Civil Engineering Reference
In-Depth Information
Figure 15.27
Erection of unbalanced end spans
The mid-span stitch is best made thin enough so that it does not need to be reinforced,
imposing a maximum width of approximately 250 mm. This gives very little tolerance
to correct alignment errors. It also gives little margin to accommodate tolerance on
the length of segments. When such a short stitch is adopted, it is important to keep a
record of segment lengths during casting, so that action may be taken in good time if,
for instance, segments are being cast fractionally long.
The principal diffi culty in making this stitch is the fact that the cantilever ends to
be joined are constantly moving. The completed viaduct to which the new double
cantilever must be attached will be expanding and contracting on a daily cycle.
Furthermore, the cantilevers will defl ect downward during the day under the action
of the sun shining on the top surface, and rise at night. If the stitch were cast without
precaution, these temperature effects would crack the fresh concrete before it was
possible to stress the continuity prestress.
The simplest case is when the spans do not exceed about 40 m and the joint width
is less than about 250 mm. Temporary stressing bars are installed just above or just
