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Re: Bent Gusset Plates on MN I-35W Bridge

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On Mar 25, 2008, at 9:25 AM, Christopher Banbury wrote:

I don’t think I would find this alarming unless analysis and testing showed some evidence of fatigue by stress reversal under moving live loads. I suppose a settlement at an abutment could also cause this type of rotation at a truss joint. It is difficult to tell if the beam is loaded anywhere except at the joints.
As you might imagine this made big news here in the Twin Cities, since 'bent' gusset plates were found on another similar bridge southwest of Minneapolis, which was subsequently closed. you may also have seen a rough analysis of some other gusset plates that concluded the plates were too thin (installed thickness was 1/2 inch; should have been 1 inch).

Releasing information in bits and pieces is probably necessary for PR purposes, but it's not giving a very clear picture of what happened. I'd be real careful about making inferences at this point. I'm no bridge designer, but there are some odd things about the failure, that are going to require some pretty serious analysis to draw any conclusions--

--The most salient fact of the failure is the way the center span simply dropped flat without warning, almost as though all four center span supports failed simultaneously, like you'd expect from a sudden overload. --Although one lane had a lot of construction material in place, the traffic load and speed was low because of the repairs in progress. I haven't seen any figures of the actual loading at the time of the collapse, but it's hard to imagine how undergauge gussets in place for 40 years would not have failed earlier.

Any proper failure analysis needs to address these two issues, and it's not something you can figure out from a couple of photographs. I think they're looking in the right place, but a lot more needs to be done. It's a pretty hot political issue here. There is some high visibility litigation in the making and allegations of misconduct by the former DOT commissioner (Lieutenant Governor Molnau who had been filling in at the post for several years, without any real experience other than being a pal of the governor) and squabbling over highway funding levels. Every snippet of information like the gusset plates and the Hwy 23 bridge closing is being microscopically examined and spun about multiple axes. So wait til the real reports (there are two in progress currently) come out.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)

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