Civil Engineering Reference
In-Depth Information
which may span typically up to 45 m, may also be built on a semi-mechanised rig,
although there will be fewer re-uses. Assuming that each span of the wider bridge takes
on average four weeks to build, the twelve spans will be built in about 50 weeks.
If the bridge was over diffi cult terrain or over water, cast-in-situ balanced
cantilevering of box girders could be adopted. The spans could be 70 m, 3 × 120 m,
70 m, for instance. Due to the slow rate of progress of this method, it would be well
worthwhile considering building the deck as one wide box or investing in at least two
sets of travellers.
16.4.2 Precast Tee beam
A very economical method of construction applicable to bridges of this length is the
precast post-tensioned custom-built Tee beam (
10.3
). As they weigh generally between
60 tons and 140 tons, they are normally built in a purpose-built casting yard on site,
and are launched into place with the minimum of handling. Generally a minimum of
50 beams is required to amortise the precasting facility and the launcher.
Six beams at approximately 4 m centres would be appropriate for the full width.
With 14 spans of 35.5 m, there would be 84 beams weighing some 80 t each. On such
a relatively small site, the beams would be placed at the rate they are precast, avoiding
the need for storage and double handling. Thus with say six precasting beds and one
steel mould moved from bed to bed, it should be possible to cast the six beams in
12 working days, although 18 days would be a more relaxed target. The beams would
then all be launched in 3 days. The following 18 days would be used to build the
top slab for this span, and to precast the next set of six beams, giving a construction
rhythm of a span in 4 weeks, or about 62 weeks for the entire deck including an
additional 6 weeks as a learning curve.
16.4.3 Precast segmental
For a two-deck scheme, some 280 box section segments 3.5 m long, weighing up
to 50 tons each would be required, which is at the lower limit of viability for this
method (Chapter 14). A single casting cell working 5 days per week and producing
4.7 segments per week on average would cast these segments in about 60 working
weeks, or 50 weeks for a 6-day week. Erection could be either span-by-span, or
balanced cantilever.
Span-by-span construction of segmental decks is best suited to statically determinate
spans, with dry joints if this is allowed (
15.3.3
). Erection would be expected to proceed
at a rate of between one and two 40 m spans per week on average on this relatively
small site, giving a deck erection period of about 20 weeks. Continuous bridges may
also be built by this method, which was employed for the construction of the 400 m
long twin box glued segmental deck of the Stanstead Abbotts Bypass (
15.3.4
and
Figure 10.4) at a rate of approximately one span per week. However, even for a small
site span-by-span construction needs a relatively sophisticated gantry that requires a
design and construction time of probably 9-12 months.
The decks may be erected in balanced cantilever by crane or by shear legs. Crane
erection can be very fast, erecting up to six segments per day, or three segments per
day on average, taking into account the time taken to place the pier segment, build
the mid-span stitch, hold-ups and maintenance, giving an erection period of some 20
