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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.