Civil Engineering Reference
In-Depth Information
extend by 15 mm, and a bar by only 8.75 mm. Consequently, a considerable portion
of the jacking force in short tendons will be lost by the pull-in of wedges or the set of
nuts. Vertical prestress should not be used on decks less than 2.5 m deep, and close
supervision of the stressing operation is necessary, with checks made on the residual
force in a sample of the tendons.
9.5.4 Transverse bending moments applied to webs
The webs of box girders are subjected to signifi cant bending moments derived from
the actions of the top slab and side cantilevers. These moments are calculated from
a frame analysis, and diminish rapidly down the web. The web is also subjected to
bending moments due to the distortion of the box under eccentric live loading. The
reinforcement for these moments may need to be added to the shear steel, particularly
at the top of the web. However, care should be taken that incompatible load cases are
not added together; it is unlikely that the maximum bending moments on the web
occur concurrently with the maximum shear.
The design of the bending reinforcement in the web should take into account the
effect of compression due to the weight of the slab and of any co-existent live loading.
For further advice on the design of web reinforcement, subject both to shear and
bending, see [1].
Figure 9.30
Detailing shear links
