2011•35th Annual Symposium of IABSE / 52nd Annual Symposium of IASS / 6th International Conference on Space Structures: <i>Taller, Longer, Lighter - Meeting growing demand with limited resources</i>, London, United Kingdom, September 2011Requires access
Bearing replacement and strengthening of Forth road bridge approach viaducts
Barry Colford, Manuela Chiarello, Chris Hendy, Jessica Sandberg, Paul Smout
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Abstract
The existing Forth Road Bridge spans the Firth of Forth in Scotland. Construction of the bridge was completed in 1964. The main structure is a three span suspension bridge with a central span of 1006m and side spans of 408m. At each end of the bridge there are two multi-span approach viaducts leading up to the main crossing. The deck of the approach viaducts comprises a pair of longitudinal steel box girders supporting a series of transversely spanning steel girders, both act compositely with a reinforced concrete deck. The steel girders of the approach viaducts are supported on steel roller and rocker bearings. The bearings are fixed to reinforced concrete portal piers which are founded on rock. These piers vary in height between 11m and 40m. An initial study of the bearings identified that the rollers had locked up due to corrosion and distortion, and the concrete beneath the bearings and elsewhere on the pier tops had deteriorated due to chloride contamination. Assessment showed that structural deficiencies in the pier were exacerbated by both the concrete deterioration and change in articulation. These factors lead to the decision to replace all the bearings on the viaducts. The original structure was not designed to facilitate replacement of the bearings so the structure had to be strengthened and modified by the addition of jacking stiffeners and corbels to the pier tops. Initial design was carried out to BS5400 leading to substantial amounts of strengthening being required. This was then repeated using Eurocodes to gain better structural efficiency and reduce the amount of strengthening required to the deck and piers. This paper outlines the design of the strengthening and modifications to the bridge to facilitate bearing replacement, together with a detailed description of the design of the temporary works needed to maintain the bridge’s articulation during jacking. Lessons learnt during construction are discussed and the structural benefits of design to Eurocodes are emphasised.