Poor design blamed for bridge collapse
Cutting corners in the design of the US bridge which collapsed into the Mississippi killing several people may have contributed to the disaster, British experts said today.
The engineers pointed out that the interstate bridge in Minneapolis had “low levels of redundancy”.
In other words it may not have been built robustly enough to survive the failure of one of its main elements.
A number of theories have been suggested for why the eight-lane Intestate 35W bridge buckled and broke apart while it was jammed with rush-hour traffic, pitching dozens of vehicles into the river.
One suggestion is that maintenance work may have weakened the bridge. Others point to the weight of the bumper-to-bumper traffic and unusually high temperatures that could have caused the bridge’s steel sections to expand.
However, more than one UK expert believed the structure’s lack of redundancy was likely to have played a key role in its final collapse.
The bridge had three spans, the centre one crossing the wide expanse of the river without supporting piers to avoid obstructing shipping.
Beneath the “deck” of the bridge carrying the road were two lattice-like trusses made from criss-crossed steel girders. It was these that effectively held the bridge up.
Dr Robert Crouch, from the University of Durham, said: “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 redistribution of the forces in the remaining structure. If those adjacent members cannot support the additional loads, then collapse can occur.”
James Parsons, a fellow of the Institution of Structural Engineers – who specialises in bridge design, said: “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.”
Dr Crouch stressed the importance of maintaining bridges properly and taking account of the extra loads they might be expected to bear during their lifetimes.
Like other experts he was surprised by what happened. “It is a surprise when you see a major structure collapse in that sort of way,” he said. “In a developed country you don’t expect bridges to collapse. Clearly it’s a disaster.”
Tom Foulkes, director general of the 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 inquiry.”
Mr Parsons 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.”
He said many US bridges had concrete deck slabs without a surface covering such as bitumen. If the maintenance work had involved replacing these slabs it may have had some effect on the strength of the bridge.
Dr Crouch said he understood the maintenance work involved minor repairs and was not extensive enough to have affected the bridge in this way.
He added that it was vital to establish the cause of the collapse as soon as possible.
“Whenever you have a major structural failure like this, engineers will want to look at the root cause very quickly,” he said. “There may be some element that hasn’t been considered. Very often it’s a combination of a series of events.”
