Civil Engineering Reference
In-Depth Information
weeks. However it can only be used on sites suitable for crane access. Shear legs may
be used universally, as for the Byker Viaduct, Figure 15.31. However, this method is
relatively slow, erecting generally not more than one pair of segments per day giving
an average rate of about four pairs of segments per week, and a deck erection period
of 35 weeks. Balanced cantilever construction requires only simple falsework that does
not control the mobilisation period.
It would not be economically viable to adopt a single wide box, as 140 segments
is well below the economical threshold for segmental construction, particularly as the
segments would be 22 m wide and, at a length of 3.5 m would weigh over 100 tons.
16.4.4 Incremental launching
If the viaduct alignment and spans were suitable, this method of construction would be
very economical, as described in
15.8
. A length of 500 m is well within the capabilities
of the method, which has been used for viaducts over 1,000 m long. The deck could
be either two adjacent boxes, or a single box.
Two small boxes would limit the costs of the mould and jacking equipment, although
this option would lengthen the construction programme. 20 m long segments built at
a rate of one per week, would give a construction period for the fi rst deck of about 35
weeks, including a learning curve. The mould would then have to be moved sideways,
and the second deck built, giving a total deck construction programme of about 65
weeks, exclusive of fi nishing works.
A single box, Figure 16.2, would economise on substructure and on time, although
the mould and the launching nose would be more expensive. Adopting a 15 m segment
on a weekly cycle for the much larger, more complex section would yield a construction
programme of approximately 45 weeks. The single box option is likely to be the most
economical for this length of bridge, due principally to the shorter programme.
Alternatively, the wide deck could be built in two phases, the launched box fi rst,
followed by the addition of the strutted side cantilevers. This would take considerably
longer, but would need a simpler, more economical launching site, and a smaller labour
force.
When necessary, this construction method may be speeded up as described in
15.8.7
for the Sizewell project where 30 m long complete spans were planned to be built at
the rate of one per week, for an 800 m long deck, adopting complete prefabrication
of the reinforcing cage.
Figure 16.2
Constant depth with strutted cantilevers
