Civil Engineering Reference
In-Depth Information
Figure 11.5
Solid slab portal bridges
11.4.2 Reinforced concrete portals
A good starting point for sizing a reinforced portal structure carrying highway loading
would be to choose a deck slab thickness equal to span/30, with haunches 50 per
cent thicker, extending some 7 per cent of the span, if this does not interfere with the
clearance diagram. Clearly, if the portal carries a great depth of fi ll, the slab depth will
need to be greater. Haunches are always economical. They provide the twin benefi ts
of attracting moment away from mid-span and then providing a greater lever arm to
resist this moment economically. Even very short haunches are valuable in reducing
the hogging reinforcement. Figure 11.7 shows a section of the Singapore Central
Expressway, designed by Benaim. However, if the haunch is too steep, it will not be
entirely effective; the maximum useful slope being about 35˚, Figure 11.8 (a).
