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Lab tests raise fears over bridge parapet safety

FULL SCALE laboratory tests have raised serious doubts over the safety of the UK's 40,000 masonry and brick bridge parapets, a leading academic told NCE last week.

Latest results from more than four years of research at Teesside University suggest that only a minority of such parapets could resist the impact from a slow moving HGV, although most would stop a car.

But even if the parapet restrained the vehicle, the impact could result in sections of masonry being dislodged.

Teesside University director of research and development Professor Brian Hobbs said that there was still no reliable way of assessing the safety of this type of parapet.

'There are big gaps in our understanding of how these masonry structures behave and what is the best way to assess them. Quite frankly, no one has any idea how many would meet today's safety criteria, ' he added.

Hobbs is heading a six year research programme funded by a £330,000 government grant and backed by a number of interested organisations, including Railtrack, London Underground, brick manufacturer Marshall and consultants Mott MacDonald and Curtins. A full scale parapet testing rig has been constructed at Teesside's heavy structures laboratory in Middlesborough, capable of simulating vehicle impacts on test walls up to 20m long and 1.1m high.

These tests are used to validate parapet assessment software developed by the Teesside team (See box). The same rig is used to evaluate reinforcement techniques.

Most were effective in increasing the ductility of the original parapet. However, Hobbs warned that increasing the flexural strength of the parapet too much could make overturning the reason for failure. Adding extra buttresses to the parapet could eliminate this particular risk, Hobbs added.

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