Civil Engineering Reference
In-Depth Information
As box section bridges are torsionally very stiff, it requires theoretically a misalignment
of only 1 or 2 millimetres between the two slide bearings on a pier for the entire deck
reaction to be carried by one bearing only. In reality this rigidity is mitigated by the
use of laminated rubber (
15.8.10
) sliding pads, which have a small degree of vertical
fl exibility, and sometimes by the transverse fl exibility of the pier. However, it should
be assumed that each slide bearing may carry 75 per cent of the deck reaction, unless
it can be demonstrated that the fl exibility of the pier makes this impossible. The web
should be checked at the ULS under the enhanced launch bearing reaction with a
load factor of 1.33. Also, as it is undesirable that the web should crack signifi cantly
in bending during launching, the tensile stresses in the bending reinforcement of the
web should not exceed 220 MPa under nominal launch bearing reactions. It is also
necessary to check the shear in webs during launching, under the enhanced launch
bearing reaction.
15.8.7 Casting area
The casting area is normally set back some 30 m behind the abutment, so that it is not
affected by the defl ections of the deck as it spans from the abutment to the fi rst pier. If
it is impossible to extend the construction site by this amount, one or two temporary
falsework towers may be placed within the fi rst span to reduce the deck defl ections.
The most critical components of the casting area are the beams that carry the
formwork for the strips of the soffi t that engage with the launch bearings. These must
be aligned to a tolerance of the order of a millimetre as they defi ne the shape of the
bridge deck in both the vertical and the horizontal planes. They usually consist of
substantial steel or reinforced concrete beams, and must be either very solidly founded,
or mounted on levelling jacks.
Usually a deck segment is one-third or one-half of a complete span. For a deck with
regular spans, this simplifi es construction, as special features of the deck such as web
thickenings and pier diaphragms occur at the same place in the formwork for each
successive span. The box deck is usually cast in two phases, bottom slab plus webs,
followed by the top slab. Some contractors prefer to build each phase in consecutive
casting areas as the labour force is less crowded and thus more productive, and the
bottom slab concrete that is engaged by the launching system, is two weeks old when
it is fi rst put under stress, Figure 15.39. However this requires a longer casting area
which may not be available.
The formwork is steel, and the internal shutter must be designed to cope with
thickenings of the web and bottom fl ange, anchor pockets, tendon deviators,
diaphragms and other items. This can become complicated when the bridge spans are
not regular. It is worthwhile using high strength concrete for such bridges to allow
webs and bottom fl anges to remain of constant thickness, as this not only simplifi es the
mould but also reduces the interruptions to the programme.
In order to achieve the weekly cycle, some contractors prefer to prefabricate all or
part of the reinforcing cage. The design for tender of an 800 m long viaduct for the
rear access road to Sizewell B power station, prepared by Benaim for the contractor
(Fairclough), was based on the construction of 30 m long complete spans on a weekly
cycle, with prefabrication of the entire cage.
