History suggests bright future for steel bridges

Movable, suspension and cable stayed steel bridges feature in the second part of Alan Hayward’s history of steel bridge building in the UK, published in the April edition of Structural Engineer.

Bridge design specialist Mr Hayward’s articles are based on his 2015 James Sutherland History Lecture. At the start of the period under review box girders fell out of favour and demand for steel bridges was affected until steel contractors developed their own alternative designs.

A key factor in the development of steel bridge design after that period was the investment in automation by fabricators in the early 1980’s which dramatically reduced the cost of plate girders. Steel designs were now cheaper than concrete alternatives and were being selected for highway bridges.

Developments in higher capacity cranes meant that girders could be erected as braced pairs rather than singly, which demanded provisions against buckling. Steel has since then become the natural material for movable bridges, with swing and bascule types predominating.

The most revolutionary development in bridge design during the period was the ‘aerofoil box’ deck of the Severn Bridge of 1966 which was also used on the Humber Bridge which held the world record for a single span – of 1,410 metres – for a number of years.

Cable stayed bridges have since come to be favoured where a suspension bridge might have been previously. Landmark cable stayed bridges included the Erskine and Kessock bridges in Scotland, the QE2 bridge at Dartford and the Second Severn bridge.

Weather resistant steel is widely used in the 21st century, together with ‘integral bridges’ which eliminate bearings and joints. Relaxation of a 7.5 metre headroom restriction meant that weather resistant steel could be used on highway bridges.

Numerous ‘millennium type’ footbridges have been built. Many have been cable stayed and/or curved in plan, often built to an architectural concept, such as the Gateshead Millennium Bridge.

Weather resistant steels, extended life protection systems and integral spans have secured the competitiveness of steel bridges over concrete. Mr Hayward said: “Composite construction exploiting the attributes of steel and concrete seems likely to remain a logical solution over most ranges of span, especially under future site conditions which will be increasingly congested.”