Civil Engineering Reference
In-Depth Information
The need to clad both faces of each web and each face of the top and bottom slabs
with a mesh of reinforcement inevitably leads to the use of expensive small bars, slow
fi xing rates and a high rate of reinforcement, typically over 230 kg/m
3
.
As a consequence, multi-cell box decks are heavier than well-designed ribbed slab
or single-cell box section decks, have more reinforcement content, and are far more
labour intensive and costly to build.
11.7.3 Development of multi-cell box decks
The designer faced with one of these decks, perhaps specifi ed by a client, should
investigate alternatives. These may be voided slabs, ribbed slabs for cast-in-situ
construction, or precast segmental single cell boxes attached by their top slabs. The
author was faced with such a problem when asked by a contractor to assist in the
tender for the deck shown in Figure 11.19. As the deck width and the crossfall varied
along the deck, the height and width of each cell was variable and the proposed design
was virtually unbuildable. The alternative proposed was a series of identical precast
segmental boxes (Chapter 14), linked by cast-in-situ top slabs whose width could be
extended as necessary. The variable geometry of the conforming design could thus be
matched exactly with a standard precast product. Alternatively, if it had been essential
to maintain a fl at soffi t, the cross section could have been made far easier to build by
adopting circular voids.
