Civil Engineering Reference
In-Depth Information
tendon, Figure 5.13 (a). Consequently, when the duct is curved, the tendon will bunch
up to one side. The design of the prestress should take into account the position of the
centroid of the bundle of strands or wires within the duct, Figure 5.13 (b), rather than
the axis of the duct itself.
Reference [2] is useful for all aspects of prestress technology.
5.16.2 Stopping off cables
In order to save prestressing steel, it is possible to reduce the prestress force in zones
of the beam where bending moments are lower. For instance, the maximum bending
moment at the quarter point of the beam is 13.29 MNm, down from 16.51 MNm
at mid-span. Rather than raising the prestressing cables, they could have been kept
at their maximum eccentricity and the force reduced. For a stress limit of zero on
the bottom fi bre, the prestress lever arm would be the same as at mid-span, that is
1.76 m. The prestress force required at the quarter point would thus be 7.73 MN,
corresponding to 77 strands.
Practically, with the cable arrangement shown in Figure 5.11 (b) it would be
possible to reduce the number of tendons in the heel from 8 to 7. Further cables may
be stopped off as the bending moment falls. Cables 1 and 2 are shown swept up and
anchored in pockets in the top fl ange of the beam. Although reducing the total weight
Figure 5.14
Arrangement of swept up anchors
