Civil Engineering Reference
In-Depth Information
Finally, for bridges that are less than about 150 m long, the entire deck may be
built span-by-span using only half the tendons, the remainder being threaded from
end to end.
15.2.3 Typical construction details
When a falsework truss is used for construction, its rear end may be supported on the
pier foundation, or it may be suspended from the end of the bridge deck, Figure 15.4.
Clearly if the latter arrangement is used, the load applied to the cantilever will affect
the self-weight bending moments of the deck. This will be more signifi cant if the
falsework has no intermediate supports, but spans freely to the next pier foundation.
If the falsework spans from pier to pier, the weight of the fresh concrete will cause
it to defl ect at the location of the construction joint. Care has to be taken to avoid a
step in the concrete or leakage of the shutter at the joint.
When only half the tendons are stressed, the deck will defl ect downwards under its
combined self weight and prestress, and the falsework must be designed to be struck
when under load. If all the tendons are stressed the deck will defl ect upwards, and so
will tend to lift itself off the falsework. However, adjacent to the pier there may be
a short length where the defl ection under prestress is downwards, even with all the
cables stressed.
If the falsework is fl exible and has no intermediate supports, the striking must be
planned with care. Before being prestressed, the weight of the deck is carried by the
falsework which defl ects downwards. As the cables are stressed, the deck and falsework
together will defl ect upwards; the weight of the deck is now being shared between the
deck itself and the falsework. If the downwards defl ection of the falsework under the
self weight of the deck is large compared with the upwards defl ection of the deck
under prestress, the majority of the weight of the concrete will still be carried by
Figure 15.4
Support of falsework truss for span-by-span construction
