Aug 8 2007 2:36PM | Permalink | Email this | Comments (1) |
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One of our great contributors and technical resources at Design News  is Kenneth C. Russell, professor emeritus of metallurgy and nuclear engineering at MIT. I asked Ken, in view of his experience in metallurgy and failure analysis what thoughts he had off the top of his head on the I-35W bridge collapse in Minneapolis. His response is below:

“Just for the record I speak only for myself.  MIT will scissor the shorts of anyone other than the president who speaks for the institute.  I am free to express my opinions anyway I want.

My expertise is metallurgy.  I got some construction training in ROTC and as an Army Engineer Officer.  My position is that such a collapse should never occur in a prosperous democratic country.  There are two many  professional societies and licensing boards and the like in place for the collapse to occur.  (Chernobyl, for example, would have never been licensed in a democratic country.)

That said there are a bunch of things that could have gone wrong. You can start from the ground up.  Were the footings in all way properly designed and constructed, and was there any sort of erosion or shifting?  The civil guys will answer those questions.

Consider concrete.  I learned in ROTC that the concrete business is rife with corruption.  Concrete is a huge part of such a project and the financial returns on cutting corners are huge.  Some Big Dig contractors were caught adding water to concrete that was starting to set up in the Redi-Mix truck.  Such is an absolute no-no as water content must be precisely controlled to give good concrete.  The concrete should have been dumped, which would have cost $$. I am not surprised at such chicanery in such corrupt states as  Massachusetts
or Louisiana.  

 
I doubt that the steel was faulty.  The steel furnace makes many tons of steel at a batch.  If any part is off in composition or in rolling technique the entire batch is off.  A simple tensile test now and then should assure that the steel was OK.

Did the bridge deteriorate faster than expected?  Certainly the traffic was a lot heavier than expected.  Salt and traffic could have done a job on the concrete but that is not really my bailiwick.

Metal fatigue has been mentioned a lot.  I have not heard much of fatigue being a problem in bridges, but repeated stresses will give fatigue.  I might mention that steels fatigue a lot faster in a corrosive environment than they do in dry air.

The bridge was given a visual inspection a few years ago.  I consider this a copout as regards fatigue.  Fatigue cracks are closed most of the time and may easily be hidden by paint rust, or dirt.  In those cases visual inspection would be pretty well useless.   You need something that will see inside the steel such as x-rays or ultrasonics.”

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