Civil Engineering Reference
In-Depth Information
Figure 5.11
Cable arrangements
It would be possible to defi ne the locus of the centroid of the prestressing cables as
a parabola and then to arrange that each cable follows fl atter or steeper versions of this
curve. Although this is an option frequently adopted, it is not a very practical idea. The
curve will inevitably be very fl at near the centre of the beam, and every cable support
will be at a slightly different level. It is much better practice to adopt a pragmatic cable
profi le, where each cable stays parallel to the beam soffi t for a length near mid-span,
and then peels off as required, as shown in Figure 5.11 (a and b). The overall centroid
of the cable group may still be arranged to lie close to a parabola, if necessary. A simple
cable profi le, used on the Sungai Kelantan Bridge, is shown in Figure 5.12.
As the strands or wires making up the prestress tendon are usually pushed or
pulled into the empty ducts, these have to be a relatively loose fi t around the tendon.
Generally, the cross section of the duct is approximately twice the area of steel of the
