Photos of gusset plates

Last summer when there was the Minneapolis bridge collapse, I was thoroughly annoyed because I knew I had some really good photos of the bridge intact from underneath, with a focus on the steel, and I simply could not find them.  Today I randomly stumbled across them, so here we go.  From underneath, taken on the side where St. Anthony main is (and where I used to live just a few blocks from here!) and dated 24 March, 2003.

Examples of gusset plates, which have been blamed for the collapse, are clearly shown in all three images.  I’m struck by how narrow the beams look for their length compared to something like a sturdy rail bridge, and how thin the gusset plates are at all such that I’m still having a hard time accepting that as the answer to “why the bridge fell down”… but at least I know now I was not crazy, I really did have these photos somewhere.  And I want to see the finite element analysis that demonstrates the gusset plates as the culprit.

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5 responses to “Photos of gusset plates

  1. But why would the fact that the gusset plates are so thin make it hard to believe that they gave? Wouldn’t the thinness maske it easier for the bolt heads to tear through the edges of the holes under lateral (i.e., facial) stresses?

  2. It’s not that the fact that their thin makes it hard to believe that they would be the weak link. It’s the idea that since they are so thin, the idea I am having a hard time with is that they were dominant load-bearing structures in the first place. They could only be the weakest link in bridge failure if they carried considerable loads in the first place, in their role of load-transfer between the beam members in the overall truss structure.

  3. If I recall, there was concern about the plates such that the consulting engineering firm that led the maintenance inspection process explored ways to reinforce them, which sort of suggests that they recognized the fundamental design flaw (overreliance on inadequate design margin on the gusset plates) that comprised the greatest risk to structural integrity.

    The gusset plate upgrade was ruled out not due to a desire to save on or avoid cost (a common politically-driven accusation from the “raise taxes to rebuild the infrastructure” crowd, for example, the utterly moronic James Oberstar) but because the new bolt holes required would have weakened the plates still further rather than improved anything, and they were not weldable because of the particular type of steel (super-light, high-strength). They couldn’t remove and replace gusset plates with thicker ones because of the central role each one played in the overall structure of the bridge. Doesn’t that add up? Could you be reluctant to conclude the obvious because you may be horrified to think that the design was that obviously shaky?

    That confluence of factors seems to me to suggest that the gusset plates were a major cause, and the design was tragically inadequate back in 1967.

  4. The fact that the bridge was poorly designed from the outset–this has been my opinion all along. But if they were really serious that there was such a flaw in the gusset plates that the bridge was structurally unsound, they WOULD have done something about it. They would have built a secondary steel framework or removed weight from the bridge. I’m guessing they noticed some out of plane deformations in a few gusset plates, thought it odd, but that would be a symptom of other structural problems, not a primary factor. So no, I’m still not convinced. Besides, around any circular hole in a plate you have a stress concentration of three times the stress that would not be present without the hole. Those gusset plates in my photos are swiss cheese. More than anything, they are so perforated already that there is no where else to put additional holes, and I’d guess that’s to what you refer. I’d say in this case, if there is anything about the beam joining to consider more closely, think like the recent discoveries about the titanic and have a hard look at the rivets used AT the gusset plates!

  5. Pingback: Lazy blogging (but fun) « Not From Around Here

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