Civil Engineering Reference
In-Depth Information
Figure 9.5
Transverse slab joint subjected to longitudinal bending moments
The deck slab for a ribbed side cantilever will generally be 200 mm thick, requiring
ribs at centres of approximately 4 m under UK loading. Spans up to 5 m may be possible
under other codes that use lighter loading. As the slab spans longitudinally between
the ribs, its local bending moments combine with the longitudinal stresses in the slab
due to the overall bending of the deck. This will affect the design of the longitudinal
bending reinforcement in the slab, and may also reduce its ultimate bending capacity
under local loads, requiring greater thickness or a reduced span. It is benefi cial to
haunch the slab longitudinally over each rib.
If the deck is built by precast segments a transverse cast-in-situ stitch is necessary,
usually located half way between the ribs, to create continuity of the longitudinal
reinforcement, Figure 9.6 (b).
Another consideration when using a ribbed cantilever slab is to ensure that the
concentrated moment in the rib can be accepted by the web of the deck, or by the slab
beyond the web. The most logical arrangement is to provide ribs across the full width
of the deck, although this is not essential as long as the webs and slabs are designed
correctly. The rib may also be deepened so that it applies its bottom fi bre compression
directly to the bottom fl ange of the box, Figure 9.6 (c).
The economy of ribbed cantilever slabs depends principally on the loading code.
Where the design live load consists of an isolated load or vehicle, it has been the
author's experience that ribbed slabs are not economical. A solid slab has great capacity
to spread the effects of a concentrated load over a considerable length at the cantilever
root, while a ribbed slab concentrates the bending moment on to a small number of
ribs, Figure 9.6 (d) and (e). Thus the benefi ts of the greater reinforced concrete lever
arm are lost, as the moment to be carried is more concentrated.
However, when the loading is principally a uniform load, the bending moment at
the root of the cantilever is not subject to dispersion and the greater lever arm will
reduce the reinforcement required. Transverse prestress is more likely to be economical
in a ribbed slab than in a solid slab.
