Civil Engineering Reference
In-Depth Information
Much of the effort and ingenuity in the design of bridge decks is devoted to
minimising the under-employed, expensive and heavy concrete in solid webs. Thus the
spans of side cantilevers and deck slabs are made ever longer in an attempt to minimise
the number of webs. However, once designers have learnt to use light trussed webs
made of very high strength concrete, and once the use of self-compacting concrete
becomes fully acceptable to bridge designers, it may well be more cost effective to cut
down the span and hence the thickness of the top slab by using more closely spaced
trussed webs.
The use of trussed webs for bridge decks with a depth in excess of 3.5 m is a very
satisfying solution. However, it requires considerably more design expertise and effort
than a conventional solid web.
9.5.8 Steel webs
The logic of attempting to eliminate the under-employed concrete of webs leads the
designer beyond trussed concrete webs to the use of steel webs for prestressed concrete
decks. These may be in the form of stiffened or corrugated plates or of trusses. The
author has no experience of the use of such webs.
It is very well worthwhile carrying out full-scale research by designing and building
such decks, as long as all parties to the design and construction contracts are aware
that there may be no short-term economic benefi ts. It is by experimenting in this way
that progress is made.
9.6 Diaphragms
9.6.1 General
Diaphragms are transverse beams spanning between main beams or box girders, or
within box girders to stiffen the cross section. Diaphragms signifi cantly complicate
the construction of any concrete bridge deck. For instance, in cast-in-situ twin rib
bridges they impede the launching of the falsework from one span to the next. In
precast 'T' beam decks they typically add 4-5 days to the construction of each span,
while in cast-in-situ box girders, they make it impossible to launch an internal shutter
forwards, imposing a piecemeal dismantling and reassembly. They create hard points
for the loaded slabs, making it necessary to thicken them locally and increase their
reinforcement and they impede maintenance access from span to span. In precast
segmental construction their weight often governs the lifting capacity needed for
erection, even when the pier segments are shortened, and they need special moulds
which reduce the productivity of the process.
Diaphragms are classed either as 'intermediate', occurring within spans, or as
'support', situated at piers or abutments. Modern methods of bridge deck analysis
have demonstrated that intermediate diaphragms are generally unnecessary, and the
tendency in design has been to omit them. Their use within box girders has completely
ceased, as it has been realised that it is better to carry the bending moments caused by
distortion in frame action over a considerable length of box rather than concentrating
them on discrete diaphragms. For decks consisting of multiple box girders, support
diaphragms between boxes are generally only used at abutments. Consequently, the
following discussion concerns only support diaphragms within box girders. (See also
