[jeffers]

I see what you refer to now. The deflection appears to be at least 1X gusset thickness. It shows a decreasing angle between U9-U10, and L9-U10. (Those probably are the strain sensors referred to in post 45, if so, yes, that could easily be the U10 gusset.) How the gusset deflection got that way, and what having it like that did for the bridge, structurally are problematic, at least.

But your third image appears to show that critical evidence has been tampered with, THE gusset that failed the bridge, altered, SINCE the collapse.

Where did that image come from?

[Abigail Adams]

Photos from the NTSB here:

http://www.ntsb.gov/dockets/Highway/HWY07MH024/387406.pdf

The first two photos above show a slight bend in the gusset plates on both sides of the joint. A similar bridge in St. Cloud was shut down a few days ago because the same kind of bending was found in the gusset plates.

[TXnMA]

When you have visited the link(s) below, you will know as much about that third photo as I do...

The second in the series of three photos (with attempts at descriptions) is

HERE.

You can use their arrows to page backward/forward to the other two photos.

I find it to be significant that prior to catastrophic failure, there had already been a condition that induced displacement/deflection/distortion of the critical gusset plates in the area of failure.

1) What stress/relative movement between components could have bent those plates?

2) Could continuation of that deflection in the same direction have led to structural failure of that assembly? More particularly, could increasing the angles of those "bends" have caused failure of the type that you see in the post-failure components?

[TXnMA]

There is a new FR thread with at least one interesting comment re the tradeoff between the number of rivets and gusset plate thickness.

I admit that I have long been surprised at the large number of rivet holes in those visibly thin plates...