Civil Engineering Reference
In-Depth Information
Other operations may well determine the contract duration. The launching nose
and the casting area must be removed, and the two abutments completed. The deck
fi nishes, including parapets, balustrades, verges, waterproof membrane and asphalt
must be installed. When the programme is critical, the concrete parapet may be
installed during the launch. This is best carried out forward of the deck casting area,
typically over the fi rst span of the deck. As launched decks are relatively stocky, the
defl ections imposed on the parapet by the second stage prestress will be imperceptible.
If the parapet is cast in-situ it is necessary to create joints so that it is not subject to
damaging bending stresses during the launch. The fi rst stage launching prestress will
need to be increased signifi cantly to carry this extra weight, and the cost of this extra
prestress must be judged against the shorter programme. Once the parapet is in place,
the metal balustrade and the waterproof membrane may also be installed during the
launch. Verges are likely to be too heavy to be built economically during the launch,
while the asphalt is best laid in one operation once the launch is complete.
15.8.18 Variations on the theme
As with everything in this topic, the author is describing how and why things are done,
in the hope that it will stimulate the inventiveness of engineers to take matters further.
Launching a concrete bridge into place is such a sensible solution, and there are many
situations that differ from the standard where it can be adapted.
The Liu To Bridge in Hong Kong crossed a deep valley with spans of 50 m, 70 m
and 50 m and had been designed to be built in cast-in-situ balanced cantilever. The
bridge was straight in plan and elevation. Peter Baum, Technical Director of the French
contractor Dragages et Travaux Publics, offered a tender with an innovative launched
alternative, and once they had won, commissioned Benaim to carry out the detailed
design.
The deck consisted of two adjacent box girders joined by a top slab. The launching
formwork was sized to build 5 m lengths of one box at a time, on a 2-3 day cycle. A
temporary tower divided the end span into two, with launching spans of 20 m and
30 m, and no launching nose was necessary. When the 50 m side span and half the
70 m main span had been built and launched forwards, the box was slid sideways
across the pier and abutment, and the second box was built and launched. The top
slabs of the two boxes were then stitched together. The falsework was moved across
the valley and the operation was repeated. The deck was then joined at mid-span by a
cast-in-situ stitch, Figure 15.44.
The launching prestress was designed so that a signifi cant tonnage could be de-
stressed and re-used as permanent prestress. The ends of the tendons that had been
marked by the grips of the anchorages were discarded.
Each complete half bridge was statically determinate, and could be re-levelled by
jacks on the abutment and pier. Thus the diffi culty of creating an accurate longitudinal
profi le with a shutter only 5 m long was not relevant.
