Civil Engineering Reference
In-Depth Information
16.7.4 Incremental launching
If the alignment were suitable, the decks could be incrementally launched, with a
launching bay behind each abutment, and with the decks meeting in the centre. It would
probably be economical to build one wide deck rather than two decks, principally due
to the shortened programme.
16.7.5 Prefabrication of complete spans
At this length of viaduct, the prefabrication of complete box section spans becomes
viable (
15.9
). A 40 m span, 11 m wide box section deck would weigh about 600 tons.
A relatively simple mould, equipped with steam curing, can build such units at a rate
of a span every two days. Thus, a half-width bridge could be built at a rate of one span
every other day, say 12 spans per month. This was the rate of construction achieved at
the Poggio Iberna Viaduct by Ferrocemento
.
The bridge deck could be completed in a matter of 11 months from start of
casting, allowing a 12 week addition for a learning curve. As the time for design and
construction of the gantry and for the segment transporter would be similar to that for
precast segmental placing gantries, the overall length of construction of the deck from
commissioning the moulds and gantry would be about 26 months.
The spans would preferably be statically determinate, because creating continuity
slows down the erection rate, and loses some of the advantage offered by the method.
Also, it would be preferable to divide the two carriageways by a longitudinal joint. If a
linking slab was required, it would most probably be cast once the two decks had been
completed. The slab could be built at the same time as the parapet, if this had not been
cast together with the spans.
In this technique, the deck units are built at the same rate they are placed, giving the
possibility of a better use of labour, with elements of the same team being responsible
for precasting and placing the units.
This method of construction requires major capital expenditure, including the mould
and casting area, the transporter, rails and the placing gantry. However, the labour and
supervision required to build the decks is less than for any other construction method,
except possibly incremental launching.
16.8 The bridge is 10,000 m long
16.8.1 General
For a viaduct of this length, the rate of construction of both the foundations and the
decks becomes the dominating design criterion. Although decks may be erected at a rate
of several spans per week for each placing gantry, the construction of the foundations
will be much slower. A mechanised system of bridge deck erection generally takes at
least 15 months to mobilise, giving a good start to the foundation construction. It then
remains to calculate how many foundation sites must be mobilised in order to have the
piers fi nished in time to receive the deck.
In a highway deck of this length, there may be slip roads joining the main carriage-
way, and railway viaducts may even have stations or sidings, locally increasing the
width of the elevated structure. If the deck is not of constant width, the method of
