STRUCTURAL DESIGN FOR ARCHITECTURE

Fig. 4.22 Cracking of the concrete, shown exaggerated

here, is inevitable in the parts of reinforced concrete

elements in which tensile stress occurs. The width of the

cracks which form must be controlled to prevent the

ingress of water, which would cause corrosion of the

reinforcement. This places a limitation on the amount of

strain, and therefore stress, which can be tolerated in the

reinforcement. Allowable stresses in reinforcement are

normally lower than the strength of steel would in other

circumstances allow.

which carry bending-type load the reinforce-

ment is concentrated in locations where

stretching, and therefore, tensile stress, occurs

(Fig. 4.21).

The tensile internal forces which are present

in reinforced concrete as a result of bending

action, and which are resisted by the reinforce-

ment, cause tensile strain^8 to occur. This

produces cracking in the concrete surrounding

the reinforcement which affects the appearance

and durability of the elements (Fig. 4.22). The

extent of the cracking must be limited by

suitable design and is usually controlled by

restricting the amount of stress which is

permitted in the reinforcement. This limits the

amount of stretching which can occur and

therefore the width of any cracks which form

but it has the disadvantage of requiring that

the steel be greatly understressed and there-

fore inefficiently used.

Pre-stressing of concrete allows the full

potential strength of the steel reinforcement to

be realised. In pre-stressed concrete, crack

widths are limited by deliberately introducing

an axial compressive load into the concrete so

8 For definitions of stress and strain see Macdonald,

Structure and Architecture, Appendix 2.

Fig. 4.23 The principle of pre-stressed concrete. In the

top diagram the steel bar is tensioned by the bolts at the

ends of the beam and introduces a compressive 'pre-

stress' into the concrete which is uniformly distributed

within each cross-section. This neutralises the tensile

bending stress which occurs in the lower half of the beam

when the load is applied and thus eliminates tensile crack-

ing of the concrete. A quite different relationship between

steel and concrete exists here than occurs in plain

reinforced concrete.

as to limit the amount of stretching which can

develop when a bending-type load is applied.

The pre-stressing is carried out by stretching

the reinforcement during the construction of

an element and then connecting it to the

concrete in such a way that it imparts a per-

manent compressive stress into the concrete

(Fig. 4.23). If a load occurs on the structure

which tends to cause tension to develop, the

compressive pre-stress must first be overcome

before any stretching can occur. The pre-stress-

ing technique thus uses the concrete-steel

combination in such a way that the full tensile

strength of the steel is utilised without the

penalty of excessive stretching and therefore of

cracking of the concrete being incurred.

Pre-stressed concrete is either 'pre-

tensioned' or 'post-tensioned'. In pre-tension-

120 ing the reinforcing bars, which are usually

Structural Design for Architecture