STRUCTURAL DESIGN FOR ARCHITECTURE

small-diameter wires of high-tensile steel, are
stretched to a predetermined tensile stress in a
pre-stressing bed (Fig. 4.24). The fresh concrete
is then placed around these and allowed to set
and harden. The pre-tension is then released
and the friction (bond) between the concrete
and the surface of the reinforcement transfers
a permanent compressive load into the
concrete. Pre-tensioned elements are normally
precast and the technique is commonly used
for producing a range of proprietary compon-
ents such as lintels, beams of various kinds,
and floor slabs. These are used in a number of
different types of structure. The lintels are
normally used in conjunction with masonry.

Pre-stressed floor slab units are used in many

types of structure including steel and

reinforced concrete frames and masonry

loadbearing-wall structures.

In post-tensioned pre-stressed concrete,

which is usually cast in situ, the concrete is

placed into moulds which contain narrow

tubes (Fig. 4.25). After the concrete has set and

hardened the reinforcement is placed in these

tubes and tensioned against plates or jacks

which are permanently fixed to the ends of the

elements. This has the effect of introducing a

permanent compressive pre-stress.

Pre-stressing allows much higher stresses to

be used in the reinforcement in concrete and 121

Fig. 4.25 Post-tensioned

pre-stressed concrete. Where

concrete is post-tensioned,

ducts for pre-stressing

tendons are cast into the

elements concerned. The

tendons are fed into the

ducts once the concrete has

hardened and then stretched

by external jacking devices.

The resulting pre-stress is

then introduced to the

concrete via special anchor-

ing devices at the ends of the

element.

Fig. 4.24 Pre-tensioned

pre-stressed concrete. In pre-

tensioned pre-stressed

concrete the pre-stressing

wires are stretched in a pre-

stressing bed prior to the

placing of the concrete. The

wires are released from the

pre-stressing bed once the

concrete has hardened and

the pre-stress is transferred

to the concrete by frictional

(bond) stresses acting

between the concrete and

the surfaces of the wires.

Reinforced concrete structures