Civil Engineering Reference
In-Depth Information
hand, the initial stress in a component may be controlled as in a bolted connection;
the subsequent applied load may or may not affect the initial stress in the bolt.
Initial stresses may be deliberately induced in a structural member so that the adverse
effects of an applied load are minimized. In this the category is the prestressing of
beams fabricated from concrete which is particularly weak in tension. An overall state
of compression is induced in the concrete so that tensile stresses due to applied loads
merely reduce the level of compressive stress in the concrete rather than cause tension.
Two methods of prestressing are employed, pre- and post-tensioning. In the former
the prestressing tendons are positioned in themould before the concrete is poured and
loaded to the required level of tensile stress. After the concrete has set, the tendons
are released and the tensile load in the tendons is transmitted, as a compressive load,
to the concrete. In a post-tensioned beam, metal tubes or conduits are located in the
mould at points where reinforcement is required, the concrete is poured and allowed
to set. The reinforcing tendons are then passed through the conduits, tensioned and
finally attached to end plates which transmit the tendon tensile load, as a compressive
load, to the concrete.
Usually the reinforcement in a concrete beam supporting vertical shear loads is placed
closer to either the upper or the lower surface since such a loading system induces ten-
sion in one part of the beam and compression in the other; clearly the reinforcement is
placed in the tension zone. To demonstrate the basic principle, however, we shall inves-
tigate the case of a post-tensioned beam containing one axially loaded prestressing
tendon.
Suppose that the initial load in the prestressing tendon in the concrete beam shown
in Fig. 7.21 is
F
. In the absence of an applied load the resultant load at any section of
the beam is zero so that the load in the concrete is also
F
but compressive. If now a
tensile load,
P
, is applied to the beam, the tensile load in the prestressing tendon will
increase by an amount
P
T
while the compressive load in the concrete will decrease
by an amount
P
C
. From a consideration of equilibrium
P
T
+
P
C
=
P
(7.52)
Furthermore, the total tensile load in the tendon is
F
+
P
T
while the total compressive
load in the concrete is
F
−
P
C
.
Concrete,
cross-sectional area,
A
C
Prestressing tendon,
cross-sectional area,
A
T
End plates
Applied load,
P
P
F
IGURE
7.21
Prestressed
concrete beam
L
