STRUCTURAL DESIGN FOR ARCHITECTURE

Fig. 4.34 Shear failure of a reinforced concrete beam.
The beam depicted in Fig. 4.33 eventually fails due to the
formation of an inclined crack at the end of the beam
which is not effectively crossed by the reinforcement. This
type of failure is called a shear failure.

stress. This is, however, difficult to carry out in

practice and the solution which is normally

adopted is either to bend up the main

reinforcement bars in straight sections towards

the ends of the beam (Fig. 4.35a) or to form a

cage of reinforcement with links of steel (Fig.

4.35b). Both of these methods ensure that

every potential tensile crack in the concrete is

crossed by some reinforcing bars.

Figure 4.36 shows diagrammatically a typical

pattern of reinforcement for a reinforced

concrete beam. This consists of primary

reinforcement in the tensile half of the beam to

carry the tensile component of the bending

stress, and shear reinforcement to carry the

shear load caused by the inclination of the

principal stresses. Secondary reinforcement is

also normally provided (in this case longitudi-

nal bars in the top half of the beam) to hold

the primary reinforcement in position while the

concrete is being placed around it. This

secondary reinforcement also helps to control

shrinkage and to carry secondary tensile stress

which could occur, for example, due to thermal

movement.

The behaviour described above, by which

bending-type load is resisted by the composite

action of concrete and steel, illustrates the way

in which reinforced concrete functions. In large

complex structures, such as multi-storey

frameworks, the direction of bending within an

individual beam or slab may vary due to the

combined effects of structural continuity and

load configuration. The distribution of the

main reinforcement within the structure must

Fig. 4.35 Beam with shear reinforcement. Bent up bars

or links prevent shear failure by ensuring that all possible

tensile cracks are effectively crossed by reinforcement.

be such as to allow the effective resistance of

all tension which occurs within the concrete.

The pattern of reinforcement in real structures

is therefore often considerably more complex

than that shown in Fig. 4.36.

Fig. 4.36 The pattern of reinforcement in a fully

reinforced concrete beam.^129

Reinforced concrete structures

(a)

Shear reinforcement

Small-diameter bars to hold

shear reinforcement in position

Tensile reinforcement

resisting negative bending

moment at support

Tensile reinforcement

resisting positive bending

moment at mid-span