Civil Engineering Reference
In-Depth Information
Allowing for some special moulds for pier segments and a reasonable contingency,
probably at least 20 moulds would in fact be required. This compares with the
42 moulds actually mobilised to cast the 7,000 segments for the 22 km PUTRA LRT
in Kuala Lumpur. This number is not strictly comparable, as there were two basic
types of deck for single or twin track railways, with separate moulds for pier segments.
However, it is an example of how a casting yard must be equipped to cast at the rate
of erection for a long viaduct.
For a viaduct of this length, the single box scheme is likely to be the most economical
if there are not too many changes in width due to slip roads or other special events.
2,850 segments would be required, needing two placing gantries for 19 months and
at least 10 moulds.
Erection could be span-by-span or in balanced cantilever.
16.8.5 Incremental launching
In order to use incremental launching for a deck of this length, it would be necessary
to set up fi ve intermediate casting stations at 2,000 m intervals, equipped to launch
in each direction. In reality, even if the deck alignment was suitable, it is unlikely that
this would be an appropriate method of construction due to the cost of the elevated
casting areas and the large workforce required. However, it needs a relatively low level
of technology compared with precast segmental techniques or with the precasting of
complete spans and it may be competitive where labour is cheap.
16.8.6 Precasting complete spans
This method of building is highly relevant for such a long viaduct. Whereas the precast
segmental method cannot reasonably be accelerated beyond two spans per week per
gantry on average, complete span placing gantries may work at up to three spans per
24-hour day, if the supply of decks can keep pace.
The Poggio Iberna site had a very high performance gantry, but its productivity
was limited by a deck-casting rate of 12 per month. The fi rst step in accelerating the
construction rate for a bridge with twin decks would be to have two moulds feeding
the single placing gantry, achieving 24 decks per month, erecting the 500 decks in
21 months. In order to avoid the need for the gantry to have to make the return
journey along the completed fi rst deck, it would be possible to adapt it with transverse
sliding capability so that it may erect both decks in parallel.
In order to further accelerate the deck construction, it would be possible to add a
third mould, leading to a theoretical erection rate of 36 decks per month, or about
1.6 decks per working day, and an erection period of 14 months. At a rate of casting of
two days per span with three moulds operating, it is still feasible to introduce the minor
specialisations required, and to produce the decks in the order required for erection,
avoiding the need for expensive storage and sorting of decks. The three moulds would
be arranged behind an abutment, with a system of railway tracks and points leading to
the rails on the deck itself. Clearly, a very substantial and well-organised casting yard
would be required to service these moulds.
An alternative strategy would be to adopt moulds that were capable of producing
one deck per day, say 24 decks per month, as described in
15.9.2
. Two such moulds
would produce 48 decks per month, completing the erection in less than a year. The
