Civil Engineering Reference
In-Depth Information
The deck segments were cast using the pier segment as the origin, casting the
cantilevering segment in one direction, then reversing the pier segment and casting the
other fi ve span segments.
The segments were placed onto a self-launching gantry by mobile crane. More
typically, the segments for viaducts built by this method are carried to the construction
head over the completed deck.
With such a shallow deck, work inside the box was minimised, and the temporary
prestressing was located above the top slab, anchored on precast concrete blocks
stressed down to the deck, Figure 15.11. Thin plywood was used as a bedding medium
between the blocks and the deck, allowing re-use of the blocks. The bars clamping the
blocks to the deck were 750 mm long, and as they extend by only 2 mm when stressed,
had to be carefully monitored to ensure that the stressing force was not lost as the load
was transferred from jack to anchor.
The segments were handled on the gantry by four sledges equipped with vertical
jacks running on greased slide tracks. Once a segment had been attached by the
temporary bars, it was allowed to cantilever off the previous segment, maintaining a
hogging moment in the deck. As a new segment was attached, the previous segment
was packed off the girder and the sledges recovered. Although economical in sledges,
this method made it essential to align the fi rst pair of segments very accurately, as their
orientation could only be changed within very narrow limits subsequently. On larger,
more highly automated sites, each segment is carried by three or four sledges equipped
with jacks that may be adjusted at any time until the span is struck.
Once all the segments had been assembled, fi ne adjustments to the elevation of the
span were made by jacking the entire girder at the pier falsework, and the bearings on
the pier were grouted in. The tendon ducts were coupled across the 150 mm stitch, and
the joint cast. When the joint had achieved adequate strength, typically 15-20 MPa,
the prestressing tendons could be stressed, the temporary stressing removed and the
girder lowered clear.
As the girder was very fl exible compared with the deck (
15.2.3
), it was necessary
to execute a staged programme of application of the prestress and lowering of the
gantry. When the prestress was applied, the contact pressure between deck and gantry
reduced fi rstly at mid-span, throwing more reaction onto the side cantilevers either
side of this mid-span zone.
The average speed of construction was of the order of a span per week, with a
peak rate of a span in four days. However, this relatively small site was not highly
mechanised, and achieving faster construction was not a priority.
The two parallel boxes that constituted the deck were built successively. Once both
were complete, they were joined at their top slabs by a 2.5 m wide cast-in-situ stitch. At
the time the two boxes were linked, the deferred shortening of the fi rst completed box
was proceeding more slowly than that of the second box, Figure 15.12. Consequently
the completed deck behaved like a bi-metallic strip, bending sideways, damaging
the bearing plinths at the abutments. This effect may be mitigated by equipping the
penultimate piers with free sliding bearings. Alternatively, the forces may be calculated
and the substructure designed in consequence.
