Civil Engineering Reference
In-Depth Information
When the use of self-compacting concrete is fully established for the construction
of bridge decks, it is likely to eliminate the need for internal vibration and the rules on
the safe thickness of webs will need to be redefi ned.
If the prestress is external, the webs will be less congested, and it is suggested that
the absolute minimum thickness for webs for box girders should be 250 mm for precast
construction, and 300 mm for cast-in-situ decks.
Other factors may determine the web thickness. For instance, in balanced cantilever
construction where tendons are anchored on the face of the webs, the minimum
concrete thickness may be defi ned by the requirements of the anchors. However, this
constraint can be overcome by locally thickening the web at the anchor positions. On
the East Moors Viaduct for instance, the 300 mm thick webs were locally thickened
to carry face anchors that required a thickness of 450 mm, and this thickening was
integrated with the bottom corner blisters, Figure 9.28. If powerful prestress tendons
are anchored in blisters attached to the web, it may need to be sized to safely resist
these forces. Finally, if the transverse spans of the side cantilevers or of the slab between
the webs are unusually large, the bending moments applied to the top of the webs may
determine their thickness.
c) Width of webs near the supports of continuous box girders
The minimum thickness of webs at the support sections is defi ned by calculation.
In most modern codes of practice that consider shear and torsion at the ULS, this
minimum is controlled by the principal compression in the concrete.
Figure 9.28
East Moors Viaduct: face anchors in locally thickened webs (Photo: Benaim )
