Civil Engineering Reference
In-Depth Information
Figure 15.36
Self-weight bending moments during launching
alignment of the supports over which they slide. The 'differential settlement' consists
of several components. The actual shortening of piers and foundations as the weight
of the deck fi rst comes onto them should be calculated, and introduced into the deck
calculations explicitly. Once the fi rst span length of the deck has passed over a pier, the
reaction on it stays constant. However, if soil conditions vary along the length of the
bridge the foundations may suffer deferred differential settlement, and a best estimate
of this effect should be introduced into the calculations explicitly.
Temporary steel falsework towers will shorten elastically more than the concrete
piers. Either this should be calculated and included explicitly in the deck calculations,
or the launch bearings on the towers should be carried by hydraulic jacks that can
adjust their level.
There remain the unquantifi able misalignments of the soffi t which may consist of
steps at construction joints, movement of the form under the weight of concrete, a
slightly incorrect vertical radius given to the whole deck or errors in the levelling of
the launch bearings. A typical value adopted for misalignment is ±3 mm, applied on
successive piers in the most unfavourable way.
