Engineers puzzle over bridge collapse

Minor repairs were under way at the time of the collapse

Engineers are trying to understand what caused the catastrophic collapse of the bridge over the Mississippi river in Minnesota.

Resurfacing work was taking place, but the bridge was last inspected in 2006 and no significant structural problems were found.

Such complete bridge collapses are a very rare occurrence.

If they happen, it is either because the load is too heavy, or the connections between the bridge's structural elements are too weak, Keith Eaton, chief executive of the UK's Institution of Structural Engineers, told the BBC.

"The engineers will have to see where the collapse started. Clearly a failure occurred somewhere which imbalanced the whole thing," he said.

Speculation that hot weather contributed to the accident by weakening the concrete or expanding the steel framework was not a likely explanation, he added, as modern bridges are built to cope with extremes.

A crack in the steel making up the bridge's structure was the most likely explanation for the disaster, he said.

Corrosion

The I-35W highway bridge (Bridge 9340) was built using a framework of rafters, posts and struts - a structure known as a truss bridge.

In 2005, it was one of thousands across the US rated as "structurally deficient" on the federal National Bridge Inventory database.

It rated 50 on a scale of 100 for structural stability in that study, White House press secretary Tony Snow said.

About 140,000 cars are thought to have used the bridge every day, but a 2001 report by University of Minnesota's civil engineering department found traffic levels were below those the bridge was designed for.

See graphic of the bridge collapse

The report went on to express concerns that a single crack in the main truss could "theoretically" lead to the entire bridge's collapse.

However, it also said that even if there was a crack, the load could "theoretically" be redistributed along the steel trusses or the concrete deck of the bridge, keeping the bridge aloft.

It added that no fatigue cracking had occurred, and that the bridge "should not have any problems with fatigue cracking in the foreseeable future".

The bridge crossed the Mississippi River near downtown Minneapolis

The state need not "prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project".

The truss bridge was built in 1967, with eight lanes over a span of 581 meters (1,900ft). It had no piers in the water, allowing easy passage for river traffic.

While no longer the cutting edge of bridge design, truss bridges are relatively cheap to build, and were a very popular structural choice in the US in the 1960s and 1970s, Mr Eaton said.

They have a downside, however.

"They are made of lots of complex pieces of metal, interconnected bolts or rivets," Mr Eaton told the BBC.

"They have little corners between two pieces of steel where water can collect and cause corrosion."

Nesting pigeons could also be an issue.

"Their droppings are very corrosive, which can be a problem," he said.

Return

---