Civil Engineering Reference
In-Depth Information
9.4.6 Out-of-plane forces
When a bridge deck has variable depth, the bottom slab that is compressed longitudinally
will be subject to out-of-plane forces. If the variation of depth is circular or parabolic,
the compression in the bottom slab creates a distributed force normal to the slab P = F/
r where
F
is the compressive force in the slab due to all loads including prestress, and
r
is the local radius of curvature of the bridge soffi t, Figure 9.19 (a). This force must be
carried in transverse bending of the slab.
If the bridge deck has a linearly haunched profi le, the forces in the bottom slab
either side of the angle change give rise to an out-of-plane resultant, Figure 9.19 (b).
It is quite likely that the bottom slab will not be strong enough to carry this force in
transverse bending, in which case it will be necessary to provide a transverse beam at
this location.
Even if the bridge soffi t is fl at, out-of-plane forces in the bottom slab are created at
changes in its thickness, either tapered or stepped, Figure 9.19 (c).
Figure 9.19
Out-of-plane forces in bottom slab
