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Strain gages were installed during a joint (sorta) U of M and MnDOT study, and are well documented on the MnDOT documents page. The conclusion was that live loads stressed those points nowhere near design limits. Their engineering and methodology looked sound to me.

If I bolt two pieces of steel together, with an extra long bolt, and the protruding end gets bent, that doesn’t affect the fastening, at least not directly.

I feel the same way about the bent gusset plates shown in the images. The bent areas do not affect the gusset’s prime design function, which is to fix the ends of the connecting members.

However...

The bent areas tell me two things, indirectly.

1. U9-U10, at least once since construction, changed its angle with respect to L9-U10, in response to a load.

2. The change was of a magnitude that exceeded the gusset’s elastic limits, the gusset did not return to its original shape.

Item 2 above touches on a critical property of bridge steel, once you change its shape permanently, you have at least approached rupture or fracture levels of stress.

Item 1, and the strain gauges, and the preponderance of interest in the four similar points on the main span, and my long standing interest in the U10 point all stem from the same source. This is a critical area of the bridge. These four points are where stresses reverse, and the tensile stresses, more likely to fracture steel than compressive stresses, max out there.

Most scenarios fall into one of two categories. One, a single max load event sometime before those images were taken in 2002-2003, or two, repetitive flexing over an extended period.

Both could be bad news. One max load event could approach, and possibly exceed design limits, damaging the molecular bonds that give steel its strength, making eventual catastrophic failure a given unless detected. Repetitive stresses can induce metal fatigue, with similar result, and can also be just as difficult to spot.

Any competent bridge inspector should have been able to understand the implications of the bent gussets, without worrying about the direct effect of the bent gussets. The clue lies in what bent the gussets, and why they stayed bent, not the fact that they are bent. A bend right where member meets member would be sigificant, but a bend in the free plate of the gusset is not, not directly.

The free area of the gusset plate DOES contribute to the overall strength of the connection, BUT...if the condition of THAT free area was to have become a problem, it wouldn’t have bent under compression, it would have parted under tension. As seen in the images, it is not under enough tension to straighten the bend, and therefore clearly not under enough tension to fracture there. Gussets plates generally are not used to resist compressive forces, the steel members do that themselves. The gussets plates connect members via tension, and to a much smaller degree, hold alignment through complex internal stresses.

In my opinion, those bent gusset plates indicate something significant happened somewhere besides in the gusset itself, and the visible bending itself is not a primary concern.

Further, again in my opinion, the bent gusset plates indicate to me that the U10 nexus, on both trusses, indicate that the U10 centroid moved enough to change the angle between U9-U10 and L9-U10, permanently. The stresses that caused that and the stresses that resulted from that probably, (certainly, in my opinion) set the gusset up for fracture under tensile stress.

This is a very fine point, and I’m not comfortable with my explanation so far. In simple, non engineering terms, the bent area of the plates is or was in compression, no problem.

However, whatever bent those plates atered either the members and/or the gusset, such that it failed in tension. The deflection of the plates is an indicator that something went wrong, but the failure did not originate in those deflected areas. Instead, the failure originated in a different area of the gusset contradictory to the compressive force that bent them originally, since the gussets failed in tension. There are bolt line failures evident in the imagery, but there are others as well. It would take an expert some time, in close proximity to the plates themselves, to determine which fracture led the pack.

I still stand by earlier statements. U10E failed under tension and precipitated the collapse. The new images reinforce this belief, though I have not yet studied them exhaustively.

I am still amazed at the similarity of fractures between the east gusset of U10 west and the east gusset of U10 east. Its like finding two broken pieces of glass, from two different broken windows, with the exact same shape and dimensions. I understand how similarly the two different plates were stressed, over 40 years, but I’ve never seen two different fracture configurations that similar. Once steel starts ripping, molecular level considerations define the topology of the fracture, and when I post side by side images you will see just how similarly they failed.

If you already know where to look, there are numerous clues to this failure sequence, including the west truss bottom chord, originally shoreward of pier six, which briefly withstood a MASSIVE bending moment, as if the entire mainspan was loaded onto it. Also of interest are the difference between the UxLx verticals on either side of the U10 point. South of U10, the verticals fell over sideways. north of U 10 the verticals endured a sudden maximal compressive load and pretzyled when they absorbed the ground impact of the superstructure.

After I finish my studies, I will post some relevant images with labels, but so far, U10 east still looks like the culprit to me.

Remember, government PDFs should be open source, and you can always screen cap them, convert to jpg, and post them here for reference.