353-23 Principles and Concept of Structures

7/30/2019 353-23 Principles and Concept of Structures

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INDEX

Unit I .............................................................. 3

Lesson 1: Design Process ............................... 4

Lesson 2: Structures and Buildings ............. 16

Unit II .......................................................... 37

Lesson 3: Structural Requirements ............. 38

Lesson 4: Structural Forms and Materials .... 52

Unit III ......................................................... 78

Lesson 5: Basic States of Stress ................... 79

Unit IV .......................................................... 92

Lesson 6: Beams .......................................... 93

Unit V ......................................................... 109

Lesson 7: Structural Arrangements ............ 110

Books for Further Reference ......................... 131


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Unit

IDesign ProcessLesson-1: Design ProcessLesson-2: Structures and Buildings


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Lesson 1: Design Process

Objective:To understand the design process and the stepsinvolved.

Structure:1.1 Introduction1.2 Synthesis

1.3 Analysis1.4 Theory

1.5 Conceptual Design1.6 Preliminary Design1.7 Final design1.8 The design process1.1 Introduction

1.1 IntroductionA process of synthesis of an object (a product,

building, city etc.) from given data, by employingDesign tools and Design criteria and subject to

Constraints.The process like any human activity, and

particularly a creative one, is extremely complex.The discussion that follows is an attempt to outline

the essentials of the process in as objective manneras possible. A step-by-step description of theprocess is given below. The process is applicable to

the design of any object.


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1.2 SynthesisA process, in which an object is assembled, created

or generated, from basic components or data. A

problem in synthesis typically has multiple solutions.

1.3 AnalysisA process of disassembling or dissecting an existing

entity (object, phenomenon, idea etc.) into its basiccomponents. A problem in analysis typically has asingle solution (although the solution may consist of

several parts).

1.4 TheoryAn analytical framework providing a systematicdescription of a system or class of existing entities('structures' in the present context).

Design toolsThe means employed in the design process. The

main tools, in order of significance and precedenceare:


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'Blank page'.Common sense. Experience (gained through thepractice of design).

Theory.Design codes.Design aids, such as design guides, product

catalogues, computer programs (e.g. CAD).


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Types of designThe design of a large object, such as a building,

typically consists of the three phases listed below.

The process is general for the design of any product,but a building or another type of structure is used

as a model.

1.5 Conceptual DesignConceptual design a design phase beginning with

the initial data (the 'brief') and ending with a

number of concepts for the 'product'. For example,when the 'product' being designed is a building, theresults of the conceptual design may include suchfeatures as the general shape, layout of spaces, and

the types of the main supporting elements of the


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structure and their locations without details or

accurate dimensions. The design tools which featuremost prominently in conceptual design are the three

listed earlier (blank page, common sense,experience).


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1.6 Preliminary DesignA design phase beginning with the concepts

developed in the conceptual phase and ending with

fewer variants (usually one or two), includingapproximate sizes and rough details.

1.7 Final designThe final design phase, starting with the preliminarydesign of the selected variant, and ending with the

working drawings, shop drawings and other projectdocuments.

The design tools featuring in preliminary and finaldesign are mainly the last three listed in 1.2(theory, codes and design aids), but common senseand experience play an overriding role. The present

work is concerned mainly with conceptual design.1.8The design process step by step

1.8.1 Preliminaries

Step 1 Blank pageEmpty the mind of any preconceived ideas,intuition, prior experience (e.g. of similar projects).

It is important to embark on a new project with a

fresh outlook. This is probably the most importantstep and the hardest to accomplish. Experience and

intuition will play their role willy-nilly, but it isimportant to rein them in and subject them to the

constraints of the problem at hand (see below).The development of design alternatives is based on

the fact that any design problem (as distinguishedfrom analysis problem)The process can be summarised as follows:

Based on the data and the constraints, formulateat least two substantially different general


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concepts (see 'Conceptual design' above). To

follow the chair example, two possibilities couldbe a soft padded chair or a solid (e.g. wooden)

one.Each of the solutions will generate its own

problems, for example the material for theframework of the chair. Provide at least two,

substantially different, solutions for each

problem. For example metal frame or woodenframe, sponge padding or elastic membrane, etc.


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Repeat the process until a sufficient number of

conceptually distinct alternatives are obtained.The process is a geometric progression and a

large number of alternatives is quickly produced.What is a 'sufficient' number will depend on the

scale of the project, and on the inclination of thedesigner, but a rough guide for conceptualdesign of, say, a building, is between four andeight alternatives.

Step 2Definition of design criteriaThis step can be performed at any stage after step 1

but it is presented here, at the stage when Designcriteria are needed to compare alternatives.

Design criterionA measure of some aspect of the quality of a

proposed design solution. For example, the comfortof the chair, its durability, colour fastness, ease of

sitting and getting up, weight, cost etc., can allserve as design criteria in the chair example.

It is important to distinguish between a designcriterion and a constraint. As mentioned above, asolution which violates any constraint is not in fact asolution to the given problem, and either the

solution is rejected or the problem is redefined. On

the other hand a design criterion can be satisfied toa greater or lesser extent.


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Step 3Comparison of design alternatives The comparison

can be qualitative or quantitative to varying

degrees, but even in an apparently quantitativeanalysis the assigning of weights to different

criteria, and the assignment of marks to solutions,are based on the designer's judgment and arehighly subjective.

Nevertheless, the designer must resist, as far as

possible, the temptation to assign weights so as toarrive at a favourite preselected choice. An honest,even-handed selection process can sometimes leadto unexpected and gratifying results.

Step 4Selection and update

The selection is based on the comparison of step 6,

but subject to the designer's judgment.The number of selected alternatives for the nextphase of the design depends on the size of the

project and the nature of the next phase. In the

case of conceptual design at least two alternativeswill usually be selected for preliminary design, morein a large project. It is rare for more than one

alternative to be considered in the final designphase.


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On the basis of the comparison, it is sometimes

possible to improve a selected solution, incategories indicated by the design criteria, prior to

moving to the next stage. For instance, it may be

possible to reduce the weight of the chair (thestructure) without making it too weak.

Step 5

UpdatingReturn to step 6, in the case where updating hasbeen carried out in the preceding step. This is notusually applicable to the conceptual phase since it

involves refinement rather than change of concept..


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Post processing (output)

Step 6

Presentation of resultsProduction of models, drawings, prototypes, etc.according to the nature of the project and thedesign phase (1.3).

Step 7

Proceed to next phaseFrom conceptual design proceed to preliminary

design or from preliminary to final design.Some general comments

The selection between design alternatives should

be put off as late as possible, in order to avoidthe natural inclination for prejudged preferences.

Design often has to do with shape or forD1(architectural design, product design, structuraldesign etc.). In design (as distinct from styling)shape (geometry) is, in most cases, the result

(output) of the design, not an input (data,

constraint or criterion).


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The relationship between the shape of a

structure and the principles governing itsbehaviour is the central theme of this work.

Presuming a shape amounts to dictating a modeof behaviour, which, unless the presumption is

based on thorough structural knowledge, is likelyto produce poor results.


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Lesson 2: Structures and

Buildings

Objective:To understand the types of loads and forcesacting upon a building and the role of structures

in a building

Structure:2.1 Structure2.2 Forces and loads

2.3 Force2.4 Units of measurement2.5 Load

2.6 Types of loads on structures

2.6.1 Gravity loads2.6.2 Environmental loads2.6.3 Other environmental influences

2.7 Load distribution2.7.1 Uniformly distributed load2.7.2 Concentrated or Point load

2.8 Values of loads for design purpose

2.8.1 Safety2.8.2 Serviceability

2.9 Movement

2.9.1 Displacement2.9.2 Rotation

2.10 Force couple and moment2.10.1Moment

2.11 Resultant force2.12 Summation of vectors resultant

2.13 Resultant force location2.13.1Parallel forces2.13.2General system of forces


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2.1 StructureThat part of the object (building, bridge, chair, living

body etc.) which is responsible for maintaining the

shape of the object under the influence of theenvironment.

The simplest way of describing the function of an

architectural structure is to say that it is the part ofa building which resists the loads that are imposedon it. A building may be regarded as simply an

envelope which encloses and subdivides space inorder to create a protected environment. Thesurfaces which form the envelope, that is the walls,the floors and the roof of the building, are subjected

to various types of loading: external surfaces areexposed to the climatic loads of snow, wind andrain; floors are subjected to the gravitational loads

of the occupants and their effects; and most of thesurfaces also have to carry their own weight. All ofthese loads tend to distort the building envelope and

to cause it to collapse; it is to prevent this fromhappening that a structure is provided. The functionof a structure may be summed up, therefore, as

being to supply the strength and rigidity which arerequired to prevent a building from collapsing. Moreprecisely it is the part of a building which conductsthe loads which are imposed on it from the points

where they arise to the ground underneath thebuilding, where they can ultimately be resisted.


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2.2 Forces and loadsThe influence of the environment on structures

takes the form, principally, of Loads and Forces.

Here the word 'environment' is taken to mean any-thing in contact with the structure (e.g. vehicles,

furniture, people etc.), including the structure itself.Such primary environmental influences as wind,

temperature, and earthquake affect the structure by

exerting forces on it. The remainder of the chapteris concerned chiefly with these concepts.

2.3 ForceInfluence on a body, causing (or attempting tocause) the Movement of the body or part of it, orcausing a change in its movement, if it is already in

motion.


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This is the common definition of force encountered

in the literature. It is interesting to note that even

though force is one of the most fundamentalconcepts in physics, its definition is indirect, relying

on its effect. This is an indication of the complexityof this concept and the difficulty in visualising it.

The definition of force through the concept ofmotion, a concept which is easy to grasp intuitively,

enables easy visualisation of forces, and brings forththe extremely important relationship between forceand motion. This, in fact, is the source of the force-

shape relationship which is the focal point of thiswork.A force is a Vector.


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A vector is a parameter (a physical quantity)

characterised by a magnitude, (or 'intensity') and adirection. Relying on the correlation between force

and motion, it is convenient to visualise a vector interms of motion: when an object moves from point

A to point B, the magnitude of the distance travelledis not enough to define the position of point B,relative to A. We need to know the direction as well.Distance, like force, is a vector (see Displacement

below).A vector is described graphically as an arrow,pointing in the direction of the vector and having alength representative of the magnitude.

2.4 Units of measurementForce, like distance, is one of the fundamental

physical entities, measured in one of the basic units.

The basic force unit is the Newton (denoted N) andits multiples -kilo-Newton (kN, one thousandNewtons) and Mega-Newton (MN, one million

Newtons). As a rule, the international system ofunits is used throughout this text, with some

exceptions. This system employs the Newton (N)and its derivatives for force units, and the metre

(m) or millimetre (mm) as length units. Centimetre

(cm) is also used occasionally.2.5 Load:A force applied to a structure by the environment or


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by any object (including the structure itself or other

structures). Alternative definition: any Externalforce applied to the structure, other than a Reaction

force.

2.6 Types of loads on structuresThe structures in question are buildings, bridges,monuments, signposts etc. There are two majortypes of loads: Gravity loads, which are usually

vertical, and Environmental loads, which are often

horizontal (e.g. earthquake) but can generally takeany direction. Note that although all loads weredefined as arising from the influence of theenvironment, the term Environmental load refers to

a subclass of loads defined below.

2.6.1 Gravity loadsGravity loads are the effect of the weight of objects

on the structure, including the weight of thestructure itself (weight is a force). Two kinds aredistinguished:

Dead load: Load resulting from the self weight

(SW) of the structure and of any permanentlyattached components, such as walls, flooring,permanent partitions etc.

Live load: Load arising from the function of thestructure, including attached components whose

location is not fixed, such as movable partitions.Live loads are a result of the weight of the loading

objects (vehicles, furniture, goods, people etc.) andare mostly vertical (snow load is also considered live


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load). In some cases, however, loads may be

applied in non-vertical directions, for instance loadsdue to braking of vehicles, loads transmitted

through pulleys, earth or hydrostatic pressure etc.

2.6.2 Environmental loadsEnvironmental loads are not a direct result of theweight of objects, but of movement in thestructure's environment. The most common

environmental loads are Wind load and Earthquake

load. Wind load is a result of moving air tilting the

structure. Earthquake load is a result If themovement of the earth in which the structure isfounded.The force-movement relation is reciprocal. In thesame way that force causes movement, force can be

caused by movement. In the above instances, themovement (of the air or the ground) causes forces

on the structure and these forces, in turn, causemovement of the structure and of parts of the


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structure relative to one another.

2.6.3 Other environmental influences

Other environmental influences are movementswhich may cause Internal forces in certain

structures. In other cases they only causeDeformations. These influences include temperature


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effects -change of temperature or temperature

difference over parts of the structure, e.g. betweenthe inside and the outside; Support settlement -

settlement (sinking) of foundations by differingamounts; and so on.

Some other influences affecting dimensions ofcomponents of the structure are also consideredenvironmental effects because of the similarity tothe influence of temperature and settlement. These

include statistical variation in component dimensions('lack of fit'), and deliberately induced Deformations

2.7 Load distributionSo far, load has been described in general terms, as

the overall force acting on the structure, causingmovement in it. In practice, a load applied to a

structure is distributed, or 'spread', over its surface

in certain ways, for instance snow over the roofsurface, vehicles over a bridge deck etc. A loaddistributed over a portion of the structure is termed

Distributed load.Two major types of load distribution are mostcommon:

2.7.1 Uniformly distributed load

The load is distributed uniformly over the surface, orover a projection of the surface. The load on a unitarea of the surface (or its projection) is the same,

no matter where this unit area is taken.


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2.7.2 Concentrated or Point load

This is a load distributed over a very small portion ofthe structure's surface. It is considered as a forceacting at a point. Such loads are often exerted by

one structural member on another.

2.8 Values of loads for design purposeLoad values are specified in Codes or Standards.

Codes and standards are design aids, as mentionedearlier.

Codes and StandardsThese are documents produced by authorisednational institutes, which prescribe certain


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requirements to be satisfied by various 'products'

including structures.More specifically, regarding structural design, codes

and standards prescribe procedures aimed atensuring the Safety and the Serviceability of the

structure. Part of these procedures is thespecification of the values of loads (andcombinations of loads of different types) required tobe applied to commonly constructed structures.

For practical purposes the words 'code' and'standard' are synonymous. The difference is in theirlegal status which varies from country to country.

2.8.1SafetyThe ability of the structure and every part of it tosupport the load without collapsing, taking into

account uncertainties in the values of actual loads

and in the strength and behaviour of the structure.2.8.2 ServiceabilityThe ability of the structure to ensure its satisfactoryfunctioning. This implies particularly limitations on

the magnitude of movements under various appliedloads (Deflection, vibration etc.).

2.9 Movement

Movement is the result of the action of force, or acombination of forces. In general, movement caninclude such parameters as distance, speed, time,

acceleration. In the context of this work only thedistance is of interest, both in its own right and asan indicator of the force causing it.


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2.9.1 DisplacementDisplacement is the distance through which a body,

or a point on the body, moves as a result of theaction of force. This distance is a vector. It is

characterised by a magnitude -the amount of travel-and a direction.

2.9.2 RotationRotation is a kind of movement (displacement) but

it is more complex than the linear movementimplied so far. When an object rotates there is a

point in it which does not move at all and differentpoints on it have different displacements -different

magnitudes and directions of distance.


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2.10 Force couple and momentA rotation cannot be affected by a single force

vector of the type we have encountered. Since the

body as a whole does not move, there can be no netforce acting on it (see force Resultant below). We

can imagine a rotation of a body if the body is actedupon by two forces of equal magnitude (say P) andopposite direction, such that the lines of action of

the two forces are offset by a certain distance (a,say). Such a pair of forces is termed a Force couple,or Couple for short.The body as a whole cannot move, because the two

forces act in opposite directions. But at each of thetwo points of application of forces, thecorresponding force moves the point in its direction.The result is that the two points move in opposite

directions, causing the rotation of the body.


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2.10.1MomentThe effect of a force couple is clearly dependent, not

only on the magnitude of the two forces, but also on

the distance by which they are offset -the Leverarm. If the arm was zero -the forces were collinear -

there would be no rotation The effect of lever armlength on such activities as bolt tightening orreleasing is well known.

In order to express the effect of the force couple

which takes into account both force magnitude andlever arm a parameter termed Moment is defined(denoted M), whose magnitude is the product of the


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force magnitude and the lever arm length: M = p x

a. It is customary to display a moment graphicallyas a curved arrow showing the sense of rotation,

instead of showing the system of force couple. Thisarrow does not represent a vector -it has only a

sense, not a magnitude and not a specific direction.Units of measurement of moment are force x length,such as Newton-millimetre (Nmm), kilo- Newton-metre (kNm) etc.

The force couple defines a plane (two parallel lines).It is intuitively clear that the rotation is not affectedby the direction of the forces in this plane, but onlyby the relative sense of the forces forming the

couple, which determines the sense of rotation -clockwise or counter clockwise.Nevertheless, a moment is, in fact, a vector whose

magnitude is defined above and whose direction isperpendicular to the plane of the force couple, and

with a sense related to the sense of rotation in a'right handed' manner. Any operation on vectors, asdetailed in subsequent sections, is applicable also tomoments, but due to the difficulty in three-

dimensional visualisation, this topic is not pursued

further. Furthermore, the vectorial nature ofmoment is not essential for the understanding of

structural behaviour at the fundamental level.

2.11 Resultant force


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Normally a structure is not subjected to a single

force, but to .a combination of several loads andother forces, in different directions and locations. In

order to understand how the structure responds tosuch load combinations, it is necessary to know how

to handle such combinations -how to operate withvectors.

2.12 Summation of vectors -resultant

When a number of forces (or any vectors) act on anobject simultaneously, the Resultant force (or

Resultant vector) is a single force (vector) which, ifacting alone on the object would have the sameeffect as the combined forces (vectors). It is said to

represent the sum of the vectors, or the Vectorial

sum.

It is easy to visualise a resultant vector and a way

to derive it if we think of displacements rather thanforces. If we think of each vector as a corresponding


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displacement, and instead of applying them

simultaneously apply them sequentially (the finalresult being the same), then the resultant

displacement is the distance from the starting pointto the final point.

To obtain the resultant graphically, plot theindividual vectors tail to head. The resultant is thevector joining the tail of the first vector with thehead of the last.

2.13 Resultant force location

2.13.1 Parallel forcesThe magnitude of the resultant of a set of parallelforces is simply the sum of the forces and thedirection is parallel with the forces. The question isthe location of the resultant relative to a reference

point.To obtain the location of the resultant force, apply

at the reference point imaginary forces of equalmagnitude and opposite sense to the given forces.These imaginary forces form couples with theoriginal forces. Their sum forms a couple with the

resultant force.

The location of the resultant force is determinedfrom the condition that its moment is equal to thesum of the moments of the given forces. This is

because the effect of the resultant has to be thesame as that of the given forces in every respect,

including rotation with respect to any point.

2.13.2 General system of forcesThis expression can be used to obtain the location ofthe origin (the application point) of the resultant ofany set of forces (not necessarily parallel), byworking with their components. Each force is

replaced by its components, having the same point

of application as the force.


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The components parallel to any axis (x,y) form a set

of parallel forces and so the expression above givesthe location of the component of the resultant

parallel to the same axis (i.e. its distance from theaxis). The origin of the resultant is at the inter-

section of the directions of the two components.


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Summary:The influence of the environment on structures

takes the form, principally, of Loads and Forces.

Here the word 'environment' is taken to mean any-

thing in contact with the structure (e.g. vehicles,furniture, people etc.), including the structure itself.Such primary environmental influences as wind,

temperature, and earthquake affect the structure byexerting forces on it. The remainder of the chapteris concerned chiefly with these concepts. The

components parallel to any axis (x,y) form a set ofparallel forces and so the expression above givesthe location of the component of the resultantparallel to the same axis (i.e. its distance from the

axis). The origin of the resultant is at the inter-section of the directions of the two components.Revision Points:

Gravity loads :Gravity loads are the effect of the

weight of objects on the structure, including theweight of the structure itself (weight is a force). Two

kinds are distinguished:Dead load: Load resulting from the self weight (SW)

of the structure and of any permanently attachedcomponents, such as walls, flooring, permanent

partitions etc.Live load: Load arising from the function of thestructure, including attached components whose

location is not fixed, such as movable partitions.

Key Words:


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Summation of vectors -resultant - When a number

of forces (or any vectors) act on an objectsimultaneously, the Resultant force (or Resultant

vector) is a single force (vector) which, if actingalone on the object would have the same effect as

the combined forces (vectors).

Intext questions:1. Explain the following terms:

a. Designb.

Synthesisc. Analysis

Terminal Exercises:1. Discuss in detail the different types of designs.2. How does a design process help a designer to

work in a organised and systematic manner.3. What is the purpose of a structure? Explain in

detail why you as an Interior Designer need tohave a good understanding of its principles andconcepts.

4. What is force? Explain in detail its impact on astructure.

5. What is a load? Explain in detail the varioustypes of loads exerted on a structure.

6. Write short notes on the following:a. Movementb. Displacementc. Force couple and momentd. Resultant force locatione. Rotation

Assignments / Learning Activities:

1. How to develop a design.2. What steps do the students follow during design

process?3. The instructor will explain the actual design

process and stage involved.4. Students will be explained the purpose of each

design step followed.

5. Discussion about structures and its various parts.6. The forces and loads acting on structures.


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7. Explanation of different movements anddisplacements that occur in structures becauseof those forces.

8. Discuss about different vector forces andresultant forces and how do they act on

structures.

Supplementary Material / SuggestedReading:1. Time Savers Standards For Building Types2.

Structure in Architecture by Salvadori and Heller


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Unit

II

Structural RequirementsLesson-3: Structural Requirements

Lesson-4: Structural Forms andMaterials


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Lesson 3: Structural

Requirements

Objective:To study in detail the structural requirements ofbuildings.

Structure:

3.1 Introduction3.2 Equilibrium

3.2.1 Conditions for equilibrium

3.2.2 Equilibrium and structures3.2.3 Overall equilibrium of a structure

3.3 Stability3.3.1 Geometric stability

3.4 Strength3.5 Functionality3.6 Economy3.7 Aesthetics

3.1 Introduction

To perform its function of supporting a building inresponse to whatever loads may be applied to it, a

structure must possess four properties: it must becapable of achieving a state of equilibrium, it must

be stable, it must have adequate strength and itmust have adequate rigidity. The meanings of these

terms are explained in this lesson. The influence of

structural requirements on the forms which areadopted for structures is also discussed.

3.2 EquilibriumStructures must be capable of achieving a state ofequilibrium under the action of applied load. This

requires that the internal configuration of thestructure together with the means by which it is

connected to its foundations must be such that all


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applied loads are balanced exactly by reactions

generated at its foundations.Despite the famous statement by one celebrated

commentator buildings are not machines.Architectural structures must, therefore, be capable

of achieving equilibrium under all directions ofloads.When a body is subjected to the action of several

forces, the combination of forces can be such thatthe body does not move -the forces 'cancel' one

another's effect. The simplest example is two forcesof equal magnitude and opposite senses acting on

the body along the same line. When such a situationexists, the body or the forces are said to be in

Equilibrium (a Latin word meaning 'equal weight' ason the two arms of a scale).The study of forces in equilibrium is termed Statics,indicating the absence of motion.

3.2.1 Conditions for equilibriumThe condition of no movement implies that tomaintain equilibrium, the resultants of all forces and


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of all couples must vanish. Since couples and forces

are different entities, having different units, theresultants must be considered separately.

In graphical terms, the first condition (vanishing ofthe force resultant) implies that, in a state of

equilibrium, forces, when drawn tail to head, form aclosed polygon, i.e., the head of the last forcevector touches the tail of the first. This can be auseful tool for force analysis in certain simple cases.

3.2.2 Equilibrium and structuresIn most cases, architectural structures, or any part

of a structure, do not move once loads have beenapplied (dynamic situations such as duringearthquake or vibration are not considered here).

An architectural structure and every part of it is in

equilibrium. This simple and apparently obviousstatement is the principal tool enabling the analysis


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of the behaviour of structures and of the forces

acting on and in them.

3.2.3 Overall equilibrium of a structureThe loads applied to a structure are, in general, notin equilibrium. Furthermore, some of the loads are

changing and variable, and yet the structure is

(usually) stationary, i.e. in equilibrium. In order toensure this state of affairs, it is clear that otherforces act on the structure, which are always inequilibrium with the applied loads. These forces are

termed Reactions they 'react' to the loads to keep

the structure in equilibrium. The reactions areprovided by the structure's supports usually thefoundations, or by another structure, considered

separately.A force causing motion can be considered an acting

force. A force restraining motion can likewise be

considered as reacting.


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Stability and Rigidity are basic concepts. However

they relate to a structural system as a whole andnot just to parts of structures. There is a lack of

consistency in the technical literature with regard tothese concepts, and the meaning sometimesdepends on the context or on the background of the

practitioners. Other terms are sometimes employed

to indicate the same thing. The definitions thatfollow are intuitively clear. They are general and, atthe same time, rigorous enough to characteriseproperly the desired properties of structural

systems.

3.3 StabilityStability is the ability of a structure to support load

while undergoing limited deformations anddisplacements. The limit of deformation or

displacement which determines if a structure isstable or not depends on the type of structure (see

Rigidity below).Stability is a qualitative term a structure is stableor unstable. It cannot be 'more stable' or 'less

stable'. Two kinds of stability are distinguished -


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geometric and elastic -depending on the source of

instability, if it occurs.

3.3.1 Geometric stabilityGeometric stability is the ability of a structure tosupport any load at all. This is a property of the

geometry of the structure (hence the term). It is not

related to the magnitude of the load or he strength

of the components of the structure. It is sometimestermed general stability or overall stability.Geometric stability is the property which preservesthe geometry of a structure and allows its elements

to act together to resist load. The distinctionbetween stability and equilibrium is illustrated bythe framework shown in Fig. 2.1 which is capable of

achieving a state of equilibrium under the action ofgravitational' load. The equilibrium is not stable,however, because the frame will collapse if

disturbed laterally.


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This simple arrangement demonstrates that the

critical factor, so far as the stability of any system isconcerned, is the effect on it of a small disturbance.

In the context of structures this is shown very

simply in 'fig. 2.2 by the comparison of1ensile andcompressive elements. If the alignment of either of

these is disturbed, the tensile element is pulled backinto line following the removal of the disturbing

agency but the compressive element, once itsinitially perfect alignment has been altered,progresses to an entirely new position. Thefundamental issue of stability is demonstrated here,

which is that stable systems revert to their original

state following a slight disturbance whereasunstable systems progress to an entirely new state.

Fig. 2.1 A rectangular frame with four hinges iscapable of achieving a state of equilibrium but isunstable because any slight lateral disturbance to

the columns will induce it to collapse. The frame onthe right here is stabilised by the diagonal elementwhich makes no direct contribution to the

resistance of the ravitational load.


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The requirement of "rigid-body" stability isconcerned with the danger of unacceptable motions

Fig. 2.2 The tensile element on the left here isstable because the loads pull it back into linefollowing a disturbance. The compressive

element on the right is fundamentally unstable.


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of the building as a whole. When a tall building is

acted upon by a hurricane wind, and is not properlyrooted in the ground or balanced by its own weight.

it may topple over without disintegrating. Thebuilding is unstable in rotation. This is particularly

true of tall narrow buildings, as one may prove byblowing on a slim cardboard box resting on a roughsurface (lest it should slide).The danger of rotational instability is also present

when a building is not well balanced or is supportedon a soil of uneven resistance. If the soil under thebuilding settles unevenly, the building may rotate asthe Leaning Tower of Pisa still does, and may

eventually topple over.A building erected on the side of a steep hill may, byits own weight, have a tendency to slide down its

slope. This may happen either because the buildingskids on the soil, or because a layer of soil adheres

to the foundations and slides on an adjoining layer(Fig. 4.8). The second occurrence is not uncommonin clay soils when water seeps through the ground,transforming the clay into a soapy material.


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All these cases of instability are related to the soil,

and to the building foundations. From the viewpoint

of economy and usage foundations are a "necessary

evil": moreover, they are out of sight so that thelayman is seldom aware of their importance andcost. For example, the foundations of a heavystructure erected on loose sand permeated by water

must allow the building to "float" on such a soil:they are built by means of 'rafts" which in structureare similar to the hull of a ship (Fig. 4.9).Elaborate precautions against soil failures are

extensively taken to guarantee the stability ofstructures. Wood, steel or concrete piles can be

driven into the soil to depths which permit the

building to be supported by friction against thesurface of the piles or to reach solid rock (Fig.


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4.10). The piles may be rammed into the soil or

may be made to slide into it by rapid vibrations.Soils may also be consolidated by chemical means.

Usually, an improvement of soil conditionsaccompanies the design of proper foundations, but

soil mechanics is as yet a difficult and uncertain art,to this day, most of the damage to buildings comesfrom faulty foundations, even though their cost mayreach 10 per cent, or more, of the total cost of the

building.

3.4 StrengthThe requirement of strength is concerned with theintegrity of the structure and of each of its parts

under any and all possible loads. To this purpose,the structural system is first chosen, and the loads

on it are established: the state of stress is then

determined at significant points of the structure andcompared with the kind and amount of stress thematerial can safely stand. Factors of safety of

varying magnitude are used to take into accountuncertainties in loading conditions and materialpropertiesRigidity should not be confused with strength: two

structures may be equally safe, even though onedeflects more than the other under the same loads.

Although it is often a measure of strength againstloads, rigidity may be a sign of weakness in a

structure subjected to temperature changes, unevensettlements. and dynamic loads.

Certain structural weaknesses may lead to modest

damage, while others may produce the collapse ofthe structure. Hence, the designer must checkstrength under a variety of loading conditions to

obtain the worst stress situation at significant pointsof the structure. The structural optimist is inclined

to believe that a structure collapses only if faultydesign is compounded with faulty construction, and

helped by an act of God. The cautious pessimistbelieves, instead, that structures collapse at the


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slightest provocation. In practice, structures do

collapse, although in small numbers; moreover,owing to the plastic behaviour of structural

materials, most collapses do not occur suddenly,and they seldom take human lives.

The strength of a structure is often evaluatedaccording to the rules and regulations of codes.These procedures are usually safe, but may becomeuneconomical when they ignore recently developed

techniques and materials.The responsibility for strength rests squarely on theshoulders of the structural engineer. Every day his

job is made more complex, and safer, by the

increased theoretical knowledge and the improvedtools at his disposal. Among the new tools, theelectronic computer deserves special mention.

These "electronic brains', allow the performance ofotherwise impossibly lengthy calculations in a

matter of a few seconds or minutes, and areparticularly useful in the kind of basic calculation tobe performed daily by the structural engineer.

3.5 Functionality

Structural functionality is concerned with theinfluence of the adopted structure on the purposefor which the building is erected. For example, long-

span floors could be built by giving them curvature,as in the dome of a church; their thickness and theircost would be greatly reduced. But, since the pull of

the gravity is vertical, floors must be horizontal.

Suspension bridges are flexible structures. TheGolden Gate Bridge in San Francisco sways as mushas thirteen feet under strong wind gusts. Such

motions obviously must be limited, not only so thatfast travelling cars are not swayed from their paths,

but also because the pressure of a steady windproduces aerodynamic oscillations capable of

destroying a bridge if it is too flexible.


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The excessive flexibility of a structure may impair its

functionality, if the deflections under static loadsmake it difficult or uncomfortable for people to

move in it. Aluminium, which is three times asflexible as steel, in many cases requires design for

deflection rather than for strength. Worse conditionsmay arise under resonant loads: a stream of trafficmay produce a continuous and uncomfortablevibration throughout a structure, seriously impairing

its usefulness. Buildings over subway or railroadtracks are often supported by lead insulation pads tostop such vibrations.

3.6 EconomyEconomy is not always a requirement of

architecture. Some buildings are erected for

monumental or symbolic purposes: to aggrandizethe owners in the eye of the public, or to enhancespiritual values. Monuments to the state or to

"corporate images" fall in the first category:churches belong in the second. Their cost has littlerelation to their financial value

But the utilitarian character of structure is so

fundamental that even the structural systems ofnon-utilitarian buildings are influenced by economy.In other words. a strict structural budget must

always be contended with unless the structure itselfis an advertising display: an aluminium structuremay be required, regardless of cost, in order to

emphasize the ownership of the building by an

aluminium manufacturer.

3.7 AestheticsThe influence of aesthetics on structure cannot be

denied; by imposing his aesthetic tenets on theengineer, the architect often puts essential

limitations on the structural system. In actuality,

the architect himself suggests the system he


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believes best adapted to express his conception of

the building.In some cases the architect consults with the

engineer from the very beginning of his design, andthe engineer participates in the conception of the

work, making structure an integral part ofarchitectural expression. The balance of goals andmeans thus achieved is bound to produce a betterstructure and a more satisfying architecture.

The influence of structure on architecture and, inparticular, on aesthetics is more debatable. It wasremarked in Section 1.1 that a totally sincere andhonest structure is conducive to aesthetic results,

but that some architects are inclined to ignorestructure altogether as a factor in architecturalaesthetics. Both schools of thought may be correct

in their conclusions, provided their tenets be limitedto certain fields of architectural practice. No one can

doubt that in the design of a relatively smallbuilding the importance of structure is limited, andthat aesthetic results may be achieved by forcingthe structure in uneconomical and even irrational

ways. At one extreme, the architect will feel free to

"sculpt" and thus to create architectural formsinherently weak from a structural viewpoint,

although realizable.At the other end of the scale, exceptionally largebuildings are so dependent on structure that the

structural system itself is the expression of theirarchitecture. Here, an incorrect approach to

structure, a lack of complete sincerity, and a misuseof materials or construction methods may definitely

impair the beauty of the finished building.


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Lesson 4: Structural Forms

and Materials

Objective:To study the various structural forms andmaterials.

Structure:

4.1 Introduction4.2 Stone4.3 Bricks

4.4 Timber4.5 Iron and steel4.6 Reinforced concrete4.7 Pressurised concrete

4.1 IntroductionThe relationship between structural form and

material properties is complex, but unavoidable. Therelationship for a particular material must beunderstood by designers if they are to producegood, economical designs. It will not be possible in

this book to explore this relationship very deeply: itis a subject for a book in itself. What we can do is to

attempt to explore some facets of this interaction by

considering some examples.As a first example, we shall consider a case wherethere is only a single available material to satisfy

very stringent structural and functionalrequirements. This example will take us to the arcticwinter, where the Inuit traditionally follow anomadic existence. Temperatures are many degrees

below zero, combined with wind and blizzard. Inthese circumstances, shelter is required from the

elements in a structure that can be built rapidly with

readily-available materials. The only such material issnow and the structure that has resulted fromgenerations of experience is the igloo. This is


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actually a very sophisticated structure. Typically it is

made of blocks of snow about 400-500 mm thick.These are cut and laid in a spiral form until the

classical hemispherical dome has been completed.The structure is self-supporting at all stages of the

construction, so needs no temporary supports ofany kind. The resulting form is excellent for resistingwind forces and is highly insulated, allowing thebody heat of the occupants, supplemented by a

fairly small blubber stove, to maintain an insidetemperature some 40C above the outsidetemperature. The construction has to take accountof the structural properties of snow, which has some

compressive strength but negligible tensile strength.The dome is ideal for this, as it is a compressionstructure. It also takes advantage of another

property of snow: its excellent thermal insulation.The form of structure developed for this very

exacting circumstance is thus the result of acombination of the function that the structure isrequired to serve, and the properties of theavailable material.

We can now move on to another apparently

primitive construction material, though it is one thathas been very extensively used in the past and still

is in many parts of the world. This material is mud,which can be used for building in many ways. It canbe made into mud bricks by being compacted into

moulds and then left to dry in the sun, it can be castlike concrete into formwork, or it can be plastered

onto some type of supporting material such aswattle. Mud bricks were used extensively by the

ancient Egyptians and, most particularly, by thevarious civilisations in early Mesopotamia, an areawhere there was no stone and little timber. Mudbricks are not particularly strong but, if used for

thick walls, can be used to build structures of

considerable height. The walls of Babylon, built byNebuchadnezzar in the sixth century BC, for


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example, were some 10 metres high and were said

to be broad enough for three chariots to driveabreast. The main disadvantage of mud as a

building material is its susceptibility to damage bywater. This is probably why its major use has been

in relatively arid climates. Nevertheless, it can beused in wetter climates if properly protected. Mudwas, for example, once used fairly extensively inthis country for the construction of cottages and

some can still be found standing. Their successdepended on having a good over- hang to the roof;so that rain was thrown well clear of the walls, anda coating of protective lime wash over the outside.

The mud brickwork used in Mesopotamia tended tobe protected either by fired and glazed brick on theouter face or by an imported stone facing. It is a

feature of all masonry that it has minimal tensilestrength and that it therefore cannot be used for

beams, where, as has been seen in the last chapter,large tensions must develop. In the absence oftimber to span large gaps, the arch developed.Another aspect of the successful use of mud is in

the construction of dwellings in arid regions. Here

the thick mud walls of houses, usually lime washedto give a light, sun-reflecting surface, provide a

means of controlling the environment within thedwelling. A problem with arid regions is the verylarge temperature changes that occur during the

course of the day, often being very cold at night andvery hot during the middle of the day. The air

temperature within a building is governed largely bythe temperature of the inner face of the walls. With

mud brick dwellings, such as the traditional housesof the Pueblo Indians, the walls take a very longtime to heat up during the day and a long time tocool down at night. As a consequence, the

temperature of the inner parts of the walls stays

relatively constant and this maintains the interior


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temperature at a relatively constant and

comfortable level.Mud, therefore, can be used successfully to fulfil

most of the construction needs of a society; it canbe used for the construction of massive public works

such as fortifications, temples or palaces, but is alsoa highly versatile material for domestic use. Allthese applications have to recognise the propertiesof mud in the forms of structure that are developed;

mud has reasonable compressive strength,permitting the construction of walls, platforms andarches, but is susceptible to the effects of water,requiring the surfaces to be protected. High thermal

capacity confers advantages in environment control.A major advantage of mud is its ready availabilityand its ease of use.

This lesson has started with the consideration of twomaterials, mud and snow, which most readers

probably would not have considered as structuralmaterials at all yet, if used properly, even theseunconsidered materials can produce highlysuccessful structures. The lesson is that almost any

material can be used structurally if its properties are

properly understood and are used to developappropriate forms of structure. We shall now move

on to consider more commonly recognised,structural materials.


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4.2 StoneIn many areas where people live there is an

abundance of stone and this is an obvious

construction material. It is not, however, without

disadvantages. While forming mud into bricks is aneasy process, needing little specialised equipment or


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expertise, stone will often require quarrying and

cutting and, frequently, transporting some distancefrom a suitable outcrop to the construction site. It is

thus generally a more expensive building material.This is not always the case, of course, and there are

situations where good building stone has beenreadily available in an easily-worked form. Anexample of this is the construction of GreatZimbabwe. Figure 4.1 shows the quality of the dry

stone construction in these mysterious ruins. It isclearly superb. The builders were fortunate that thearea has frequent granite outcrops. This graniteweathers by sloughing off thin sheets of stone that

can easily be broken into suitably sized slabs ofuniform thickness for building. In general, however,except for the poorest quality of rubble masonry,

stone is a substantially more expensive buildingmaterial. To lay stone effectively, it is necessary to

bed the stones in some form of mortar. Historically,various materials were used for this: gypsumplaster in ancient Egypt, natural bitumen inMesopotamia and, most commonly, lime mortar.

Nowadays, the mortar will normally be made using

a mixture of lime and Portland Cement as this setsmuch more quickly than lime mortar. The mortar

should not be seen asglue sticking the stonestogether but rather as a bedding. The structural


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properties of masonry may be considered to be the

same as those of mud brick, but much better.Masonry is very strong in compression, but has

minimal tensile strength. It has the great advantageover mud of being very durable.

FIG-4.2 Pont Du Gard- A Roman AdequateShowing Semicircular Arches

As civilisations developed, so did skills in masonry

construction, and stone masonry became thematerial of choice for prestige construction. The lackof tensile strength means that the most immediatelyobvious use for masonry is for the construction ofwalls and columns. The problem of using the

material to span gaps was solved by thedevelopment of the arch. We have already

mentioned that these were used with mud brickconstruction, but it is really with stone masonry that

the arch came into its own and the most prolificearly developers of arched structures were the

Romans. The semicircular arch is probably the mostcharacteristic feature of Roman architecture (Figure4.2).

Two other developments used extensively by theRomans, though they were not the originators, were


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the vault, where an arch is extended in breadth to

cover a large area, and the dome, where an arch isrotated about a vertical axis through mid-span.

Domes were possibly most highly developed in theeastern part of the Roman Empire in structures such

as Santa Sophia in Istanbul, and then in Muslimarchitecture. Roman masonry architecture remainsvery heavy and the highest levels of masonrydevelopment probably belong to the Gothic period in

European architecture. At least in the view of theauthor of this chapter, the very pinnacle of thetechnical development of stone masonry wasreached in the perpendicular style of architecture

developed in England around 1350 (Figure 4.3).It is probable that modern tools and machinerycould make some economies in masonry

construction over the methods used in thefourteenth century, but it is doubtful if any real

technical advances in design have occurred since

FIG-4.3 York minister


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that time. The structural forms developed by then

were as near a perfect exploitation of a technologyas it is possible to achieve.

4.3 BricksA development parallel to the development of stone

masonry was that of fired brickwork. It has alreadybeen noted that fired brick was used inMesopotamia to form a weatherproof skin to mud

brick construction, but fired bricks could be used as

a strong building material in their own right. Thefiring process makes them more expensive thanmud brick but substantially stronger and moredurable. Brick has never attained the architectural

cachet of stone but its use became common- placein areas of good clay but little stone. Its structural

properties are basically the same as stone masonry

and so the same basic structural forms are used. Itis a dominant material in domestic housing withinthe UK and many parts of Europe.


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4.4 TimberThe disadvantage of masonry (stone or brick) is in

the expense of using it to span gaps. We have seen

that this can be done by arches, vaults and domes,but these are very expensive forms of construction

and are only really viable in structures such asbridges or prestige buildings. The problem witharches was not only the direct expense resulting in

part from the need to build a supporting structure

for the arch or vault during construction, but alsothe construction depth required from the springingof the arch to the top, and the problems withresisting the out- ward forces developed at the

supports of arches. A material that could span bybending was necessary for more normal

construction and, up until the Industrial Revolution;

the only material that met this requirement wastimber.


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Timber is the first material we have discussed that

has good tensile strength as well as compressivestrength, enabling it to resist bending. Also, its

nature as the trunk of a tree meant that it wasavailable in long, relatively thin sections that were

ideal for beams. Timber was thus the ideal material

for spanning medium-sized gaps. It was, and still is,

the preferred material for supporting flat roofs and

floors in small to medium-sized buildings. It couldalso be used for small span bridges (Figure 4.4).


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For larger structures, timber could not be obtained

in the necessary sizes or strengths, but hereanother property of timber could be exploited: it can

be cut and jointed relatively easily, leading to thedevelopment of more complex structures formed

from interconnected smaller members. The resultwas the development of the use of timber forframed structures and trusses. These types ofstructure were very important for the development

of structures using more modern materials, buthave been used in timber for centuries. England andother parts of Europe are particularly rich in timber-framed buildings developed from medieval times

until relatively recently. They are particularlycharacteristic of areas that were forested and wereshort of other materials such as stone. Figure 4.5

shows an example of a fifteenth-century timber-framed structure. The areas between the framing

were infilled with cheaper materials such as wattleand daub (mud or cow dung plastered onto a panelmade of woven twigs). No doubt it was discoveredat a very early stage that the tendency of the

rafters in a pitched roof to spread could be stopped

by providing a tie between them. From this insightit was a short step to the development of the

trussed roof.Trussed roofs were developed fairly early in theMedieval period and evolved over the years into

highly complex forms. Even today, it is notnecessarily clear how these timber roof structures

actually function. The triangulated truss, as weunderstand it, was probably a development of the

Renaissance; in fact Palladio is often credited withthe first clear illustrations of trusses. Timber is thusan ideal material for a truss, though thedevelopment of joint details, which could handle

either tension or compression or both, could lead to

some complexity. Timber trusses are still used veryextensively and are the almost universal form of


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supporting structure for the roofs of domestic

housing. Jointing has, however, become much moresophisticated in recent years.

4.5 Iron and steelIn 1779 Abraham Oarby constructed his famouscast iron bridge at Ironbridge in Shropshire (Figure4.6). This bridge was the public demonstration of a

revolution that was to change the world. Abraham

Oarby had developed a means of using coal for thesmelting of iron and the economic large-scaleproduction of cast iron. This ready availability ofrelatively cheap iron is often considered as one of

the prime factors leading to the IndustrialRevolution. It is interesting that the first

demonstration of this new ability was the

construction of a bridge. The development ofrailways, roads and mills during the early years of

the Industrial Revolution presented designers ofstructures with problems that were not readily

solvable using the traditional materials. Cast iron,wrought iron and, later, steel provided the solutions.Iron and steel have the same basic properties as

timber ii} that they are strong in both tension andcompression. They are both much stronger and


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much stiffer; however, due to the industrial nature

of their production, they pre also much moreexpensive than timber. Iron and steel are also

heavier than timber. Even more than timbertherefore; iron and steel were, from the start, used

in frames and trusses where the material could beused in the most economical way.In the early years of the development of railways, asubstantial number of cast iron bridges were built.

However, cast iron was largely abandoned after thecollapse of the Dee Bridge in 1847. The problemwith cast iron compared with wrought iron or steel isits brittleness, Brittle failures are something that

engineers try to avoid, as they occur withoutwarning and also provide no opportunity for forcesand moments to redistribute to other stronger parts

of a structure when failure of one member isimminent.

Wrought iron, which was a fairly expensive material,dominated the field until close to the end of thenineteenth century. The first major structure builtusing carbon steel was the St Louis Bridge over the

Mississippi, completed in 1874. The first major steel

structure in the UK was the Forth Bridge; completedin 1889. After this, the cheapness and convenience

of rolled steel sections led to the fairly rapiddisplacement of wrought iron. The hot rollingprocess resulted inevitably in the standardisation of

section sizes and shapes. Furthermore, thenecessity for specialised equipment to cut, shape

and drill rolled steel sections led to the membersbeing formed ready for erection in a factory and

then delivered to site ready for erection. Thisresulted in the potential for steel frames to beerected very rapidly. There has also been atendency for the detailed design work to be carried

out by steel fabricators rather than by consulting

engineers.


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A further fundamental development was the

production of sheet and plate steel. Thin sheetmetal was a major cladding material during the

twentieth century, initially in the ubiquitous form of'corrugated iron' and more recently in a variety of

forms. Steel plate allows the fabrication of non-standard elements but, more critically, it can beused to form large box structures, which have beenused with great success in recent years in bridge

design.In summary, in steel we have the first newstructural material to arrive on the constructionscene for, possibly, millennia. It is a highly versatile

material of high strength and stiffness. Themanufacturing process inevitably makes itexpensive and, as a consequence, steel structures

tend to be designed to minimise the quantity ofmaterial used, resulting in its use in frames and

trusses or as thin sheet material strengthened eitherby the addition of stiffeners or by profiling thesheets. The manufacturing process and resultingproperties also leads to the production of

standardised sections, design by specialised

fabricators, and the use of prefabricated elements.

4.6 Reinforced concreteWe shall now consider the second major new

construction material to arrive over the last centuryor so. This is reinforced concrete.Concrete is essentially artificial stone and, as such,

has the same basic properties as stone. Its greatadvantage is that, as a man-made material, it canbe poured into moulds of any shape where it sets,thus removing the necessity to form the material by

carving, as is the case with stone. A furtheradvantage is that its properties may be tailored to a

considerable degree to meet different situations.


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The basic ingredients of concrete are: gravel

(usually stone in the sizes in the range of 5-20mm), sand, Water and cement. The cement is theonly industrially produced ingredient and is used in

relatively small quantities compared with the sand

and gravel (typically about 15% by weight of the

concrete). This makes concrete a very cheapconstruction material. The two basic types of

cement are: hydraulic cements and pozzolans.Pozzolans were the earlier forms of cement and theycan be found naturally as volcanic earths. If mixed

with lime (calcium hydroxide) and water, pozzolansset to form a very effective concrete. Pozzolanic

concrete was used extensively by the Romans:many of their great monuments were built by

constructing a masonry skin and then filling thiswith concrete (the Colosseum, for example, is

largely made this way). The most impressive Romanconcrete building is probably the Pantheon in Rome.This is covered by a concrete dome 143 feet (43

metres) in diameter. This appears to have been castin much the same way as we would today, by


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making a mould (formwork) and then pouring the

concrete and, after hardening, removing theformwork to reveal the concrete surface. The long

life of Roman structures illustrates the inherentdurability of concrete. The second type of cement,

hydraulic cement, reacts when water is added andrequires no lime. The best-known hydraulic cementis Ordinary Portland Cement invented by JosephAspdin in 1811. This is now the most used

commodity on Earth after water. Because hydrauliccements set rather faster than pozzolanic cements,they have largely displaced them; howeverpozzolans are used as replacements for some

Ordinary Portland Cement in mixes for some uses.Though concrete alone has great potential as aconstruction material, it shares one major weakness

with stone. Stone is strong (often very strong) incompression, but, in tension, it is weak and brittle.

If you have ever considered why there are so manycolumns in the Egyptian temple at Abu Simbel orwhy the columns in the Parthenon are so closelyspaced, the reason is that it is impossible to make

long span, reliable, stone beams. As you will

remember high tensile stresses are developedwithin beams and this means that stone beams can

only be short. Concrete has the same problem andcannot be used economically in any situation thatrequires it to resist bending. Wilkinson in England

and Lambotte in France independently and at aboutthe same time (in the 1850s) discovered how to

circumvent this weakness. Wilkinson's 1854 patentfor reinforced concrete explains how the steel ropes

or bars were to be arranged in the formwork so thatthey finished up in the parts of the concretemembers that would be subjected to tension underload. The concrete was thus used to support the

compressive stresses and the steel to carry the

tension. Steel bars are probably the cheapest meansof supporting tension, while concrete is certainly the


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cheapest means of withstanding compressive forces.

Reinforced concrete is thus an example of acomposite material where ideal use is made of the

materials.Despite its invention in the 1850s, reinforced

concrete was not really used to any great extentbefore the early years of the twentieth century.There was significant use in the years 1920 to 1939but it was the Second World War that really led to

the development of reinforced concrete as the pre-eminent structural material. This was mainly due toan extreme shortage of structural steel, whichprobably lasted from the war until the late 1960s.

The result was that the great rebuilding throughoutthe world after the war was mainly done withreinforced concrete. It was probably only in the

1980s and 1990s that structural steel, due to aworldwide overproduction and a consequent major

drop in price, started seriously to regain ground. Inmany, if not most, countries, reinforced concretestill turned out to be a greatly versatile material,able to be handled reasonably competently by a

largely untrained workforce throughout the world. It

is not, however, without its disadvantages. Two maybe particularly mentioned. The first is its

appearance. Concrete is a uniformly grey material,susceptible to staining from the environment, andlarge masses of exposed concrete can look deeply

unattractive. The move to use exposed concrete inthe 1960s led to some truly awful buildings that

have given concrete a bad name that it has yet tolive down. Concrete in buildings is nowadays usually

covered discreetly by cladding. Concrete can, infact, look stunning if designed, detailed and builtcorrectly, but this requires inspired architecture andvery careful construction. The second problem is

with durability. As has been seen, concrete itself is

highly durable and can last for centuries withoutserious degradation. There are some conditions that


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can lead to the degradation of concrete, and these

will be discussed later, but they are relativelyuncommon.

The real problem arises when steel is incorporatedwithin the concrete since there are circumstances

when this steel can corrode. Rust actually occupiesa greater volume than the steel from which it isformed and, as a result, if the reinforcementcorrodes, it tends to force off the surrounding

concrete, leading to disintegration of the surfaceparts of the structure. There is also obviously a

safety problem. Corrosion can be avoided by careful

design and detailing but in the days when reinforcedconcrete construction was booming, the

understanding of the corrosion processes and the

necessity to design to avoid problems were not fullyrealised. Consequently, much money has beenspent in recent years on the repair of corrosion-

damaged reinforced concrete structures.


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Like structural steel, reinforced concrete is a highlyversatile material; it probably comes closest of any

major construction material to being a material that

can be used for any form of structure. This breakingof the linkage between structural form and materialproperties is a major feature of reinforced concrete

that designers may exploit in the development ofeconomical or imaginative structures; more thanwith any other material, the possibilities ofreinforced concrete are limited only by the

designer's imagination.4.7Prestressed concrete

There is a second method of overcoming the

weakness in tension of concrete. The principle maybe seen by considering the problem of trying to lift a


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row of books (Figure 4.8). If we just lift the end

books in the row, we shall lift only those two books.If, however, we provide a compressive force and

'squeeze' the line of books as we try to lift them,then we shall find that the books can be lifted. By

providing an axial compressive stress we haveconverted our line of books into a book beam thatcan carry bending moment. The possibility ofimproving the performance of concrete by providing

a longitudinal stress was recognised in the 1880s,but no practical working system of providing thelongitudinal force developed. The reason was thatconcrete creeps under load. Creep is an increase in

strain with time in a material subjected to constantstress. The effect of the creep is to reduce themagnitude of the longitudinal force with time,

resulting in failure of the beam. In the end, thisproblem was solved by Freyssinet in France in the

1920s after many years of experiment. He realisedthat the creep problem could be over- come byusing high-strength concrete, with steel wires ofvery high strength to provide the force. Two basic

systems of prestressing (as this system of imposing

a longitudinal compression on concrete memberscame to be called) developed: pre-tensioning and

post-tensioning.


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Pre-tensioning is usually a factory process because

it requires the facility to stretch wires and hold themunder tension for some time. High tensile wires are

stretched along the length of the casting floor. Theformwork for the members (the mould) is

constructed around the wires and then filled withconcrete (Figure 4.9a). When the concrete has setand gained sufficient strength, the wires are cut.Since the wires should have become bonded to the

concrete, this transfers the tension to the concrete.The resulting prestressed beam is then transportedto site and erected.Post-tensioning is usually a site process and is used

for larger or more complicated structures. Thestructure is cast with ducts (tubes) set in where theprestressing wires are required. When the concrete

has hardened, high tensile steel wires or cables arethreaded through the ducts and anchored at one

end. A jack is fixed to the wires at the other endand the wires are tensioned (Figure 4.9b). Once thishas been done, anchors are fixed on so that whenthe jack is removed the tension remains in the

wires. The ducts may now be filled with grout (liquid

mortar) to protect the wires and bond them to thebeam, or may be left ungrouted so that, if

necessary, the wires can be removed for inspectionor replacement in the future.The pre-tensioning process lends itself to the


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production of numbers of similar units under factory

conditions. The nature of the process tends tofavour straight members. The elements need to be

small enough to transport from the factory to thesite. Typical pre-tensioned products are: railway

sleepers, standard beams for bridges, and floorplanks for making precast floors for commercial andresidential buildings.Post-tensioning lends itself to use in structurally

much more exciting situations. It is probably mostcommonly used in large bridges. One way it is oftenused is in the construction of segmental bridges.This type of bridge is made up of units that are

precast, usually on or near the site because of theirsize. Each new segment is hoisted into positionagainst a previous segment and then prestressing

tendons are threaded through the unit andconnected on to the previous unit. The tendons are

then tensioned to pull the unit tightly against theprevious units. This is just the same procedure aslifting a pile of books by providing a compressiveforce on the ends.


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Summary :Stability is the ability of a structure to support load

while undergoing limited deformations and

displacements. The limit of deformation ordisplacement which determines if a structure is

stable or not depends on the type of structure (seeRigidity below).The requirement of strength is concerned with the

integrity of the structure and of each of its parts

under any and all possible loads. To this purpose,the structural system is first chosen, and the loadson it are established: the state of stress is thendetermined at significant points of the structure and

compared with the kind and amount of stress thematerial can safely stand. Factors of safety of

varying magnitude are used to take into account

uncertainties in loading conditions and materialpropertiesTwo other developments used extensively by the

Romans, though they were not the originators, werethe vault, where an arch is extended in breadth tocover a large area, and the dome, where an arch isrotated about a vertical axis through mid-span.

Revision Points:The basic ingredients of concrete are: gravel

(usually stone in the sizes in the range of 5-20mm), sand, Water and cement. The cement is the

only industrially produced ingredient and is used inrelatively small quantities compared with the sand

and gravel (typically about 15% by weight of theconcrete). This makes concrete a very cheap

construction material. The two basic types ofcement are: hydraulic cements and pozzolans.Pozzolans were the earlier forms of cement and theycan be found naturally as volcanic earths. If mixed

with lime (calcium hydroxide) and water, pozzolans

set to form a very effective concrete.


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Key Words:Stability - is the ability of a structure to support

load while undergoing limited deformations and

displacements.

Intext questions:1. Explain in detail the following structural

materials:

a. Stoneb. Mudc. Bricksd. Timbere. Iron and steelf. Reinforced concrete

Terminal Exercises:

1. What do you understand by the termequilibrium? Why is it required in a structure?

2. What is stability? Explain in detail the differenttypes of stability.

3. Why does a structure require strength? Explainby giving examples.

4. When is a structure considered to be functional?Explain in detail by giving examples.

5. How does economy effect structures? Explain6. How important is aesthetic for a structure?7. What is the relationship between structural forms

and materials?

Assignments / Learning Activities:An assignment to be prepared by the students onhow the material governs the structure and thechanging era of designing structures according tothe materials.

All the materials to be discussed one by one statingadvantages and disadvantages and drawbacks ofeach one over the other.


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Supplementary Material / Suggested

Reading:

1.Time Savers Standards for Building Types2. Structure in Architecture by Salvadori and Heller


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Unit III

Basic States of StressLesson-5: Basic States of Stress


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Lesson 5: Basic States of

Stress

Objective:To study the basic states of stress involved in astructure.

Structure:

5.1 Simple Tension5.2 Simple Compression5.3 Types of buckling

5.3.1 General buckling5.3.2 Local buckling

5.4 Simple Shear

5.5 Simple Bending

IntroductionStructures deform whenever loaded. Although thesedeformations can seldom be seen by the naked eye,the corresponding stresses have measurable values.Stress patterns may be quite complex; each,

however, consists at most of only three basic states

of stress: tension, compression, and shear.

5.1 Simple TensionTension is the state of stress in which the particles

of the material tend to be pulled apart. The steelcables lifting or lowering an elevator have their

particles pulled apart by the weight of the elevator.

Under the pull of the weight the cables becomelonger: lengthening is typical of tension. Theelongation of a unit length of cable is called its

tensile strain.Provided the material is not stressed beyond itselastic range the lengthening of the cable dependsonly on its cross section, its length, and the load.

The larger the diameter of the cable, the smaller theunit elongation: the tensile strain is proportional to


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the load carried by each unit area of the cable

cross-section, or the tensile stress in the cable. Theratio of tensile stress to tensile strain is a

characteristic of the material called its elasticmodulus in tension.

Certain materials, such as concrete, may be easilytorn apart by tension; others, such as steel, arevery strong in tension. A high-strength steel cable,one square inch in area (1.2 inches in diameter) can

safely carry a load of 100.000 pounds, and willbreak only under a load of 200,000 pounds or more.A cable of aluminium alloy, with the tensile strengthof steel and a unit weight one-third that of steel,

could be three times as long: it could hang for 15.34miles. Because it would be three times as long and,moreover, because aluminium stretches under

tension three times as much, the aluminium cablewould stretch nine times as much as the steel cable.

Elongation is the most important, but not the only,deformation accompanying simple tension. Carefulmeasurements of the cable before and after theapplication of the load show that as the load

increases and the cable elongates, its diameter

decreases.5.2 Simple CompressionCompression is the state of stress in which theparticles of the material are pushed one against the


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other. A column supporting a weight is under

compress

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353-23 Principles and Concept of Structures