Civil Engineering Reference
In-Depth Information
in the bottom member are
P
= ±
M
/
l
a
, and the shear force carried by the bottom fl ange
is:
S
R
=
P
tan
α
, or
S
R
= (
M
/
l
a
)tan
α
.
For the bottom fl ange to carry all the shear
= tan
-1
(
Sl
a
/
M
).
The web may then be omitted close to the support, Figure 9.23 (c), which is one step
in the logical development that leads to trussed webs.
(
M
/
l
a
)tan
α
=
S
, or
α
9.4.8 Prestress anchor blisters on the bottom slab
It is often necessary to anchor prestress tendons in blisters in the corner between
the bottom slab and the web. The design of these anchors is discussed in Chapter
6. However, it should not be overlooked that these anchors can create large tensile
equilibrium forces in the thin slabs, which may cause severe cracking if they are not
transversally reinforced or prestressed appropriately.
9.5 Webs
9.5.1 General
The functions of the webs are to carry the shear force in the deck, to support the top
slab, and to carry the reactions of curved prestress tendons. For internally prestressed
bridges, they also house the prestressing cables as they travel between the extreme
fi bres of the deck. In a box section deck, the webs are usually subjected to transverse
bending moments imposed on them by live and dead loads on the top slab and side
cantilevers, and to a lesser extent by the self weight of the bottom slab.
9.5.2 Width of webs
As has been argued in Chapter 8, it is important for the economy of a project that
the number of webs is limited, and their thickness minimised. In the central part of
the span of most bridges, the web thickness is not controlled by shear forces, but
by the minimum that can reliably be built. This minimum depends on whether the
bridge deck is precast or cast in-situ, on whether the concrete vibration is internal or
external, on the inclination of the webs, on whether the webs carry prestressing ducts,
and whether there are prestress anchor blisters at the bottom of the web. Casting and
compacting the concrete in the webs and ensuring that it correctly fi lls the bottom slab
or heel is perhaps the most critical activity in the construction of prestressed concrete
bridge decks. It is certainly one of the activities most prone to give rise to construction
failure and contractual claims. However success or failure is under the control of the
designer.
The width of webs in twin rib bridge decks is subject to a different logic to that of
other deck types, and is considered in Chapter 12.
