Civil Engineering Reference
In-Depth Information
of delay to the programme. Consequently, it was necessary to invest in a storage area,
serviced by a multi-wheeled pneumatic-tyred transporter, which would create a buffer
between prefabrication and erection, and which would also give the opportunity to
carry out any repairs to the decks.
Groups of up to nine statically determinate spans were linked together to create
expansion lengths of some 300 m. They were joined by stainless steel Macalloy bars
anchored on the end diaphragms. These bars were sized to resist the bearing drag,
braking and seismic forces. They were to be stressed against a joint fi ller poured
between the top slabs of the spans. This fi ller had to be strong enough to resist the
prestress of the bars, fl exible enough to accept the rotation of the decks ends in service,
and had to set in a few hours. The fi ller used was an appropriate grade of Sika Rail
currently marketed under the name ICOSIT [7], providing the required combination
of fl exibility and strength.
The gantry designed by Benaim for this project consisted of a 6 m deep steel truss
75 m long and weighing 300 tons, with legs at each end plus a third set of mobile legs,
Figure 15.49. When waiting for a new deck unit, the gantry was carried by its front
leg resting on the forward pier, and on its central leg resting on the rear pier. The rear
leg was lifted to allow the new span to roll beneath the gantry on its rail transporter,
Figure 15.49 (a). The rear leg was then lowered, and the centre leg disengaged and
nested with the front leg. The gantry was now spanning 67 m. Two mobile crabs
running on its top fl ange then lifted the new deck unit, Figure 15.49 (b). The crabs
rolled forwards carrying the unit until it was over its bearings, and then lowered it into
place. Every action involved in adjusting the position of the new span and placing it on
the laminated rubber bearings had to be analysed and refi ned to fi t in with the 8-hour
cycle. The gantry launched forwards resting on its rear leg that rolled on short rails
placed on the deck, and on the central leg that was equipped with rollers at its head,
Figure 15.49 (c). All the operations comprising the placing and launching sequence
were analysed in detail, and it was concluded that the cycle time was at most 8 hours,
most probably reducing with practice.
Full working drawings of the decks, foundations and placing gantries were completed
by Benaim while the design of the casting yard was completed to an advanced stage.
Two of the moulds for the decks, Figure 15.50, were completed by Ninive Casseforme
[8] of Italy and shipped to Hong Kong, and some of the heavy plant for the casting
yard had been purchased. When many kilometres of the foundations for the viaducts
had been completed, the project for the deck was modifi ed to allow it to be built by
local contractors rather than by a highly specialised company. It was fi nally built to a
different time scale with a deck consisting of precast T beams, also designed by Benaim
(
10.3.8
).
It is hoped that this brief description of the project makes clear that achieving a rate
of deck erection of several spans per day per placing gantry is possible, but requires
careful integration of the design of the deck and substructure with all stages of the
construction process.
Benaim have carried out preliminary schemes that involve launching decks weighing
up to 1,500 tons, such as that proposed to launch into place prefabricated two-level
decks for Hopewell Holding's Bangkok Elevated Rapid Transit System, Figure 15.51.
