Lecture 2 - Structures Basics

8/3/2019 Lecture 2 - Structures Basics

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STRUCTURAL AND ARCHITECTURE

REQUIREMENTS

Vitruvius defined the 3 basic components of architecture as:

1 ± FIRMITAS (firmness)

2 ± UTILITAS (commodity)

3 ± VENUSTAS (delight)

Firmness is the most basic quality. It is concerned with the ability of the building to preserve it¶s physical integrity and survive inthe world as a physical object. The part of the building, which satisfies the need for firmness is the structure. Structure is

fundamental, without structure there is no building and therefore no commodity; without structure there can be no delight.



STRUCTURAL REQUIREMENTS

REQUIREMENTS

To perform its function of supporting a building in response to whatever loads may be applied to it, a structure must possess 4

properties:

1 ± EQUILLIBRIUM

2 ± GEOMETRIC STABILITY

3 ± STRENGTH

4 ± RIGIDITY



STRUCTURAL TECHNOLOGY

EQUILLIBRIUM

EQUILLIBRIUM

The internal configuration of the structure together with the means by which it is

connected to its foundations must be such that all applied loads are balanced exactly by

reactions generated at its foundations.




EQUILLIBRIUM - FORCES

EQUILLIBRIUM

For a structure to maintain equillibrium, it must resist all the applied forces with equal opposite forces

ACTIVE FORCE ACTIVE FORCE

REACTION FORCE

BEAM ON END SUPPORTS SLAB ON CONTINUOUS SUPPORT

LOADS

The forces which act on a building are termed loads. Loads can be classified as:

1. DISTRIBUTED OR UNIFORM LOADS

Loads which are applied over the full area or length of a structural member

2. CONCENTRATED OR POINT LOADS

Loads which are concentrated at one point over a structural area




EQUILLIBRIUM - FORCES

LOADS

There are 3 types of load which acts on structures.

1. DEAD LOADS

These are permanent loads made up of the selfweight of the

building fabric.

2. LIVE OR SUPERIMPOSED LOADS

Temporary loads in the fact that they are not always present.

Live loads can be loads caused by snow, weight of occupants,

furniture etc.

3. WIND LOADS These can be lateral or horizontal loads caused by prevailing

wind. These loads will affect the geometric stability of the

structure.

Dead and live loads in commercial buildings are often approximately equal. For normal office loadings, dead and live loads are roughly equal in

proportion but higher live load allowances will be necessary in the areas of plant to accommodate special requirements such as storage or heavy

equipment. The optimum structural solution is to locate any heavier loadings close to columns or where the floor spans are shorter.




GEOMETRIC STABILITY

GEOMETRIC STABILITY

Geometric stability is the property, which preserves the

geometry of a structure and allows its elements to acttogether to resist load. In other words, the ability of a

structure to return to its natural state. A stable system

reverts to its original state following a slight disturbance,

whilst unstable systems progress in an entirely new

state




STRENGTH AND RIGIDITY

STRENGTH AND RIGIDITY

The application of loads to a structure generates internal forces in the elements and external reacting forces at the foundations. The

elements and the foundations must have sufficient strength and rigidity to resist these forces. This is chiefly a matter of providing

elements with cross sections of adequate size, given the strength of the constituent material. The determination of sizes and rigidity

are formulated by structural calculations.



4. SHEAR IT (SHEAR FORCE)

Cause one part of the member to slide past

another part


STRESS

APPLIED FORCES

The types of internal forces the structural members have to resist can be classified as:

A. STRESS

A force on a structural member may:

2. COMPRESS IT (COMPRESSIVE FORCE)

1. STRETCH IT (TENSILE FORCE) 3. TWIST IT (TORSIONAL FORCE)

Cause the member to twist




STRESS

A. STRESS

The effect of these various types of force is to put the material of a structural member into a state of stress and the material of the member is then said to

be in a state of compression, tension, torsion or shear.

The material in the structural member must therefore be able to resist the forces which are being applied on to it:

If a tension force is applied the material wants to lengthen, to resist this the material must thus exert and inward pull or reaction.

If a compression force is applied the material wants to shorten. Therefore, under a compression force the material must exert an outward push.

If a force is applied which wants to cause a sliding of one part to another, the material must exert an opposite an equal force in the direction of the shear

force.

MEASURING STRESS

The measure of the intensity of loading is expressed as a load or force per unit area.

STRESS (F) = W / A

W = THE APPLIED LOAD IN KN

A = THE CROSS SECTIONAL AREA.



A. STRESS

As with the loads causing them, the stresses may be tensile, compressive or shear stresses.


STRESS

B. BENDING STRESS

Compression and tension stresses at right angles to the direction of the load are called bending stresses

A. DIRECT STRESS

1. When the stresses are caused by axial loads stretching or compressing a member in the direction of the load, they are called direct stresses.



B. STRAIN


STRAIN

MEASURING STRAIN

The deformation or dimensiovnal change in a member per unit of length which occurs under load is found by dividing the change in length by the

original length.

STRAIN = d / l

d = THE CHANGE IN LENGTH OR DISTORTION

l = THE ORIGINAL LENGTH

MODULUS OF ELASTICITY

Within certain limits of loading it is assumed stress is proportional to strain . This assumption is only applicable within the ³elastic limit´ of the mater ial. In

other words stress is pr opor tion to strain only within the range of loading in which the mater ial retur ns to its or iginal for m after the removal of the load.

The ratio of stress /strain is constant for any given mater ial and is known as its MODULUS OF ELASTICITY (E)

WHAT IS A MATERIALS MODULUS OF ELASTICITY AND WHAT DOES IT MEAN

The modulus of elasticity is a pr oper ty of a mater ial and is basically a measure of its stiffness.

The higher the E value the stiff er the mater ial is and the lar ger the stress necessary to pr oduce a given strain.

Strain is the geometrical expression of deformation caused by the action of stress on a physical body

Strain is calculated by first assuming a change between two body states: the beginning state and the final state. Then the difference in placement of two

points in this body in those two states expresses the numerical value of strain. Strain therefore expresses itself as a change in size and/or shape



C. MOMENTS

In certain circumstances a force can cause turning or rotation.

W

THE TENDENCY TO ROTATE DEPENDS ON THE FOLLOWING:

1. THE MAGNITUDE OF THE FORCE = W

2. THE PERPENDICULAR DISTANCE BETWEEN ITS LINE OF ACTION AND THE POINT OF ROTATION = L (LEVER ARM)

L

DEFINITION OF A MOMENT

The measure of the rotational ef fect is given by the product of these two factors, and is known as the MOMENT of the force ABOUT A POINT.

The moment is expressed in units of force and distance I.e Nm (Newton meters)


MOMENTS

The greater the lever arm the less the force required to rotate a given structure

The greater the force the smaller the length of the lever arm to rotate a given structure



C. MOMENTS AND EQUILLIBRIUM

To avoid the rotation from ocurring one must achieve equillibrium. (no rotation)

EQUILLIBRIUM IS OBTAINED BY THE FOLLOWING:

THE CLOCKWISE MOMENTS (+) ARE BALANCED BY ANTI-CLOCKWISE MOMENTS (-)

wL


MOMENTS

lW

wL = Wl or wL ± Wl = 0

wW

l L

ANTI-CLOCKWISE MOMENTS (-) CLOCKWISE MOMENTS (-)




BENDING MOMENTS

BENDING MOMENTS

When the tendency to rotate is resisted, bending occurs.

wW

l L

ANTI-CLOCKWISE MOMENTS (-) CLOCKWISE MOMENTS (-)

W W

ROTATION RESITED

A MOMENT WHOSE ROTATIONAL EFFECT IS RESISTED, IS CALLED A BENDING MOMENT (BM)

BM = wL ± Wl




FORCES ACTING ON BUILDINGS



FORCES ACTING ON A BUILDING AND THEIR EFFECTS

The forces acting on a building may be considered as:

1 - FORCES HAVING AN OVERALL EFFECT ON THE BUILDING OR STRUCTURE WHICH TEND TO MOVE IT AS A WHOLE

2 - FORCES WHICH EFFECT THE BUILDINGS PARTS WHICH TEND TO DEFORM THEM BUT NOT MOVE THEM OUT OF POSITION


APPLIED FORCES

FORCES AFFECTING OVERALL MOVEMENT

1. VERTICAL FORCES - DOWNWARD

Vertical downward forces caused by the dead weight of the

building and the applied live loads tend to force it down into the

soil.

The soil must be sufficiently strong to exert an upward force or

reaction equal to the weight of the building and live loading.

If this does not occur then settlement will occur.

If the soil is weak in certain sectors then differential settlement will

occur.

2. VERTICAL FORCES - UPWARD

Vertical active forces may also be upward as in the upward

suctrion caused by wind passing by a roof.

These forces tend to raise the roof and its structure.

It is therefore essential that the structure is correctly and

adequately secured as well as that the roof structure has sufficient

weight to prevent it from being lifted




APPLIED FORCES


3. HORIZONTAL FORCES

Horizontal forces, which may be exerted by wind or soil against

the side of a wall or building, tend to make it slideon its base or overturn.

The tendency to slide must be resisted by the friction between the

base and the soil on which the structure rests OR by the passive

pressure of the soil on the opposite side.

The tendency to overturn must be resisted by the weight of the

struture itself, by a strut or by a suitabl tension element of which

would cause a counter moment.




APPLIED FORCES


4. OBLIQUE FORCES

Oblique forces have an effect similar to that of horizontal forces.

The diagram illustrates some circumstances in which obliqueforces are generated and their effects.

The angle of the force will largely determine the type of effect. The

smaller the angle of inclination (closer to horizontal) the greater

the tendency for sliding and overturning to occur.

Methods similar in principle to those for horizontal forces are

adopted to maintain the equilibrium of structures under oblique

loading.



FORCES AFFECTING DEFORMATION

1. VERTICAL FORCES

VERTICAL FORCES ON THIN WALLS OR COLUMNS

Under a vertical load a column may crush or buckle.

How it behaves depends primarily upon:

1. The material of which it is made

2. The shape of the column

3. The slenderness of the column

4. The relation of its thickness to its height. If the height is small

relative to its thickness then it will remain stable until it is

crushed. If height is great relative to thickness then it will

become unstable due to buckling.

BUCKLING

If the column is too slender or it is too thin in relation to its height

then the vertical forces tend to make them bend. This is termed

buckling.

A column may buckle in any direction under a vertical load and

this has a significant effect upon which form it may take.

CRUSHING

If a column is overstressed and the safe compressive strength of the

material is exceeded then the column may be CRUSHED.

The effect of vertical forces and related column behaviour will be

tackled later once we have dealt with framed structures.


APPLIED FORCES



1. VERTICAL FORCES

VERTICAL FORCES ON WALLS

Under a vertical load a wall may crush, buckle or settle.



2. The thickness of the wall

3. The type of loading exerted

4. The foundations upon which it rests

5. The nature and quality of the soil

6. The construction of the wall, I.e. cast or modular

CRUSHING

If a wall is overstressed and the safe compressive strength of the

material is exceeded then the wall may be CRUSHED.

This can be avoided by ensuring adequate thickness in all points of

the walls and by reducing ECCENTRIC LOADING.

BUCKLING

This will occur when the thickness of the wall is small relative to its

height. Refer to SABS requirements

SETTLEMENT

The downward force of a wall must be resited by an equal, upward

reaction from the soil on which it rests. All soils are subject to

consolidation however, strong soils resist high stresses with little

consolidation whilst the same stresses would cause excessive

consolidation in weak soils. This consolidation causes a vertical

downward meovement of the wall which is known as SETTLEMENT


APPLIED FORCES



1. VERTICAL FORCES

VERTICAL FORCES ON BEAMS

Under a vertical load a beam may bend, deflect(buckle) or shear (crack)



2. The shape of the beam

3. The ratio of span to depth (thickness)

4. The means of support I.e simply supported or cantilevered

5. The method of connection

BENDING

This occurs within cantilvered beams. If the cantilever is firmly fixed to a

solid support so that rotation could not occur, then under load it would tend

to bend. The beam would tend to pull away from the support at the top and

push in at the bottom to maintain equillibrium.

The resistance and various behaviours of beams will be dealt with later.

DEFLECT

With the application of a load the beam will tend to deflect.

The amount that the beam will deflect will depend on the span, load,

stiffness, shape and size of the beam. The material and its elasticity also

plays a crucial part.

SHEAR (CRACK)

The application of a load may cause one part of the beam to slide past

another part or induce bending which will cause horizontal shear.


APPLIED FORCES



FORCES AFFECTING DEFORMATION


APPLIED FORCES

1. HORIZONTAL FORCES

HORIZONTAL FORCES ON WALLS

Under horizontal loading a wall may tend to slide or overturn.

SLIDING

Sliding is more likely to occur in a free-standing retaining wall than in a

wall forming part of a building which can provide the weight necessary to

assist stability.

To prevent sliding, friction and the passive pressure of the soil on which

the wall rests are the key concerns.

The frictional resistance existing between the base of the wall and the soil

depends upon the weight exerted on the soil and also upon the degree of

smoothness of the surfaces. Thus by increasing the frictional resistance

and the roughness one can effectively counteract sliding.

Frictional resistance may be increased by increasing the weight of the wall

by manipulating its height or thickness.

When sufficient frictional resistance cannot be achieved, then the next

step is to address the passive soil pressure. This may necessitate that the

wall is taken deeper into the soil.

OVERTURNING

Overturning may be caused by rotation or by settlement.

ROTATION

Overturning by rotation occurs when the counter-moment of the wall is too

small to resist the applied moment. This can be remedied by:

1. Increasing the weight of the wall by increasing its height or

thickness. The latter is the more beneficial because it also

increases the width of the base.

2. Change the shape of the wall. The wall may be made trapezoidal

to shift its centre of gravity relative to the base towards the

overturning force, thus reducing the eccentriciity.

3. Introduce buttress walls.

4. In tall walls you can increase tensile strength by changing the

material I.e. reinforced concrete.

SETTLEMENT

Overturning due to settlement may occur through overstressing of

the soil causing excessive consolidation under the soil.

This can be overcome by reducing the eccentricity of the resultant by:

1. Increasing the thickness of the wall

2. Increase the width of the foundation

3. Make the wall trapezoidal in shape




APPLIED FORCES

ECCENTRIC LOADING

CENTRE OF GRAVITY

Any object or body can be regarded as being composed of innumerable particles all acting downwards due to the force of gravity.

For practical purposes, it is often useful to regard the weight of the body as acting as a single force through one point.

This point is known as the CENTRE OF GRAVITY.

It will not always fall within the material of the body itself. The centre of a frame, for example, will be located in the space within it.



ECCENTRIC LOADING

Loading which is not applied other than through the centre of gravity is known as ECCENTRIC LOADING.

This type of loading can have various effects on structural members and should largely be avoided.


APPLIED FORCES

ECCENTRIC LOADINGS ON WALLS

Loading applied other than through the centre of gravity of the wall has the

effect of increasing the compressive stress in the wall on the loaded side

and decreasing it on the opposite side. This tends to cause bending in the

wall irrespective of its thickness.

In effect compression is caused on one side of the axis of the wall and

tension on the other side.

This loading can result in the following:

1. The increased compressive stress could become greater than the

safe compressive strength of the walling material.

2. If the eccentricity is too great then tensile forces will be set up in

the opposite unloaded side



ECCENTRIC LOADINGS ON COLUMNS

The application of a load eccentrically on the column has exactly the same

effect on the column as on a wall. The overall stress is reduced on the one

side and increased on the other due to the moment created.

In columns, however, the eccentricity of load is often much greater than in

the case of walls because beams are commonly fixed to a side of a

column so that the point of application of load is at a greater distance from

the column axis


APPLIED FORCES




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Lecture 2 - Structures Basics

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