Civil Engineering Reference
In-Depth Information
15.9.3 Comparison between precast segmental and the whole-span methods
This comparison is extrapolated from work carried out in 1996 by Benaim for a
project in South East Asia. The costs of moulds for the two systems were based on fi rm
quotations at the time, and have not been updated. All the other equipment costs have
been estimated. The number of men required for segmental construction is based on
experience, while that for the whole-span site is estimated.
The project required the construction of two parallel viaducts 6 km long, each
consisting of 170 spans of 35 m. The comparison was based on a requirement to build
the deck in 8 months, a rate of 42.5 spans per month.
For the segmental option, 12 segments per span gave a total number of 4,080. It
was assumed that segments could be produced at the average rate of 20 segments/
mould/month. Thus for 510 segments per month, 25 moulds would be required. Each
casting cell cost £130,000, and required a team of 15 men. Additional equipment,
including foundations for the mould, jigs for reinforcement, roofs over the casting
cells, overhead cranes for handling the reinforcing cages and delivering the concrete,
survey equipment, vibrators etc were estimated at £60,000 per mould. Two straddle
carriers were assumed, for handling the segments in storage at a cost of £100,000
each.
The segments were to be placed by overhead gantries weighing 150 tons that were
assumed capable of placing two spans per week on average. Thus fi ve
gantries were
required, each operated by an erection team of 10 men. Each gantry was assumed to
cost £450,000 and was supplied with segments by purpose-made transporters costing
£100,000 each.
For the whole-span method, it was assumed that each mould, supported by a
production line, would be capable of producing one deck per day, or about 20 decks
per month, giving a requirement of two
moulds for this comparison. Each mould cost
£350,000, while the supporting production line, including foundations for the mould,
jigs for reinforcement, two 50 t capacity gantry cranes to handle the precast bottom
slab and the reinforcing cages and the steam curing and concrete delivery equipment
may be estimated to cost £500,000 per production line. It is estimated that 50 men
would be required for each production line.
With only two moulds operating, it may be assumed that there is no need for a
storage and sorting area; each precast unit moves directly onto the deck. Thus the
only deck-handling equipment consists of the single rail mounted transporter weighing
some 120 tons, and costing some £500,000. The transporter requires 6,000 m of rail,
which may be second hand and sold on afterwards. As the rails weigh 50 kg/m each
and four are required, there is a total of 1,200 tons at £200/ton, costing £240,000.
Only one gantry capable of placing two decks per day of the type described in
15.9.2
would be necessary, weighing 300 tons, costing about £900,000 and operated by a
team of 10 men.
