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US bridge bash exposes pier protection risk


AMERICAN HIGHWAY engineers ignored bridge protection guidance which could have prevented the collapse this week of the Arkansas river bridge at Webbers Falls, Oklahoma .

American Association of State Highway and Transportation Officers (AASHTO)'s 1991 design code recommends that engineers provide protection for all bridge piers rather than those on either side of a shipping lane.

Twelve people are thought to have died on Sunday after a barge struck a pair of piers supporting the side spans of the bridge carrying Interstate Highway 40 (I40), bringing down a section of the deck. Those killed are thought to have been crossing the bridge at the time of the accident.

Built in 1967, the bridge at Webbers Falls is constructed from concrete piers, steel beams and a concrete deck. It was protected by a pair of dolphins upstream of the piers supporting the span which crosses the main 91m wide navigation channel. No other pier protection is evident.

The AASHTO guidance, published 24 years after the I40 bridge was built, warns engineers that shipping is more likely to crash into multi-span bridge piers outside main navigation channels.

The four lane bridge's supports are generally full width piers. But supports hit by the barge were close to the bank and comprised weaker pairs of twin columns.

This is the sixth such bridge bash in the US over the past 22 years, bringing total lives lost in such incidents to 95 (NCE 20 September 2001).

The 150m long unladen barge which struck the bridge was travelling upstream at a speed of around 8km/h. Its pilot is believed to have suffered a seizure, causing it to veer off course.

Once struck, a 150m section of the fourlane bridge deck immediately fell 20m into the river.

'The mass of the barge was only 20% of a fully laden, full size tow that often use this route, ' said ICE maritime board chairman and Arup associate director, Greg Haigh.

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