On August 1, 2007, in the middle of evening rush hour, over a thousand feet of the Interstate 35-W bridge in Minneapolis, Minnesota’s deck truss collapsed. Part of it fell 108 feet into the Mississippi, killing 13 people and injuring 145. The culprit was determined to be a failed gusset plate, which is a flat heavy piece of steel bolted in pairs to join the end of the steel members that make up the truss. The gusset plates were determined to be about half as thick as they should have been, as a result of a design error made decades before.

A design flaw was clearly the major factor in the disaster, however routine inspections of bridges never required an inspection of gusset plates. Bridge capacity, or load rating, was never considered to be effected by gusset plates until this disaster. It was assumed that gusset plates were properly sized to be stronger than the members they connect. In order to assist with the process of analyzing gusset plate strength, guidance was sought in order to “load rate” gusset plates.

Because there was little information on the failure modes of gusset plates, The Federal Highway Administration (FHWA) along with the National Institution of Standards and Technology (NIST), began building full-scale models of bridge gusset plate joints and pulling them apart using a hydraulic test machine. There was a need to measure how the gusset plates failed, which brought about a technology that has been used forever – Photogrammetry. Through the use of photographic measurement of paint specks, digital images allows engineers to watch the failure of key bridge components in great detail.

There is a lot to gain from correlating the digital image method to the FHWA actual measurement methods. First, it tells the FHWA the normal ranges of stress in which the plates can handle and will spring back into their original shape. Second, the method really shows what happens past the normal stress point – when the plate starts to permanently deform and eventually fails. With this new technology, the FHWA has learned a lot about how to predict what loads will cause gusset plates to fail.

The FHWA is working to translate the findings into a language that can be adopted into the Bridge Design Specification and Manual for Bridge Evaluation, two documents that are used in the US for designing and load rating bridges.

See Article: http://www.asminternational.org/portal/site/www/NewsItem/?vgnextoid=49ab76a8253d4310VgnVCM100000621e010aRCRD

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