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OT: I-35W bridge collapse in Minneapolis

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erikem,I think your assessment is fairly accurate.  As you point out, the bridge design was relatively unique, pushing the envelop on the efficient use of materials, but thus lacking prudent reserve or redundancy.  I speculate that, for this design to be viable, it must be perfectly inspected and maintained, which did not happen.  So it appears that the design may have been structurally viable, but impractical, given the high probability of inadequate maintenance.   I too have wondered about the structural effect of removing the concrete deck, which was underway at the time of the collapse.  I would assume that the structural performance of the bridge would be rated and stipulated completely independently from the deck, but I wonder about that.  If the concrete deck were cast in a way that it became one with the top chord of the truss structure, it would have added compression strength to the top chord, which would stiffen it against downward buckling of the truss.  If this were the case, removing the deck would reduce strength.  It would also remove loading, so it is hard to say how a reduction of strength and a reduction of load would tally up.I can see this possible scenario:  The deck removal would reduce the load, partially offsetting a weakening effect, but leaving a net weakening of some amount.  That amount would be considered acceptable because it was not necessary to the bridge design capacity and its proper safety factor.  However, the bridge had already been weakened by a lack of maintenance, and had lost its safety factor or worse.  So the amount of weakening caused by removal of the deck made the difference between holding and failing.  In other words the strength added by the deck was not an essential part of the design, but became essential as the design failed over time due to structural decay.
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