Prestressed Concrete - Design of Reinforced Concrete - page 600

so the two parts will act together as a unit, they are called composite sections . Examples
of such members were previously shown in parts (e) and (f) of Figure 19.8. In composite
construction, the parts that are difficult to form and that contain most of the reinforcing
are precast, whereas the slabs and perhaps the top of the beams, which are relatively easy
to form, are cast in place.
The precast sections are normally designed to support their own weights plus the
green cast-in-place concrete in the slabs plus any other loads applied during construc-
tion. The dead and live loads applied after the slab hardens are supported by the com-
posite section. The combination of the two parts will yield a composite section that has
a very large moment of inertia and thus a very large resisting moment. It is usually
quite economical to use (a) a precast prestressed beam made with a high-strength con-
crete and (b) a slab made with an ordinary grade of concrete. If this practice is fol-
lowed, it will be necessary to account for the different moduli of elasticity of the two
materials in calculating the composite properties (thus it becomes a transformed area
problem).
Continuous Members
Continuous prestressed sections may be cast in place completely with their tendons run-
ning continuously from one end to the other. It should be realized for such members that
where the service loads tend to cause positive moments, the tendons should produce nega-
tive moments and vice versa. This means that the tendons should be below the member's
center of gravity in normally positive moment regions and above the center of gravity in
normally negative moment regions. To produce the desired stress distributions, it is possi-
ble to use curved tendons and members of constant cross section or straight tendons with
members of variable cross section. In Figure 19.14 several continuous beams of these
types are shown.
Another type of continuous section that has been used very successfully in the United
States, particularly for bridge construction, involves the use of precast prestressed mem-
bers made into continuous sections with cast-in-place concrete and regular reinforcing
steel. Figure 19.14(d) shows such a case. For such construction the precast section resists
a portion of the dead load, while the live load and the dead load that is applied after the
cast-in-place concrete hardens are resisted by the continuous member.
Partial Prestressing
During the early days of prestressed concrete, the objective of the designer was to propor-
tion members that could never be subject to tension when service loads were applied.
Such members are said to be fully prestressed . Subsequent investigations of fully pre-
stressed members have shown that they often have an appreciable amount of extra
strength. As a result, many designers now believe that certain amounts of tensile stresses
can be permitted under service loads. Members that are permitted to have some tensile
stresses are said to be partially prestressed .
A major advantage of a partially prestressed beam is a decrease in camber. This is
particularly important when the beam load or the dead load is quite low compared to the
total design load.