Civil Engineering Reference
In-Depth Information
Figure 15.12
Differential creep shortening of boxes
15.4 Cast-in-situ balanced cantilever construction
15.4.1 General
Cast-in-situ balanced cantilever construction is ideally suited to box section bridges of
medium or long span, where there is insuffi cient repetition to justify precasting. The
method becomes economical for bridges with a main span of 60 m and above, and
remains viable up to the largest span that may be built, currently about 300 m.
Early cantilever built bridges were pinned at mid-span. However, these bridges have
in several instances exhibited unexpectedly large creep defl ections at mid-span (
7.2.4
).
Furthermore, the presence of an expansion joint at mid-span affects the ride of the
bridge, and is a maintenance liability. Most modern cantilever built decks are made
continuous by casting a mid-span stitch.
15.4.2 Proportions of balanced cantilever built bridges
The method of construction leads to a self-weight moment diagram that is predominantly
hogging, and it is clearly economical to provide a greater structural depth over the
supports and to minimise the weight of the deck towards mid-span. Furthermore, the
construction of the deck in short lengths on a weekly cycle makes it relatively easy to
change the geometry of the shutter for each cast. Consequently, most bridges built by
this method have variable depth.
The depth of the deck for large bridges is inevitably a compromise between economy
of materials, appearance and ease of construction. For spans less than about 100 m,
for economy the depth of the deck at the support should ideally be approximately
span/14. At this depth, the prestress is economical, the webs do not usually need to be
thickened to carry shear, and the bottom slab can also stay at its mid-span thickness.
However, the depth of the deck may be forcing the road alignment higher than is
desirable, and there may also be consequences for the appearance of the bridge. The
support depth may be reduced to span/20, with the consequent greater consumption
of prestress, and the greater complication of the construction as the webs and bottom
slab will need to be thickened at the supports.
Large bridges generally adopt a support depth of about span/20 to overcome the
disadvantages of the great structural depths otherwise attained. For a 300 m span for
instance, span/15 would yield a depth of 20 m.
