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I-35W collapse leads to call to beef-up US inspections

United States bridge engineers should undertake a greater degree of structural analysis when carrying out routine inspections, America's leading civils body said last week.

The call followed publication of the United States National Transportation Safety Board's (NTSB) interim report on last summer’s I- 35W bridge collapse in Minneapolis, Minnesota.

The 139.6m span steel truss bridge suffered a catastrophic collapse last August killing 13 people.

At the time, an inadequate inspection regime was identified as one of the chief reasons why a possibly critical fault or deterioration of the structure went unchecked (NCE 9/16 August 2007).

Last week the NTSB said that gusset plates holding together eight of the 112 joints on bridge's main truss were under designed, with some of them having a load to capacity ratio of more than two.

The NTSB's interim report on the collapse claims that bridge inspections would have been unable to identify the design fault because they focused on detecting cracks or corrosion rather than errors in the original design.

But American Society of Civil Engineers (ASCE) president David Mongan said: "Maybe a lesson to learn from this is that we should examine all the critical elements of bridges during inspection and before [capacity] alteration, and analyse the bridge as it stands today, not as it was designed."

This extra analysis, added Mongan, would add little to the cost of bridge inspections and alterations. Standing Committee on Structural Safety secretary John Carpenter said the finding showed that in the UK, where much of our bridge infrastructure is ageing, inspection contracts should not be awarded on a cost basis.

"It is important that the money is spent so that bridge inspections are carried out by engineers with experience, and that they are given sufficient time so that they are not doing just minimal checks," said Carpenter.

The I-35W bridge opened in 1967 and was designed by consultant Sverdrup & Parcel, now part of Jacobs Engineering. Jacobs was unavailable for comment.

The bridge was subject to annual non-intrusive visual inspections, and only the beam was checked before two major renovations. Two renovations in 1977 and 1998 increased the average thickness of the road deck from 6.5 inches (165mm) to 8.5 inches (203mm) and central reservation and outer impact barriers were also increased in size.

As a result of the NTSB's interim findings, US secretary of transportation Mary Peters issued a safety guidance that all states should calculate how possible changes in bridge weight or capacity will affect gusset plates on non-load path redundant steel truss bridges.

Of the US's 13,000 steel truss bridges, only 700 are thought to be non-redundant structures similar to the I-35W, so wholesale gussetplate inspection was unlikely, said Mongan.

But the State of Minnesota has begun a review of gusset plates on all of its 59 steel truss bridges. The interim report's identification of a fault in the gusset plates placed design and engineers rather than publicly funded maintenance in the spotlight.

As a result it drew criticism from University of Pittsburgh assistant professor of civil engineering Kent Harries. "They haven't identified the mode of failure," said Harries. "The structure stood for 40 years without deformation or distortion, why did it fail when it did? This report tells us very little."

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