Engineers Examine Minnesota Bridge Collapse - KLTV.com - Tyler, Longview, Jacksonville |ETX News

8/2/07-Longview

Engineers Examine Minnesota Bridge Collapse

An expert in building and structural design at Letourneau University said there are any number of reasons why the Minnesota bridge could have collapsed.  The terrible scene of the I-35 Minneapolis bridge collapse, according to engineers, doesn't have a simple explanation.

"It really is intriguing to me what happened in this case.  Because something must be different yesterday than had ever occurred prior to that," said Letourneau University Professor Robert Warke.

An expert in metallurgy and a former investigator into these type of disasters, Warke said questions need to be asked.

"It has to be built a certain way and it has to be maintained a certain way.  If there is an issue of neglect, if a bridge is designated as structurally deficient and then nothing is done about it for two or three years, that bodes poorly for those maintaining the bridge," Warke said.

TXDOT estimates there are around 50,000 bridges in Texas, all on a two year rotation of inspection. A 2006 TXDOT study listed 2,125 bridges as structurally deficient.

"It doesn't necessarily mean the bridge is in immediate danger.  The term structurally deficient isn't a term that has a hard and fast definition, and in this case is means that there's something needing attention.  It doesn't mean there's an imminent failure," Warke said.

And over 7,800 of them are functionally obsolete. But that also doesn't mean it is dangerous.

"Let's say a bridge is designed to carry horse and buggy traffic, and 100 years later people are trying to drive cars across it.  In that sense it's functionally obsolete" Warke said.

Although the materials used play a big part in whether a bridge is structurally sound, the weather plays an even bigger part. "

Traffic loads, wind loads, snow loads ... all of these things.  The weight of the concrete that's being put on top of the bridge.  All of these things have to be accounted for in the calculation of allowable stresses" Warke said.

But as with any structure, there is a formula for failure.

"Many many bridge's in this country are corroding away, and as they corrode away they're losing their load carrying capability. If one person makes a mistake and two or three people overlook it and assume that no mistakes were being made, that's when you get this accumulation of errors, and you'll have a catastrophe" said Warke.  

Bridges and other large structural designs take into account variables, such as building materials, stress points and the elements.  They are usually doubled in safety standards, depending on specifications.

Bob Hallmark bhallmark@kltv.com

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