a tensile crack
propagates through the units and the mortar in the direction of the applied load
The tensile stresses inducing the crack are developed at the mortar-unit interfaces and are
due to the restrained deformation of the mortar. In most cases masonry strength
inconsiderably less than the strength of the individual unit. It can, however, be
considerably higher than the mortar strength. The apparent enhancement in the strength
of the mortar is due to the biaxial or triaxial state of stress imposed on the mortar when it
is acting compositely with the units.
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b) Flexural Strength
The non-isotropic nature of masonry results in two principal modes of flexural failure
which must be considered:
i. failure parallel to the bed-joints
ii. failure perpendicular to the bed-joints
The ratio of flexural strength parallel to the bed-joints to flexural strength perpendicular
to the bed-joints is known as the orthogonal ratio (μ) and has a typical value of 0.33,
calcium silicate and concrete bricks and 0.6 for concrete blocks. Research indications are
that the flexural strengths of clay bricks are significantly influenced by the water
absorption characteristics of the units. In the case of concrete blocks the flexural strength
perpendicular to the bed joints is significantly influenced by the compressive strength of
the units. There does not appear to be any meaningful correlation between the strength of
calcium silicate bricks, concrete bricks or concrete blocks parallel to the bed-joints, with
any standard physical property. In all case the flexural strength in both directions
independent on the strength of the mortar used and in particular the adhesion between the
units and the mortar. The adhesion is very variable and research has shown it to be
dependent on properties such as the pore structure of the units and mortar, the grading of
the mortar sand and moisture content of the mortar at the time of laying. [7]
c) Tensile Strength
As mentioned previously, the tensile strength of masonry is generally ignored in design.
However, in Clause 24.1 the code indicates that a designer is permitted to assume 50% of
the flexural strength values given in Table 3 when considering direct tension induced by
suction forces arising from wind loads on roof structures, or by the probable effects of
misuse or accidental damage.
d) Shear Strength
29
The shear strength of masonry is important when considering wall panels subject to
lateral forces and structural forms such as diaphragm and fin walls where there is the
possibility of vertical shear failure between the transverse ribs and flanges during
bending. Shear failure is most likely to be due to in-plane horizontal shear forces,
particularly at the level of damp-proof courses.
The characteristic shear strength is dependent on the mortar strength and any Pre-
compression which exists.[7]
2.7 Structural Elements
A structure is made up of structural elements (load carrying elements) such as beams,
columns and non-structural elements such as partitions, doors, ceilings.
The stuctural elements are supposed to effectively resist the gravitational, lateral and
environmental loading and transmit the resulting forces to the supporting ground, without
significantly disturbing the geometry, integrity and serviceability of the structure,
2.7.1.SLABS
Slabs resist the gravity loads (dead loads and live loads) acting on it and transmit these to
the beams and columns. The floor system is primarily subjected to flexure and transverse
shear, while the vertical frame elements are subjected to axial compression often coupled
with flexure and shear. The floor also serves as a horizontal diaphragm connecting and
stiffening together the various vertical frame element.
Slabs may be solid, ribbed, precast or in-situ and if in-situ they may span two-ways. i.e
one way spanning and two way spanning. In practice, the choice of slab for a particular
structure will largely depend upon economy, build ability, the loading conditions and the
length of the span.
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a) Ribbed
Ribbed floors consisting of equally spaced ribs are usually supported directly by
columns. They are either one-way spanning systems known as ribbed slab or a two-way
ribbed system known as a waffle slab. This form of construction is not very common
because of the formwork costs and the low fire rating. A 120-mm-thick slab with a
minimum rib thickness of 125 mm for continuous ribs is required to achieve a 2-hour fire
rating. A rib thickness of greater than 125 mm is usually required to accommodate tensile
and shear reinforcement. Ribbed slabs are suitable for medium to heavy loads, can span
reasonable distances are very stiff and particularly suitable where the soffit is exposed.
b) Flat slab
A flat slab is a two-way reinforced concrete slab that usually does not have beams and
girders, and the loads are transferred directly to the supporting concrete columns. The
slab thickness varies from 125mm to 300mm for spans of 4-9m.
2.7.2. Beams
They are described as structural members that have larger spans in length when compared
to their lateral dimensions (width and depth). They are subjected to transverse forces that
induce bending to the members in axial plane.
Beams are classified according to their supports, such as simply supported beam,
cantilever beam, continuous beam
Types of beams
The three common types of reinforced concrete beam section are
1. Rectangular section with tension steel only (this generally occurs as a beam section in
a slab)
2. Rectangular section with tension and compression steel
3. Flanged sections of either T or L shape with tension steel and with or without
compression steel
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2.7.3Columns
A column is a structural member that transmits axial loads from roof and floor members
to the foundation. Column that resist axial load are typically subjected to axial load, shear
forces and bending moment. A column subjected to large axial force and minor moment
is design mainly for axial load and the moment has little effect. A column subjected to
significant bending moment is designed for the combined effect.
Columns may be classified as short or slender, braced or unraced, depending on various
dimensional and structural factors which will be discussed below.
2.7.4 Walls
(a) Bearing walls—walls that are laterally supported and braced by the rest ofthe
structure that resist primarily in-plane vertical loads acting downward on the top
of the wall. The vertical load may act eccentrically with respect tothe wall thickness,
causing weak-axis bending.
(c) Non-bearing walls—walls that do not support gravity in-plane loads other than their
own weight. These walls may resist shears and moments due to pressures or loads acting
on one or both sides of the wall. Examples are basement walls and retaining walls used to
resist lateral soil pressures.
(d) Tilt-up walls—are very slender walls that are cast in a horizontal position\adjacent to
the structure. They are then tilted into their intended vertical position and fastened to the
foundation, to the roof or floor diaphragm, and to the adjacent panels.
They are designed to resist vertical and lateral loads.
(e) Although they are not walls as such, plates that resist in-plane compression, such as
the compression flanges or the decks of box girder bridges, display some of the
characteristics of walls. [7]
32
3.1.4 Shear wall
Shear wall is a structural member plane vertical element made up of reinforced concrete.
They resist lateral forces. These walls are more important in seismically active zones
because during earthquakes shear forces on the structure increases. Shear walls should
have more strength and stiffness. When a building has a story without shear walls, or
with poorly placed shear walls, it is known as a soft story building. Shear walls provide
adequate strength and stiffness to control lateral displacements. Shear walls that are
perforated with openings are called coupled walls. These walls act as isolated
cantilevered walls connected by coupling beams designed for bending and shear effects
2.7.5 Foundations/ Footings
A foundation is defined as a substructure that supports the weight of the structure and
transmits the load to underlying soil or rock. Foundation engineering applies the
knowledge of soil mechanics, rock mechanics, geology, and structural engineering to the
design and construction of foundations for buildings and other structures. The most basic
aspect of foundation engineering deals with the selection of the type of foundation, such
as using a shallow or deep foundation system. Another important aspect of foundation
engineering involves the development of design parameters, such as the bearing capacity
of the foundation.
There are many types of foundations which are commonly used, namely strip, pad and
raft. The choice of foundation type will largely depend upon
(1) Ground conditions i.e. strength and type of soil
(2) Type of structure, which involve the layout and level of loading.
Pad footings are usually square or rectangular slabs and used to support a single column.
The pad may be constructed using mass concrete or reinforced concrete depending on the
relative size of the loading.
33
Continuous strip footings are used to support load bearing walls or under a line of closely
spaced columns.
Strip footings are designed as pad footings in the transverse direction and in the
longitudinal direction as an inverted continuous beam subject to the ground bearing
pressure.
Where the ground conditions are relatively poor, a raft foundation may be necessary in
order to distribute the loads from the walls and columns over a large area. In its simplest
form this may consist of a flat slab, possibly strengthened by upstand or downstand
beams for the more heavily loaded structures
Where the ground conditions are very poor that it is not practical to use strip or pad
footings but better quality soil is present at lower depths, the use of pile foundations
should be considered
The piles may be made of precast reinforced concrete pressurised concrete or in-situ
reinforced concrete. Loads are transmitted from the piles to the surrounding strata by end
bearing and friction.End bearing piles derive most of their carrying capacity from the
penetration resistance of the soil at the toe of the pile, while friction piles rely on the
adhesion or friction between the sides of the pile and the soil.
a) Retaining walls
Sometimes it is necessary to maintain a difference in ground levels between adjacent
areas of land. Building vertical walls which is capable of resisting the pressure of the
retained material in a slope allows an immediate change in ground levels to be effected.
These structures are commonly referred to as retaining walls. Retaining walls are
important elements in many building and civil engineering projects. There are different
types of retaining walls
b) Gravity walls
Where walls up to 2 m in height are required, it is generally economical to choose a
gravity retaining wall. Such walls are usually constructed of mass concrete with mesh
34
reinforcement in the faces to reduce thermal and shrinkage cracking. Other construction
materials for gravity walls include masonry and stone. Gravity walls are designed so that
the resultant force on the wall due to the dead weight and the earth pressures is kept
within the middle third of the base. Generally, the width of base should be about a third
of the height of the retained material. It is usual to include a granular layer behind the
wall and weep holes near the base to minimise hydrostatic pressure behind the wall.
Gravity walls rely on their dead weight for strength and stability. The main advantages
with this type of wall are simplicity of construction and ease of maintenance.
c) Flexible walls
These retaining walls may consist of two basic types, namely
(i) cantilever
(ii) counterfort.
(i) Cantilever walls
Cantilevered reinforced concrete retaining walls are suitable for heights up to about 7 m.
They generally consist of a uniform vertical stem monolithic with a base slab. A key is
sometimes incorporated at the base of the wall in order to prevent sliding failure of the
wall.
The stability of these structures often relies on the weight of the structure and the weight
of backfill on the base.
(ii) Counterfort walls.
In cases where a higher stem is needed, it may be necessary to design the wall as a
counterfort. Counterfort walls can be designed as continuous slabs spanning horizontally
between vertical supports known as counterforts. The counterforts are designed as
35
cantilevers and will normally have a triangular or trapezoidal shape. For cantilever walls,
stability is provided by the weight of the structure and earth on the base. [3]
2.8 Loads
Loads that act on structures can be divided into three general categories:
2.8.1 Dead Loads
They are those that are constant in magnitude and fixed in location throughout the
lifetime of the structure such as: floor fill, finish floor, and plastered ceiling for
buildings and wearing surface, sidewalks, and curbing for bridges.
2.8.2 Imposed Loads
They are those that are either fully or partially in place or not present at all, may also
change in location; the minimum live loads for which the floors and roof of a
building are designed as specified in building code. (BS 6399, Pt 1) contains schedules of
imposed floor loads that would normally be expected for different classes of occupancy.
36
2.8.3 Environmental Loads
Environmental Loads caused by occurrences in the environment the structure is located.
They consist of wind, earthquake, and snow loads. such as wind, earthquake, and snow
loads.
2.8.4 Earthquakes loads
Earthquake ground motions impart vertical and horizontal accelerations to the base of a
structure. If the structure was completely rigid, forces of magnitude would be generated
in it, where m is the mass of the structure. Because real structures are not rigid the actual
forces generated will differ from this value depending on the period of the building and
the dominant periods of the earthquake ground motions. The determination of the seismic
force, E, is made more complicated because recorded earthquake ground motions contain
a wide range of frequencies and maximum values of base acceleration. [1]
3.0 METHODOLOGY
3.1 STURUCTURAL ANALYSIS
The analysis that is carried out to justify a design can be broken into two stages as
follows:
a) Analysis of the structure;
b) Analysis of sections.
In the analysis of the structure, or part of the structure, to determine force distributions
within the structure, the properties of materials may be assumed to be those associated
with their characteristic strengths, irrespective of which limit state is being considered. In
the analysis of any cross-section within the structure, the properties of materials should
be assumed to be those associated with their design strengths appropriate to the limit state
being considered. The methods of analysis used should be based on as accurate a
representation of the behaviour of the structure as is reasonably practicable. The methods
37
and assumptions given in this clause are generally adequate but, in certain cases, more
fundamental.
The primary objective of structural analysis is to obtain a set of internal forces and
moments throughout the structure that are in equilibrium with the design loads for the
required loading combinations. Under design ultimate loads, any implied redistribution of
forces and moments should be compatible with the ductility of the members concerned.
Generally it will be satisfactory to determine envelopes of forces and moments by linear
elastic analysis of all or parts of the structure and allow for redistribution and possible
buckling effects. Alternatively plastic methods, e.g. yield line analysis, may be used. For
design service loads, the analysis by linear elastic methods will normally give a
satisfactory set of moments and forces. [5]
3.1.2. Limit State Design
Limit state method ensure a structure remains useful for the purpose it was designed for
throughout its design life remaining within acceptable limit of safety and serviceability.
There are two major categories of limit state
i) Ultimate limit state
This is a condition in which a structure is can no longer be useful and the structure may
collapse. At this limit state the concern is with incipient failure thus factored loading is to
be applied. The initial response is to consider the use of plastic methods of Analysis.
However, use of such methods implies that the structural material will exhibit a large
degree of ductility after attainment of ultimate moment.
This generally applies to only members which are predominantly in flexure i.e beams,
slabs. The imposition of axial forces would significantly reduce such ductility. Thus
plastic methods should not be used for frame structures. It is permissible to use plastic
methods i.e the yield line approach for slabs provided that any membrane forces are low.
It is however possible to simulate the plastic collapse approach for framed structures by
carry out analysis of the structure under imposition of ultimate loading and then allowing
38
a redistribution of the moments such that the resulting moment field is still in equilibrium
with the applied loading. Amount of redistribution is limited to ensure sufficient ductility
in the members hence can only be carried out on flexural members.[5]
ii) Serviceability limit state
This refers to the conditions under which a structure is still considered useful. A structure
that may still be structurally sound would nevertheless be considered unfit. It refers to
conditions other than the building strength that render the buildings unusable.
Serviceability limit state design of structures includes factors such as durability, overall
stability, fire resistance, deflection, cracking and excessive vibration. It is essential that
serviceability analysis be carried out for prestressed concrete, highway structures, water
retaining structures
At ultimate loads plastic hinges form at the points of maximum moment and the moment
distribution changes from elastic to plastic. For the steel beam at collapse the hogging
and sagging moments are the same. The amount of redistribution is the reduction in the
peak elastic moment over the support. Thus it is possible to carry out a full plastic
analysis on the same basis as for steel structures. However serious cracking could occur
at the hinges in reinforced concrete frames and because of this the code adopts a method
that gives the designer control over the amount of redistribution and hence of rotation that
is permitted to take place. In clause 3.2.2 the code allows a reduction of up to 30% of the
peak elastic moment to be made whilst keeping internal and external forces in
equilibrium. [5]
39
3.2 Reinforced concrete beams
The design of beams may involve the following briefly discussed step. With loading
acting
a. Ultimate moment of resistance
It is the greatest bending moment that can be resisted by the section, assuming the beam
to be in pure flexure. It is defined as moment of resistance when the maximum
compressive strain in the concrete section reaches the limiting value of 0.0035
b. Longitudinal reinforcement
Concrete is strong in compression forces resistance but weak and unreliable in tension. It
is normally assumed that on the tension side of the neutral axis the concrete is cracked
and makes no contribution to the ultimate moment of resistance, all tension forces in this
section are carried by the steel reinforcement. In some cases it’s also necessary to provide
longitudinal reinforcement in the compression zone; thus we have singly or doubly
40
reinforced beams. Reinforcement is also provided in the sides of beams of overall depth
of more than 750mm
c. Shear reinforcement
The state of pure flexure rarely occurs, a beam is required additionally to resist the shear
stresses arising from transverse loads and torsion. The combined effect of shear and
bending moment may cause premature failure. To ensure the ultimate moment of
resistance is reached shear reinforcement are provided in the form of links at right angles
to the longitudinal reinforcement [3]
d. Minimum and maximum area of reinforcement
The minimum areas of reinforcement in a beam section to control cracking as well
asresist tension or compression due to bending in different types of beam section
aregiven in BS8110: Part 1, clause 3.12.5.3 and Table 3.2.7.where crack control is
discussed.
The maximum area of both tension and compression reinforcement in beams isspecified
in BS8110: Part 1, clause 3.12.6.1. Neither should exceed 4% of the grosscross-sectional
area of the concrete[3]
41
3.3 Slab Design
The general procedure to be adopted for slabdesign is as follows:
1. Determine a suitable depth of slab.
2. Calculate main and secondary reinforcementareas.
3. Check critical shear stresses.
4. Check detailing requirements.
3.3.1 Depth of slab (clause 3.5.7, BS 8110)
Solid slabs are designed as if they consist of aseries of beams of l metre width.
42
The effective span of the slab is taken as thesmaller ofthe same as those for beams, will
often control the depth of slab needed.
The minimum effective depth can be calculated from the basic (span/effective depth)
ratios are givenin Table 3.14 and the modification factor which is a functionof the
amount of reinforcement in the slab.
3.3.2 Steel areas (clause 3.5.4, BS 8110)
The overall depth of slab, h, is determined byadding allowances for cover and halfthe
main steel bar diameter to the effectivedepth. The self-weight of the slab together withthe
dead and live loads are used to calculate thedesign moment, M and the ultimate moment
of resistance of the slab,Mu
If Mu ≥M, which is the usual conditionfor slabs, compression reinforcement will not
berequired and the area of tensile reinforcement, As
Secondary or distribution bars are required in thetransverse direction and this is usually
based onthe minimum percentages of reinforcement (As min)given in Table 3.25 of BS
8110:
Shear resistance is generally not a problem in solidslabs subject to uniformly distributed
loads and,in any case, shear reinforcement should not be providedin slabs less than 200
mm deep.
As discussed for beams thedesign shear stress is calculated.The ultimate shear resistance,
is determined.
3.3.3 Reinforcement details (clause 3.12,BS 8110)
For reasons of durability the code specifies limitsin respect of:
1. Minimum percentage of reinforcement
2. Spacing of reinforcement
3. Maximum crack widths.
These are outlined below together with thesimplified rules for curtailment of
reinforcement.
43
a. Reinforcement areas.The area of tension reinforcement, As,should not be less than the
following limits.
b. Spacing of reinforcement
c. Crack width should beensured that they will not generally exceed 3 mm. This limiting
crack width is based onconsiderations of appearance and durability
d. Curtailment of reinforcement (clause3.12.10.3, BS 8110). Simplified rules for the
curtailmentof reinforcement are given in clause3.12.10.3 of BS 8110.
3.4 Columns
Columns primary carry axial loads from the floor and beams. Most columns are subjected
to moments as well as axial load. Most columns that are termed shortcolumns fail when
the material reaches its ultimate capacity under the appliedloads and moments. Slender
columns buckle and the additional moments caused bydeflection must be taken into
account in design.
The column section is generally square or rectangular, but circular and polygonalcolumns
are used in special cases. When the section carries mainly axial load it issymmetrically
reinforced with four, six, eight or more bars held in a cage by links.
The minimum size of a column must meet the fire resistance requirements. The covers
required to meetdurability and fire resistance requirements
44
The code classifies columns first as
1. Short columns when the ratios lex/h and ley/b are both less than 15 for bracedcolumns and
less than 10 for unbraced columns and
2. Slender columns when the ratios are larger than the values given above
3.4.1 Requirements for links
Clause 3.12.7 covers containment of compression reinforcement:
1. The diameter of links should not be less than 6 mm or one-quarter of the diameter of
the largest longitudinal bar;
2. The maximum spacing is to be 12 times the diameter of the smallest longitudinal bar
3. The links should be arranged so that every corner bar and each
3.5 Foundation design
Foundation failure may arise as a result of allowable bearing capacity of the soil being
exceeded or shear failure of the base.
The first condition allows the plan area of the base to be calculated, being equal to the
design load divided by the bearing capacity of the soil,
i.e. Ground= design load< bearing pressure capacity of soil
plan area
Since the settlement of the structure occurs during its working life, the design loadings to
be considered when calculating the size of the base should be taken as those for the
45
serviceability limit. The calculations to determine the thickness of the base and the
bending and shear
Reinforcement should, however, be based on ultimate loads. In most cases the design
process would be similar to that for beams and slabs. [3]
3.7 Wind loading design
The design of wind loads on a structure depends on wind velocity in a given area. the
pressure due to wind is also influenced by the height of building and other structures or
tree near it, the design of wind loading will be done using code BS 6399.
3.8 Design Codes and Specifications
Buildings must be designed and constructed according to the provisions of a building
code, which is a legal document containing requirements related to such things as
46
structuralsafety,firesafety plumbing, ventilation andaccessibility to thephysically
disabled. A building code has the force of law and is administered by a governmental
entity such as a city, a county, or for some large metropolitan areas, a consolidated
government. Building codes do not give design procedures, but specify the design
requirements and constraints that must be satisfied. Of particular importance to the
structural engineer is the prescription of minimum live loads for buildings. Whilethe
engineer is encouraged to investigate the actual loading conditions and attempt to
determine realistic values, the structure must be able to support these specified
minimum loads.
i) BS 8110 – 1997 part 1 gives recommendations for the structural use of concrete in
buildings and structures,excluding bridges and structural concrete made with high
alumina cement.
ii) BS 8110- Part 3 covers design charts for singly reinforced beams, doubly reinforced
beams and rectangular columns. These design charts cannot be used to obtain the
complete detailed design of any member but they may be used as an aid when analysing
the cross section of a member at the ultimate limit state. The charts have been based on
the assumptions laid down in BS 8110-1, use being made of the parabolic-rectangular
stress block throughout
iii) BS 5268 STRUCTURAL TIMBER
BS5268 is based on permissible stress based design and provides guidance on the
structural use of timber, glued laminated timber, plywood and other panel products in
load bearing members. It includes recommendations on quality, grade stresses and
modification.
iv) BS 6399 -1-1996 gives dead and minimum recommended imposed loads for use in
designing buildings. It applies to:
a) New buildings and new structures;
b) Alterations and additions to existing buildings and existing structures;
c) Existing construction on change of use.
47
It does not apply to the maintenance of, or the replacement of parts of, existing buildings
and structures where there is no change of use
v) BS 6399 -2-1997 gives methods for determining the gust peak wind loads on buildings
and components thereof that should be taken into account in design using equivalent
static procedures.
Two alternative methods are given:
a) A standard method which uses a simplified procedure to obtain a standard effective
wind speed which is used with standard pressure coefficients to determine the wind loads
for orthogonal design cases
j) A directional method in which effective wind speeds and pressure coefficients are
determined to derive the wind loads for each wind direction
vi) CP3:1972: Loading: Part 2: Wind Loads
The codes set out the design loads, load combinations and partial factors of safety,
material strengths, design procedures and sound construction practice. Other codes, to
which reference is necessary, will be noted as required
BS 5950-1-200 Structural use of steelwork in buildings, provides tables and charts for
design of universal beams and columns, joists and bearing piles
♦BS 8110 – Structural use of concrete,
♦BS 5950 – Structural use of steelwork,
♦BS 5268 – Structural use of timber,
♦BS 5628 – Structural use of masonry,
and are based on material characteristics such as the stress–strain relationship, the
modulus
of elasticity, Poisson’s ratio and the inherent variability both within the manufacture of
the
48
materials and the processes adopted during construction.
Currently the structural timber design code is a permissible stress design code and
those for concrete, steelwork and masonry are based on the ‘limit state’ design
philosophy.
3.9 DRAWINGS AND BAR BENDING SCHEDULE
3.9.1 Drawings
Drawings is means of presentation in a graphical form the shape, size and position of the
building on a site along with the instruction on materials to be used and the way there are
to be put together. Construction drawingare presented in precise, clear and with sufficient
information that could be important in implementation during construction.
3.9.2 Detailing
The general arrangement drawings give the overall layout and principal dimensions of the
structure. The structural requirements for the individual elements are presented in the
detail drawings. The output of the design calculations are sketches giving sizes of
49
members and the sizes, arrangement, spacing and cut-off points for reinforcing bars at
various sections of the structure. Detailing translates this information into a suitable
pattern of reinforcement for the structure as a whole. It is essential for the student to
know the conventions for making reinforced concrete drawings such as scales, methods
for specifying bars, links, fabric, cut-off points etc. The main particulars for detailing are
given for most of the worked exercises in the book. The bar schedule can be prepared on
completion of the detail drawings.
BS4466:1981: Specification of Bending Dimensions and Scheduling of Bars for the
Reinforcement for Concrete
3.9.3 Requirements to reinforcement construction drawings
Drawings used for off-site casting and factory production are excepted from this
provision. Reference shall be referred to accessory drawings. For drawings modified
later, all concerned drawings shall be modified as well. The following characterizations
(general information and placement information) of the reinforcement barsshall be given
on the drawing:
a) Required concrete strength class, the exposure class and further requirements to the
concrete given inreference standards;
b) Type of reinforcing steel and prestressed steel given in reference standards
c) Bar mark, number, diameter, shape and position of the reinforcement bars; distance
between the bars and overlap length at joints; arrangement, dimensions and development
of welding points by specification of the joining metal, jarring plates, position of the
concreting gap
c) Type of the pre-stressing system; number, type and position of the tendons; number,
type and position of the tendon anchoring and tendon coupling; bar mark, number,
diameter, shape and position of the accessory not prestressed concrete reinforcement;
type and diameter of the encasing tubes; specification of the intrusion grout;
50
d) Measures for securing the position of the concrete reinforcement and the tendons (e.g.
kind and arrangement of the bar chairs, as well as arrangement, dimensions and shape for
the support of the upper concrete reinforcement layer and the tendons);
e) The layer dimensionwhich derives from the nominal dimensionof the concrete cover,
as well asthe allowance in design for toleranceof the concrete cover;
f) The joint development
g) Special measures for quality assurance, if required.
h)Single length, sectional lengths and, if applicable, bending angles of the reinforcement
and in every case thereference standard mandrel or radii shall be represented on the
drawing
i)The mandrel diameters or radii.Manufacturing tolerances shall be taken into account in
dimensioning the reinforcement components, in orderto reach the desired concrete cover
in the ready-made structure. [9]
3.9.4Bar Bending Schedule
The bar schedule is the document used to specify and identify reinforcing bars. It is
divided up into shape schedules when applying shape codes and bending schedules.
Every schedule shall contain a title block containing elements. A shape schedule shall
contain the following information in the following sequence.
a) Member identification of the structural member in which the bar is located.
b) Bar mark which is a unique reference of the bar.
c) Type of steel (designation or abbreviation given in reference standards or other rules).
The bar's quality and profile can be designated by a single letter if it is properly define
d) Bar diameter (nominal diameter), in millimeters.
e) Bar length (cutting length) in millimeters or meters. It should be specified whether
there has been a correction, e.g. for bends or endhooks.
f) Number of members or number of groups of bars.
g) Number of bars in each member or in each group.
h) Total number of bars (f × g).
i) Total length (e × h), in millimeters or meters.
j) Bar shape (shape code)
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k) Definition of end hooks.
l) Bar-shape parameters (bending dimensions), in millimeters.
m) Modification index of the member. A letter shall be stated, e.g. A, B, C… [9]
4. REFERENCES :
1. CHARLES E. REYNOLDS AND JAMES E. STEEDMAN Reinforced concrete
designer`s handbook 10th edition.
2. BILL MOSELY AND J.H. BURGEY. Reinforced concrete design 6th edition
3. CHANAKYA ARYA. Design of Structural Elements 3RD EDITION
4. EUGENE J, O’BRIEN AND ANDREW S. DIXON. Reinforced and prestressed
concrete design
5. MARK FINTEL. Handbook of concrete engineering
6. ANDREW ORTON. Structural design of masonry
7. W.M.C MCKENZIE. Design of structural masonry
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8. DENNIS LAM AND SING PING CHIEW. Structural steelwork design to limit state
3rd edition
9. Construction drawings - Simplified representation of concrete
reinforcement (ISO 3766:2003)
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