Civil Engineering Reference
In-Depth Information
Figure 15.5
Hong Kong MTR Contract 304: precast end block (photo: Robert Benaim)
15.2.5 Rate of construction
The falsework may be a simple scaffold, a self-launching falsework rig, or something
in between. Figure 12.16 shows a simple falsework for the construction of the three
Doornhoek bridges each of which had six spans. A more sophisticated falsework is
shown in Figure 12.2, where the 1,200 m long twin rib, twin-track railway bridge of
Contract 304 of the Tsuen Wan Extension of the HKMTR was built at a rate of one
32 m span every two weeks. It consisted of a platform carried by girders which were
supported on the end of the completed deck, an intermediate support, and the forward
pier. The intermediate support was founded on a precast concrete pad foundation,
despite the poor ground. This was possible as the weight of concrete was only applied
for a short time before the span was prestressed and became self-supporting. In similar
circumstances the pad foundation may be preloaded to further reduce the risk of
settlement. The shuttering for the deck consisted of a series of boxes assembled on the
platform. This type of falsework can adapt easily to vertical and horizontal curves. In
order to achieve a rate of two weeks per span, it was necessary to stress enough cables
to render the span self-supporting some 36 hours after casting the concrete. An initial
prestress was applied 24 hours after casting to render the deck resistant to cracking.
The prestress anchors were encased in the precast end blocks shown in Figure 15.5.
The Viaduc d'Incarville with 15 m wide, 2.5 m deep twin rib decks of 45 m span,
Figure 12.3, was built span by span as part of the Autoroute de Normandie. The author,
when with Europe Etudes, designed and patented the self-launching falsework rig that
carried the shuttering for the deck. The rig consisted of an overhead truss weighing
some 250 tons and an under-slung steel girder weighing approximately 50 tons. The
