Civil Engineering Reference
In-Depth Information
loading). Each balanced cantilever would consist of a 7 m hammerhead, seven pairs
of 3.7 m long segments with a 1.2 m stitch in the centre of the main span, and a short
in-situ section at each abutment which could be built by the side span traveller or on
independent falsework.
The choice of one box or two is here more diffi cult to make. If two boxes were
chosen, the form travellers would be lighter and more economical. However, two sets
of substructure would be required, and critically, the construction time for the deck
would be nearly doubled.
Construction of two narrow boxes, using only one pair of travellers, would typically
follow the following programme:
•
•
6 weeks for the fi rst hammerhead;
14 weeks for the segments of the fi rst double cantilever (including an allowance
for a learning curve
15.4.3
);
1 week for repositioning the travellers on the hammerhead for the next double
cantilever (the second and subsequent hammerheads may be built concurrently
with balanced cantilevering, and hence off the critical path);
7 weeks for the second balanced cantilever;
2 weeks for the stitch;
8 weeks for the third balanced cantilever (including the transfer of the
travellers);
8 weeks for the fourth balanced cantilever;
2 weeks for the second mid-span stitch;
total time approximately 48 weeks.
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The in-situ ends of the side spans are assumed to be built on separate falsework
concurrently with the other activities. The deck construction will be followed by the
fi nishing works, including the construction of parapets and verges, road surfacing and
expansion joints. Time could be saved by mobilising more than one set of travellers,
although this would substantially increase the cost.
However, the designer should always be open to unconventional methods of
construction. The Liu To Bridge in Hong Kong, had spans of 50 m, 70 m, 50 m, was
just over 20 m wide and went to tender designed as a twin box balanced cantilever,
but was built as a variation on the technique of incremental launching (
15.8.18
and
Figure 15.44).
For a single box, a simple rectangular shape designed to carry British loading would
suffer from redundant weight due to a thick top slab and to an over-wide bottom
slab, Figure 16.1 (a), while a variable-depth, steeply trapezoidal deck would have the
complications described in
13.4.4
. The form travellers would be considerably heavier,
and the construction of such a wide deck would be diffi cult to enter into a weekly cycle
for a pair of segments. In order to lighten and simplify the form travellers, it would be
possible, for instance, to build only the box itself in balanced cantilever, adding the side
cantilevers in a second phase of construction. This would lead one to a cross section
that consisted of a relatively narrow box, rectangular or with slightly sloping webs,
with transverse beams to support the slab and the side cantilevers, Figure 16.1 (b).
