Civil Engineering Reference
In-Depth Information
Figure 15.1
Autoroute A8, France: span-by-span construction of box decks (Photo: Robert
Benaim)
Bridges built by this method may be only three spans long, or may extend for
many kilometres. Consequently, the falsework adopted varies from the simplest to
the most highly mechanised self-launching rigs. A disadvantage of the system is that it
is essentially linear; it is diffi cult to miss out spans or move to a second construction
front.
15.2.2 Prestress layout
The construction joint for span-by-span construction is normally situated at about 0.25
of the span. Whereas the optimum for minimising creep and reducing the uncertainty
on the fi nal self weight bending moments (
6.21
) is 0.22, the tendons are still too high
at this position, and a short distance further into the span makes the arrangement of
the prestress anchors easier.
The cables shown in Figure 15.2 (a) are each one span long, and they are all coupled
at the construction joint. The disadvantages of this arrangement are as follows:
•
The coupler sleeves must swaged on and the tendons coupled before the concrete
of the next span may be cast, taking time on the critical path.
The concrete must gain substantial strength before all the tendons in a span may
be stressed, delaying the construction programme.
Stressing all the tendons for each span is time consuming.
The space necessary to house couplers tends to force the cable profi le lower than
desirable.
•
•
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