Civil Engineering Reference
In-Depth Information
The position of the pier segment when the fi rst half span is cast must be ascertained
by very precise surveying, using studs or other markers cast into the top slab. When it
is rotated to act as the origin of the second half span, it must be brought back to the
same alignment about all three axes. Any error in this orientation will cause a kink in
the horizontal or vertical alignment, or introduce a twist into the half span.
An alternative manner of solving this problem is to omit the pier segment when
casting the half spans. It is then cast between the two adjacent segments. This
arrangement gives a longer base for surveying the position of the segments.
Although the long line method of building match-cast segments is relatively simple
to carry out successfully, it is now rarely used. A well-founded soffi t form may be
costly, particularly if piling is required, and the rate of casting is not high. However it
is not to be discarded, and if a good foundation is available cheaply, offers a low-tech
version of counter-cast technology that may be used for bridges much smaller than the
accepted lower economical threshold for this technique.
14.3 Short line casting
14.3.1 General
Most counter-cast segmental bridges are built by the short line method, which may be
used for virtually any shape of deck alignment. One of its principal advantages is that
the deck is fabricated in a factory environment, rather than that of a building site. The
majority of segments may be built to a daily cycle, with the labour force working to
a regular routine. The result is a high quality product, built to close tolerances, with
good fi nish and durability.
However, it depends for its success on a designer who understands the theoretical
basis of the alignment control and recognises the contractor's need for productivity.
It also requires careful preparation and execution by the contractor, and on the use
of a well-designed and engineered casting cell. Once these conditions have been met,
the system is remarkably trouble free, and even contractors with little experience of
modern bridge-building techniques may be very successful, casting segments at a rate
of one per working day, and erecting decks at the rate of several spans per week.
On the other hand, if the design or the construction has been carried out by a
team that has not understood the essence of the technology, this can be the most
troublesome of construction methods. Errors in the cast alignment are only discovered
when the deck comes to be erected, by which time half the segments have usually been
cast. Correcting such errors is in every case a very time-consuming process, which may
involve recasting segments out of sequence, or introducing additional grouted joints.
Short line precast segmental construction requires at least one complex casting cell,
storage capacity for a large number of segments and means of lifting heavy segments
in both the casting yard and at the construction front.
If the deck may be erected using cranes or shear legs, this method is generally
economical for 250 segments or more, representing a length of box between 750 m
and 1,000 m. If a self-launching erection gantry, typically weighing at least 200 tons
and costing upwards of £750,000 is required to erect the deck, the economic threshold
is more likely to be 500 segments.
It should be noted that if a contractor has the necessary kit from a previous contract,
the economic threshold may be much smaller. The Weston-super-Mare railway viaduct
