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What the heck, I’ll toss my two cents in here too.

I don’t disagree with anything Jim has posted, but because I’m not a licensed engineer, I’m willing to stick my neck out a bit further.

Between cracking, and frozen bearings, I like frozen bearings for collapse initiation better, for two reasons.

1. The cracks were inspected regularly, records kept, trends noted, and repairs made, in at least some cases. To fail the bridge, either a new crack would have had to appear since the last inspection, or else a known crack would have had to change behavior since the last inspection. Frozen bearings, however, could have failed the bridge by inducing greater than design loads, either by remaining frozen, or by popping loose at the wrong time. It’s a fine line, but in my opinion, frozen bearings are more likely to fail the bridge just by being frozen, than cracks are likely to fail the bridge, as long as they crack at historical rates. Frozen bearings do not require a change in recorded behavior to fail the bridge.

2. The most significant problem in this bridge’s history were cracks in the east end of the crossbeam/endbeam/rocker bearing assemblies, both north and south, directly caused by a frozen rocker bearing at the east end of the south crossbeam, in 1986. The bridge was closed, jacked up, the rocker bearing was replaced, crackes were drilled out, and plates were bolted onto the cracks caused by the frozen bearing.

The 2006 inspection report indicates the exact same (structurally speaking) bearing froze, except this time on the west end of the south crossbeam, but the bridge was not closed, the superstructure was not jacked up, the frozen rocker bearing was not replaced, cracks were not bolted or plated over or drilled out, in short, no corrective action was taken or recommended.

I’m also willing to localize the initial point of failure. Like Jim, I believe a tension member failed first, and that member could have resided in the first, second or third panel of the east truss north of pier six, in the main span of the bridge, as Jim suggests. It also could, however, have resided in the sway bracing between the east and west trusses at pier 6.

Failure in the west truss, one or two panels north of pier six is also possible, but less likely as a triggering event.

Failure at the pier 5 crossbeam, west end, from 2006 damage due to the frozen west rocker bearing, or east end, from the 1986 east truss damage, is also possible, but third in liklihood behind the other two.

The 2006 report also noted that the roller nest under the pier 6 east truss was possibly frozen, and this area appears to have been critical in the failure sequence.

Guesswork, speculation and supposition, but if I had to call it, I’d say that greater than design loads were imposed throughout the trussed spans, by frozen expansion bearings at multiple points, and that tension members at one of three points:

...in the sway bracing between the east and west trusses at pier 6,

...or the top chord of the east truss between pier 6 and the second strut north of pier 6,

...or the diagonal truss member in the east truss, running down towards midspan (span 7), from the pier 6 kingpost or just north of the second strut north of pier 6,

...were the triggering failures in the collapse sequence.

A gusset plate failure under tension at any of the above namjed points is included in the likely failure mechanisms.

Obviously, a crack at any of these points could have failed, but in my opinion, the triggering event was the excessive stress imposed by the frozen bearings that caused these cracks, not cracks due to rust, corrosion, or other reasons.

Am I positive?

No.

Am I positive I believe one of these three were the most likely initial trigger points?

Yes.

Does Jim agree?

It’s up to Jim to say yes or no, but reading between the lines seems to indicate that he does.

Am I positive that I believe the most likely direct cause of failure was frozen bearings, and/or resultant cracks, rather than cracks due to other causes?

Yes.

Does Jim agree?

Let’s ask Jim.

Keep in mind though, when reading his answer, that a licensed professional engineer has a lot more to lose in risking a guess, than a retired construction stiff who quit school before earning an engineer’s degree due to financial reasons.

If a working engineer guesses wrong, friends and/or clients get crushed.

If an old salt guesses wrong, sitting around the cracker barrel in idle speculation, he might look silly when NTSF issues a final result.

That’s a significant difference.

If Jim refuses to speculate, it should be taken as professional concern, and judicious restraint.

> Does Jim agree?

I am not in any position to agree or disagree. Neither one of us knows enough to be specific. Only the investigation will tell for sure.

However, it is possible your scenario is correct. I stated that there was a crack-through in a reversed-stress or tension member. The REASON for that failure could have been many other things. The frozen bearings could have been the trigger that initiated the crack-through. Frozen bearings will induce stress in the bridge. They can vary from a lot to nothing. It could be a lot of stress at one of the cracked reversed-stress or tension members.

That is possible, but I think somewhat of a stretch. Virtually EVERY bridge I have seen — even some that were less than 10 years old — had bearings that were in bad shape and well onto their way of being frozen. Older bridges almost ALWAYS have frozen bearings. Usually frozen bearings cause minor problems, such as the concrete deck spalling out at the expansion joint. I am not saying that it is impossible that it caused the failure, but I am saying I don’t know how that trigger caused the collapse.

Your ideas are not impossible, but until the report comes out, neither one of us will know for sure. BTW, the newest bridges have a completely different way of containing the expansion and contraction. In 30 or 40 years, we will know if it works better (although it cannot work worse than what we have now).

I might mention that I think the construction may have played a part in the trigger, but not like the news people are saying. They are saying that weight or vibration caused the problem. I don’t think so. They were replacing the deck, not just doing an overlay.

THIS BRINGS UP ANOTHER POINT! If they were redecking the bridge, they were planning to keep it there (not replace it) for at least another 10 to 15 years. Anybody planning that was unbelievably stupid.

Anyway, back to my scenario. What I wonder is if enough of the deck was removed to reduce the sway strength of the bridge. The bridge did curve. When vehicles drove on it, they exerted sideways forces on it. The deck took those loads. If enough of the deck was removed, it would not have had enough strength to resist those loads. That would have pushed the vertical trusses out of plumb. They are very strong vertically, but NOT strong at all towards any out-of-vertical forces. That would have led to a failure in a reversed-stress or tension member.

At this time, all this is speculation. What is NOT speculation is that the higher-ups in MNDOT are incompetent boobs for thinking they could keep that standing for another 10 to 15 years.