Civil Engineering Reference
In-Depth Information
For instance, when the depth varies continuously it is often judged that an elliptical
profi le is the most beautiful. However, this will tend to create a design problem towards
the quarter points, as at these locations the beam is shallower than optimal, both
for shear resistance and for bending strength. As a result, the webs and bottom slab
may need to be thickened locally, and the prestress increased. However, the economic
penalty may be small enough to accept. The Islamic form is likely to provide the most
fl exible method of optimising the depth at all points along the girder, but the cusp
at mid-span may give a problem for the profi le of the continuity tendons while for
long spans the greater weight of the deeper webs either side of mid-span implies a
signifi cant cost penalty. Also, the appearance may not be suitable for the particular
circumstance.
When the change in the depth of the box is not too great, haunched decks are often
chosen for the precast segmental form of construction, as they reduce the number of
times the formwork must be adjusted, assisting in keeping to the all-important daily
cycle of production. However, here again there is a confl ict between the technical
optimisation of the shape of the beam and aesthetic considerations. The beginning of
the haunch is potentially a critical design section, both for shear and bending. This
criticality is relieved if the haunch extends to some 25-30 per cent of the span length.
However, the appearance of the beam is considerably improved if the haunch length is
limited to 20 per cent of the span or less.
When variation of the depth is combined with a trapezoidal cross section, the
bottom slab will become narrower as the deck becomes deeper, Figure 13.5. This has an
important aesthetic impact, as well as giving rise to complications in the construction.
When a deck is built by the cast-in-situ balanced cantilever method, such as the 929 m
long Bhairab Bridge [1] in Bangladesh designed by Benaim, Figure 13.6, the formwork
Figure 13.5
Variable depth with trapezoidal cross section
