Prestressing for continuous beams - The Design of Prestressed Concrete Bridges - page 142

Figure 6.4 Cable transformations
6.7 Cable transformations
Consider the continuous beam stressed with a cable anchored at the ends of the side
spans, as shown in Figure 6.4 (a). If the eccentricity of the cable along the beam
is changed linearly but the point of attachment of the cables remains unaltered,
Figure 6.4 (b), the total prestress moment Pe + M p at any point remains unchanged.
The change in M p exactly compensates the change in Pe .
6.8 Control of prestress parasitic moments
As we have seen in 5.14 , the width of the cable zone tends to be close to zero at mid-
span of a simply supported beam, but widens out in areas of lower bending moment. In
a continuous beam, the greatest moments exist close to mid-span and over the supports,
and the prestress force is sized for these critical locations where consequently the cable
zone is very narrow. At all other locations the prestress force tends to be greater than
the minimum necessary, and consequently the cable zone is wider. This means that the
designer has some latitude in where he draws the cable within the zone, Figure 6.5.
By adjusting the position of the centroid of the prestressing force in the non-critical
locations, the designer has a degree of control over the size of M p . The lower the