283757893275

Structural Concrete Frames 359
cantilever, assume it is made of rubber. Under load, the rubber will bend as illustrated in
Figure 6.7. The top surface of the rubber will stretch and the bottom surface will be compressed,
indicating tensile stress in the top and compressive stress in the bottom. Under
load, a reinforced concrete cantilever will suffer similar but less obvious bending with the
main reinforcement in its top, as illustrated in Figure 6.7, with the necessary cover of reinforcement.
Under appreciable load, shear reinforcement will also be necessary close to the
point of support.
Columns
Columns are designed to support the loads of roofs, floors and walls. If all these loads acted
concentrically on the section of the column, then it would suffer only compressive stress
and it would be sufficient to construct the column of either concrete by itself or of reinforced
concrete to reduce the required section area. In practice, the loads of floor and roof
beams, and walls and wind pressure, act eccentrically, i.e. off the centre of the section of
columns, and so cause some bending and tensile stress in columns. The steel reinforcement
in columns is designed primarily to sustain compressive stress to reinforce the compressive
strength of concrete, but also to reinforce the poor tensile strength of concrete against
tensile stress due to bending from fixed end beams, eccentric loading and wind pressure.
Mild steel reinforcement
The cheapest and most commonly used reinforcement is round section mild steel rods of
diameter from 6 to 40 mm. These rods are manufactured in long lengths and can be quickly
cut and easily bent without damage. The disadvantages of ordinary mild steel reinforcement
are that if the steel is stressed up to its yield point, it suffers permanent elongation; if
exposed to moisture, it progressively corrodes; and on exposure to the heat generated by
fires, it loses strength.
In tension, mild steel suffers elastic elongation, which is proportional to stress up to the
yield stress, and it returns to its former length once stress is removed. At yield stress point,
mild steel suffers permanent elongation and then, with further increase in stress again,
suffers elastic elongation. If the permanent elongation of mild steel which occurs at yield
stress were to occur in reinforcement in reinforced concrete, the loss of bond between the
steel and the concrete and consequent cracking of concrete around reinforcement would
be so pronounced as to seriously affect the strength of the member. For this reason,
maximum likely stresses in mild steel reinforcement are kept to a figure some two-thirds
below yield stress. In consequence the mild steel reinforcement is working at stresses well
below its ultimate strength.
Cold worked steel reinforcement
If mild steel bars are stressed up to yield point and permanent plastic elongation takes place
and the stress is then released, subsequent stressing up to and beyond the former yield
stress will not cause a repetition of the initial permanent elongation at yield stress. This
change of behaviour is said to be due to a reorientation of the steel crystals during the
initial stress at yield point. In the design of reinforced concrete members, using this type
of reinforcement, maximum stress need not be limited to a figure below yield stress, to