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Steel Designers' Manual - 6th Edition (2003)
152 Bridges
prestressed. The longitudinal reinforcement, however, acts as part of the composite
section. Most composite bridges are designed as ‘unpropped’, i.e. the erected steelwork
supports its own weight and the concrete slab (including formwork allowance)
until hardened, with composite action being assumed only for superimposed dead
and live loads. Box girders tend to be used for the long spans. Popular forms include
twin box girders, multiple compact boxes and open top trapezoidal boxes.
Plate girder flanges should be made as wide as possible, consistent with outstand
limitations, to give the best achievable stability during erection and to reduce the
number of intermediate bracings. For practical reasons a desirable minimum width
is about 400 mm. A maximum flange thickness of 65 mm is recommended to avoid
heavy welds.
4.9.3.2 Intermediate supports
Intermediate supports often take the form of reinforced concrete walls, columns or
portals. Steel supports may alternatively be used, and tubular columns are efficient,
especially if filled with concrete and designed compositely. Where fewer columns
are required for multiple girders then integral steel crossheads at the supports are
sometimes used.
4.9.3.3 Bracings
For rolled beam or plate girder bridges, lateral bracings are necessary for stability
during erection and concreting of the slab. The bracings are necessary at all supports
and when required in hogging regions of continuous spans. If required by the
designer they may be assumed to contribute to the transverse rigidity of the deck
when carrying out an analysis of the transverse distribution of concentrated live
loads. Generally this is advantageous only for decks wider than 20 m. At the abutments
the bracing can be a rolled section trimmer composite with the slab and supporting
its free end. Over intermediate supports a channel section can be used
between each pair of girders up to about 1.2 m depth. For deeper girders triangulated
bracings are necessary.
Intermediate bracings in hogging regions are typically spaced at about 12¥
(bottom flange width).Where bracing is provided across the full width of the bridge,
i.e. between all girders, it increases transverse stiffness significantly. Because of this
stiffness such bracing will attract high stresses under loading that varies across the
width of the bridge. The effect of this behaviour on the fatigue life of the bracing
needs to be considered. Alternatively bracing should be provided only between
neighbouring pairs of girders as shown in Fig. 4.18. This reduces the transverse stiffness
considerably and alleviates the problem of fatigue. Such a structure is also likely
to be easier to erect. If the bridge is curved in plan with girders fabricated in straight