Civil Engineering Reference
In-Depth Information
Figure 15.34
Dagenham Dock Viaduct: gantry erection (Photo: DEAL S.r.l.; www.deal.it)
ribs; thus three segments were erected on temporary bars. These ribs were located so
that the cables could be stressed by a jack suspended from a crane located on the top
surface of the deck. The continuity tendons, also made of 31 strands, were anchored on
the ribs or on the pier diaphragms and were deviated by concrete blocks. The gantry,
which was designed and supplied by DEAL, was a rigid truss weighing some 400 tons,
and was capable of handling segments weighing up to 120 tons, Figure 15.34. As it
launched, it imposed loads on the cantilevers, for which the deck prestress had to be
designed. It built the deck at a rate of 4-6 segments per day.
15.6 Progressive erection of precast segmental decks
In an extension of the method of balanced cantilevering, precast segmental decks may
be erected by the method of progressive forward cantilevering. With the segments being
handled either by crane or shear legs, the deck is cantilevered forwards on one front
only, with each span being carried on a series of temporary props until the next pier is
reached, and the installation of permanent prestress renders the span self-supporting.
Several spans may be erected without any cast-in-situ stitches, offering the possibility
of a fast construction schedule.
Before the permanent prestress is installed, and the deck is only lightly compressed
with temporary bars, it is very fragile, and susceptible to being cracked by any movement
of the temporary props due to settlement of their foundations, to their expansion and
contraction under temperature changes, and by thermal gradients through the deck.
