Engineering and Construction

A minimum clear deck width of 1.8m is usual, increased to 3.0m or 4.0m in busy areas or if a cycleway is also present. Steel provides an efficient solution because the entire cross-section including parapets may be fabricated and erected in one unit. For this reason multiple spans tend to be simply supported and not continuous. Staircase and sloping or stepped ramp spans are also erected as complete units, and columns are often of steel.

Half-through cross-sections are often used because the shallow  construction depth is able to provide the shortest lengths of staircase or ramp approaches in urban areas. Half-through rolled beams, Warren truss or Vierendeel girders are  used with rolled hollow section (square or rectangular) members. Floors are steel stiffened plate, often surfaced with a factory-applied epoxy non-slip surfacing ap- proximately 5mm thick. Rigid connections between floor and girders provide for U-frame stability. Staircase approaches may be either steel stringers supporting  steel plate treads or a central spine box with cantilever treads of steel or precast concrete. Ramps may be similarly formed, but where the span exceeds about 10m then half-through construction tends to be used. Spiral approach ramps can be used, formed in steel using twin rolled sections curved in plan supporting a composite or steel plate floor.Where adequate construction depth is available, for example when a pedestrian bridge is required across a motorway or railway cutting, then deck cross-sections are appropriate with a composite or steel plate floor. Precast concrete  floor units are also suitable.
Primary members may be a single box girder, twin  plate girders or twin rolled beams.A single box girder provides a structure of neat appearance.

Fig. 4.12 Pedestrian bridges. (a) Twin universal beam; (b) box; (c) Vierendeel or Warren girder; (d) half-through universal beam; (e) half-through plate girder; (f) through lattice