August 2, 2007

experts respond to USA bridge collapse

The motorway bridge in Minneapolis collapsed into the Mississippi river.

Professor Roger Crouch, Chair in Civil Engineering at Durham University, said:

“It is important that the true cause of the bridge collapse in Minneapolis is quickly discovered so that civil engineers can re-examine similar constructions worldwide.

“While unlikely to be the principal cause of the failure, it was reported that the temperature in Minneapolis had been very high recently. If not properly designed, the expansion of the steel sections exposed to direct sunlight could lead to additional stresses which would exacerbate the problem.

“Structural engineers anticipate all plausible forces when designing bridges and add load factors for the material strength and applied loads to achieve the desired level of safety. However, design and construction are not the end of the story for a bridge engineer. A programme of regular inspection and maintenance is essential.

“This is the somewhat less glamorous, but vital part of engineering. Inspection requires skilful interpretation of defects caused by accidental overload or corrosion.

“There are some concerns that this bridge had a low level of redundancy. If this was the case, it could mean that the structure might not survive failure of one part of the bridge, and might lead to a progressive collapse of the complete span. Local overstressing of a compression member in the arch truss could lead to buckling and a sudden re-distribution of the forces in the remaining structure. If those adjacent members cannot support the additional loads, then collapse can occur.”

James Parsons, Fellow of the Institution of Structural Engineers, and Partner of Cass Hayward LLP, who specialises in the design of bridges, said:

“Bridge collapses of any kind are very rare and usually have an obvious cause such as a truck running into a support pier or flood water undermining the bridge support structures. That does not appear to have been the case here. In order to determine the cause of this collapse the first step will be to determine what part of the bridge gave way first and the sequence of events that brought the whole bridge down. Eye witness accounts will be a very important part of this process.

“There have been reports that maintenance work was taking place on this bridge. It is difficult to say if this was a direct cause of the bridge collapse however many bridges in the US have concrete deck slabs without a surface such as bitumen over the top and if the work involved replacing these concrete slabs this could have had some effect on the strength of the bridge. The peak hour bumper to bumper traffic may also have played a role, but the maintenance works had closed some of the lanes to traffic.

“The main portion of this bridge has three continuous spans, but only two trusses in its cross section, which means that if a part of one truss fails then as there is no redundancy and the whole bridge will come down. While this bridge is only 40 years old most of the bridges in the UK are built to last over 120 years and new bridges are designed to have redundancy in their construction to prevent just this sort of event.”

Tom Foulkes, Director General, Institution of Civil Engineers, said:

“This is an amazing collapse. I have never seen anything like it before. For the entire central span to collapse like this is most unusual, possibly unique. The design of steel lattice bridges like this one is pretty well understood these days, so a catastrophic failure on this scale is almost certainly caused by some specific event or condition that has only just occurred. Major failures are often due to a combination of factors, rather than a single cause. Fatigue cracks; corrosion; ship impact; bearing failure; scour or damage to the supports – all these could contribute. And if the bridge had been undergoing repair work or maintenance at the time of failure, then that would be another important line of enquiry.”