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OT: I-35W bridge collapse in Minneapolis

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<a href="http://www.designnews.com/article/CA6466909.html">http://www.designnews.com/article/CA6466909.html</a>Here is an interesting article from last fall that discusses the rust-frozen roller expansion bearings, which impeded expansion / contraction movement of the truss.  It says that prohibition of this movement caused the truss to deflect out of plane, and speculates as to the amount of extra stress this loading caused, and whether it pushed the bridge past its design limit.  At one point, last fall, I heard that this condition of locked expansion bearings had caused the force of expansion / contraction to tip one of the piers out of plumb.  However, that is not confirmed by this article.  The following is from the article: Seventeen years ago, the federal government first said the bridge was structurally deficient, citing significant corrosion in its bearings. Seven years later, significant out-of-plane distortion was noted in the main trusses connected to cross girders due to resistance to motion at the connection cross bearings, according to a <a href="http://www.lrrb.org/pdf/200110.pdf">report</a> released by the <a href="http://www.ce.umn.edu/">University of Minnesota civil engineering department</a>. The cracks in the girders were drilled to prevent further propagation. Support struts were also added to the cross girder.Bridge bearings, such as the multiple roller bearings used in the I-35W bridge, are used to <a href="http://www.childs-ceng.demon.co.uk/parts/bearing.html">transfer loads</a> from the deck to the superstructure as weight burdens vary or weather conditions change. Their primary purpose is to redistribute major longitudinal shifts in load.The basic design of the bridge, however, may have made it less able to withstand the pressure of the distortions caused by the corroded roller bearing plates.
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