Design of a Four Span Self-Anchored Arch Supported Stress Ribbon Footbridge

This paper describes the lessons learnt during the design of a 65 m long continuous four span self-anchored arch supported stress ribbon footbridge, with a maximum span of 21.2 m. The bridge has semi integral abutments and a 220 mm thick concrete stress ribbon deck that spans up to 12.6 m between the crests of the arch sections. The footbridge was commissioned by the South African National Roads Agency SOC Ltd. (SANRAL) to improve pedestrian safety along the N2 highway near George.

Although a simply supported or continuous beam structure was appropriate for the span lengths, the client requested a design that responded to the context of both the local coastline and the highway. The chosen form, however, offered some practical advantages. Firstly the bridge is semi-integral with no expansion joints. Secondly the structural system is self-anchored and both the concrete deck and arch sections remain in compression. This is of course achievable with any prestressed section. The advantage in this instance was that a 900 mm beam could be replaced with a slender arch and deck slab.

As a one-off structure the design approach was an iterative one that tested the assumptions made during the analysis. Parabolic versus circular arch geometries were investigated as was the required prestressing force for various deck sags. The non-linear analysis considering the long term creep and shrinkage effects was carried out in LUSAS. Methods of adding the transient load effects at various stages in the time history were investigated with the aim of reducing the analysis effort.