Civil Engineering Reference
In-Depth Information
Figure 11.12
Diagram of prestressed roof for Central Station on the Hong Kong MTR
The positive prestress parasitic moment due to the slab tendons will reduce the gross
hogging moment at the top of the uprights, Figure 11.11. This may be very valuable
where the thickness of the uprights, and thus their moment capacity, is limited such
as when they are built as diaphragm or secant pile walls. Under these circumstances,
prestressing the slab may be the only way to make the walls work. Figure 11.12 shows
diagrammatically the roof of Central Station of the Hong Kong Mass Transit, designed
in this way by the author when at Arup. Without the benefi t of the prestress secondary
moments, the diaphragm walls would have been unable to carry the hogging moments
derived from the loads on the slab, and the junction between walls and roof would
have had to be pinned, with deleterious consequences for the design.
