Civil Engineering Reference
In-Depth Information
diffi cult to build multi-cell boxes, and it is worthwhile extending the single-cell box as
far as possible before adding internal webs.
13.4.4 Variation of depth
Once the span of a box section deck exceeds about 45 m, it becomes relevant to consider
varying the depth of the beam. This is not an automatic decision as it depends on the
method of construction. For instance, when the deck is to be precast by the counter-
cast method (Chapter 14), if the number of segments is relatively low it is likely to
be more economical to keep the depth constant in order to simplify the mould. On
the other hand, if the deck is to be built by cast-in-situ balanced cantilevering, it is
relatively simple to design the mould to incorporate a variable depth, even for a small
number of quite short spans.
Clearly, this decision also has an aesthetic component. The depth may be varied
continuously along the length of the beam, adopting a circular, parabolic, elliptical or
Islamic profi le, Figure 13.4. Alternatively, the deck may be haunched. The decision on
the soffi t profi le closely links aesthetic and technical criteria.
Figure 13.4
Variable depth decks
