Civil Engineering Reference
In-Depth Information
11.7 Multi-cell box girders
11.7.1 General
The logical conclusion of the development of voided slabs is the multi-cell box. This
is understood to mean shallow decks, less than about 1.4 m deep, with contiguous
rectangular voids. Many theoretically minded engineers have been mislead by the
apparent economy and effi ciency of this form of deck. However simple considerations
of constructibility and a clearer view of material economy should convince thinking
designers that this is a deck form to be avoided. The reasons for this anathema are
described below.
The main reason for not using this form of deck is the slow, labour-intensive and
risk-prone construction procedure, due principally to the virtual impossibility of
casting the cross section in one pour. The bottom slab is cast fi rst with a kicker for the
webs from which projects the web reinforcement, Figure 11.18. The top surface of
the construction joint in the webs must be tooled to remove laitance, and to provide
a rough surface. The void shutters are erected and the deck is completed by casting
the upper parts of the web and top slab. Finally the void formwork is withdrawn
piecemeal through holes left in the top slab (or is abandoned).
The ducts for the prestress tendons, which are housed in the bottom slab near mid-
span, cross the horizontal construction joints in the web at a shallow angle. It is very
diffi cult to protect these fl imsy empty ducts during the tooling of the construction
joint. Undetected damage to the ducts will lead to leakage of grout into the tendons,
which then need major surgery before they can be stressed.
The preparation of this horizontal construction joint, with the interference of the
projecting shear reinforcement and the prestress ducts is very labour intensive.
The horizontal construction joint on the visible side face of webs is very diffi cult
to build neatly. The top surface of any pour has a very high water/cement ratio due to
bleeding of the mix, and unless the joint preparation is very thorough, gives rise to a
porous layer of concrete. Thus as well as being unsightly, this joint also represents a
Figure 11.18
Multi-cell box girder under construction (Photo: Robert Benaim)
