Civil Engineering Reference
In-Depth Information
Figure 11.13
Skew portals
11.4.4 Skew portals
Portals are very suitable for skew crossings. Their behaviour depends principally on
the ratio between the span and width of the deck. Figure 11.13 shows a narrow bridge
where the bridge spans principally on the skew and a wide bridge of the same skew.
Over the central part of the wide bridge the deck spans right while at the ends it spans
on the skew. Either the thickness of the deck must be adequate for the longer span with
locally increased reinforcement, or some kind of edge strengthening must be used,
such as structural parapets or a thickened slab.
Where the deck spans on the skew, the support moments are higher than for a right
span of the same length due to the stiffening effect of the angled support, and the
shape of the sagging moment diagram is also distorted. For the detailed design it is
necessary to carry out a carefully defi ned grillage analysis or a fi nite element analysis.
11.5 Voided slabs
11.5.1 General
In general terms, prestressed concrete decks become more economical as they become
deeper, with span/depth of 15-18 being typical. The solid slab is the only exception to
this rule; its economy derives from its extreme simplicity. However, when a prestressed
solid slab exceeds a thickness of 700 mm, providing a span of up to 23 m, its weight
starts to become excessive, and the designer should consider alternative deck types.
The most logical alternative is to abandon the slab form and adopt a ribbed slab
deck (Chapter 12). The depth of such a deck is likely to increase substantially, a 23 m
span requiring a depth generally in excess of 1.15 m. However, the depth allowable
may be limited, or a fl at soffi t may be required. For these special cases, a voided slab
deck may well be suitable.
It should be clear to the designer that by introducing voids, he has sacrifi ced the
essential simplicity of the slab. The cost of the voids, including the measures taken to
hold them steady during concreting and to resist the up-thrust of the wet concrete,
is at least as much as that of the concrete saved. The unit costs of the concrete,
reinforcement and prestressing will all increase due to the greater intricacy, and hence
to the greater labour required. Also, the quantity of passive reinforcement will be
signifi cantly increased, probably exceeding 110 kg/m
3
of concrete. The principal
