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The concrete & the play...

It may come as a surprise to know that concrete can behave plastically, yet this fact has recently been put to great use, to the benefit of millions of tax-payers, by Dr Campbell Middleton's team of bridge researchers in the Structures Research Group. 'European legislation, agreed to by the British Government, has led to the raising of the maximum load-carrying capacity of lorries from 38 to 40 tonnes in 1999. Although this change is relatively small, and in itself has little effect on the assessed capacity of bridges, it has initiated a major evaluation of the safety of the nation's 150,000 or so bridges, as many of them were designed and built at a time when even 30-tonne loads were probably not envisaged,' explains Dr Middleton. 'Current professional practice relies on elastic methods of analysis which define failure on the basis of failure of any single element of a component. This proves to be an extremely conservative and perhaps inappropriate approach for estimating the safety and reliability of some types of concrete structures.'

Well, Pooh, what do you think, is that a 38 or a 40 tonner? (with apologies to that nice Mr Shepard...)

Realistic

Dr Middleton's team has been investigating ways of providing a more realistic approach to determine the actual load-carrying capacity of concrete bridges. One outcome of this has been the development of a new plastic analysis computer program, using recent developments in computer graphics and solid modelling techniques, which can be used for plastic analysis of concrete bridges in bending. This advanced analysis program (known as COBRAS) has already been used to re-assess the strength of a number of bridges that had previously been condemned as unsafe using conventional elastic analysis. Nearly all were found to have substantial reserves of strength, when re-analysed using the collapse analysis technique, saving many millions of pounds of tax-payers' money. A series of concrete slab models has been tested to destruction to validate the theoretical predictions of the program.

This research is now being extended to cover other types of bridges and failure modes. This is a complex task because of the variability in design and amount of degradation experienced. However, costs saved have paid for the program development many times over.

Further information from Dr Campbell Middleton Tel: (01223) 332814