I really wish that the photos on page 18 (two views of the same object[s], BTW) had something to indicate scale. Even though the article referred to the "thin" plates as being 1/2 inch thick, I just can't envision the thickness of that torn gusset plate (on p18) as being anything over 1/4 inch -- if that much...
If that plate is 1/2 inch thick, then those rivet heads must be three inches or so in diameter...
Do you see anything there that would be a good indicator of scale?
Thank you both for your diligence and for sharing your expertise.
Yes, you understand correctly.
The force which bent the gussets was compressive, because the angle between top chord and diagonal lessened. If U10 had returned to original position, it would have pulled the gussets back straight, or at least straighter. But it didn’t.
Whatever moved U10 (downward) in the first place, causing the bent gussets, whether a single event or prolonged abuse, also transmitted significant tension up the U10-L11 diagonal, and thence along the U10-U9 top chord back to...well eventually back to concrete on shore.
That may or may not be the cause of the fatal fracture later on. The triggering failure, in my opinion, was the vertical fracture in both U10E gussets, just south of the U10 vertical strut.
Those plates look like half inch to me. Three quarters on some boxes and H beams, perhaps on the pier gussets. I’ve cut both and the pics look right to me. Those rivet and bolts are pretty big, bigger than your fist is not uncommon at all. Grapefruit sized hex-nuts aren’t rare either.
And here is the latest info with pictures of the bridge gusset in question, from pre-disaster photos.
Pictures show buckling, article discusses falsified inspections.
http://www.freerepublic.com/focus/f-news/1990455/posts