Civil Engineering Reference
In-Depth Information
permanent prestress, Figure 14.7. It is necessary to plan the temporary anchorage
blisters carefully if the webs of the box change in thickness, or if the bridge deck is
on a tight curve. Temporary bars may also be anchored on blisters that have been cast
onto the concrete webs or slabs after match casting, using bend-out bars or couplers,
Figure 14.8. In addition to the internal blisters, temporary steel or concrete anchor
blocks may be used to anchor bars onto the webs or slabs of the box, Figure 15.11
and Figure 15.25. Such anchorages are usually required where the temporary stressing
is intense, such as when several segments are erected on bars before installing the
permanent tendons.
For all blisters and external anchorages carrying temporary stressing, the bending
moments and direct tensile stresses introduced into the slabs or webs must be considered,
and usually additional reinforcement will need to be added, Figure 15.22. The blisters
or temporary anchor blocks should be placed on the transverse axis of the segment
or further forward. If they have to be placed towards the back of the segment, it is
essential that the force in the prestressed bars is carried forwards into the segment by
reinforcing bars in the top slab; otherwise the top slab may split in front of the anchor,
Figure 15.23. The rear anchorage point of the temporary prestress is usually tied in by
the permanent prestress. If not, the segment must be appropriately reinforced.
It may also be necessary to mobilise a small temporary prestress force near the
bottom of the segment to stop bottom fi bre tensions occurring when the permanent
prestress tendons are installed.
Figure 15.22
Typical moments and forces due to temporary prestress
