Civil Engineering Reference
In-Depth Information
Comments in
16.6.3
concerning the mobilisation period for a gantry apply also to
the mechanised falsework, which if anything is even more complex.
16.7.2 Precast Tee beam
See the comments in
16.8.3
on using precast Tee beams for very long viaducts.
16.7.3 Precast segmental
With a deck area of over 40,000 m
2
, this is prime precast segmental territory. A two-
box scheme would require over 1,100 segments 3.5 m long. On the assumption that a
gantry would place its fi rst segment in month 15 of the contract (
15.5.10
), and would
then proceed to place 75 segments per month, the fi nal segment would be erected in
month 30. As the last segment must be cast in month 29, and the fi rst in say month 5,
the required rate of segment casting must be 1,100/24 = 46 per month. At a casting
rate of 20 segments per cell per month, 2.3 cells are required, meaning in reality three
cells with one commissioned later in the programme, or a later start to the casting
programme.
As the casting starts 10 months before erection, it would be necessary to store
approximately 460 segments, a major expense. To reduce this storage requirement
it is likely that it would be economical to start casting later using all three moulds.
The casting rate would be 60 segments per month, giving a total casting period of
19 months. Casting must then start in month 29 - 19 = month 10. The storage
requirement would then be reduced to only fi ve months casting, or 300 segments. This
requirement would be further reduced if 6-day working were adopted for the casting
yard. If four moulds were used, the rate of segment casting would equal the rate of
erection, and storage would be reduced to a nominal buffer.
Clearly these fi gures are somewhat theoretical, but demonstrate the mechanics of
deciding on the number of moulds necessary and on the storage facilities required.
As an example, the 2,700 m long fi rst stage of the STAR Viaduct (
15.5.11
) required
approximately 1,000 segments and adopted six moulds which were commissioned
progressively. The 1,700 m long Dagenham Dock Viaduct (
15.5.12
), which had two
or three boxes in the cross section and a total of 1,030 segments, was cast with four
moulds.
The size of this viaduct makes a single-box scheme more viable. The extra cost
of the handling equipment to manage the heavier segments could well be more than
offset by the savings in manpower in the casting yard and on the erection front, which
include eliminating the connection between the decks. Furthermore, potential savings
in the cost of the piers and foundations would most probably make this the favoured
solution. A single-box scheme would have 570 segments 3.5 m long and the deck
could be erected in 8 months with a single gantry. Using the same mathematics as
above, erection would be from month 15 to month 23, and casting with two moulds
from month 7 to month 22, giving a requirement to store 8 months' production, that
is 320 very large segments. It would probably be necessary to adopt three moulds to
reduce the storage required. The overall deck construction time would be controlled
by the time to commission the gantry and by the erection rate it could achieve, and
may be estimated as 15 + 8 = 23 months using the above assumptions.
Erection coul be span-by span if the spans were suffi ciently short and regular, or in
balanced cantilever which can cope with virtually any span range.
