Civil Engineering Reference
In-Depth Information
Figure 9.2
East Moors Viaduct: precast verge
signifi cant length of deck, the maximum bending moment on the root is likely to occur
over a relatively short length. This means that the shear may be resisted by underused
bending reinforcement in areas that are not subjected to maximum bending moment.
However, there is no consensus on how the reinforcement for combined shear and
bending should be calculated.
A simple method of combining the reinforcement for shear and bending is to
calculate the reinforcement required for both actions. One-third of the reinforcement
required for shear is placed near the bottom fi bre of the cantilever, while the greater
of the bending reinforcement or two-thirds of the shear reinforcement is placed near
the top fi bre of the cantilever. For this approach to be valid, neither the shear nor the
bending must be exhausting the capacity of the concrete in compression.
For further consideration of how to combine the reinforcement for shear and
bending moment, see [1, 2 and 4].
The reinforcement required for shear should not be confused with that needed for
the dispersion of the prestress force into the cantilevers at the end of a deck. This is a
separate effect, requiring additional reinforcement (
5.24.4
).
If a direct tension was applied to the cantilever, additional reinforcement would be
required. This can occur due to impact on the parapet, although such tension is unlikely
to be compatible with the load cases giving maximum shear and bending. Tension will
be applied when the cantilever is supported by inclined struts (
9.2.11
). On the other
hand, compression due to transverse prestress would participate in resisting both the
bending moment and the shear friction.
9.2.4 Shear lag
Shear lag must be considered when assessing how much of the side cantilever participates
in the longitudinal bending of the deck at the SLS (
6.10.2
). The usual rule of thumb
is that any part of the cantilever beyond a width of one-tenth of the span of a simple
beam, or one-tenth of the distance between points of infl ection for a continuous beam,
should be ignored in calculating the bridge deck section properties. As the top fl ange
