Civil Engineering Reference
In-Depth Information
Figure 13.3
River Dee Bridge: effect of side cantilever on the appearance of a variable depth
deck (Photo: Edmund Nuttall)
bottom slab to be reduced. Close to the piers, the area of bottom slab is determined
by the need to limit the maximum bending stress on the bottom fi bre and to provide
an adequate ultimate moment of resistance. If the narrow bottom slab defi ned by mid-
span criteria is inadequate, it is simple to thicken it locally.
For a very wide deck that has a deep span/depth ratio, this logic may give rise to webs
that are inclined at a very fl at angle. The designer should be aware of the diffi culties
in casting such webs, and make suitable allowances in specifying the concrete and in
detailing the reinforcement.
Also, an important consideration in the design of box section decks is the distortion
of the cross section under the effect of eccentric live loads (
6.13.4
). The effect of this
distortion is reduced in a trapezoidal cross section.
Boxes may have a single cell or multiple cells. In Chapter 8 it was explained how
important it is for economy to minimise the number of webs. Furthermore, it is more
