Re your question B:
“Does this indicate that the western kingpost collapse was early in the collapse sequence?”
Yes and no. It means the road deck didn’t fall and bring the truss with it as a result, but we’ve never looked at that possibility anyway. “What’s under what else” is usually only a good indicator as to sequence when both pieces started out at the same elevation level. The kingpost began life under the road deck, it ended life under the road deck, it’s hard to draw useful conclusions from this.
Re question C:
“What does this say about the various moments in play here?”
Indeterminate. Not sure what that member is, therefore speculation about it’s failure mode is difficult and inconclusive. From size and placement, it could be part of the sway bracing between the east and west pier 6 kingpost, or it could be part of a deck truss, or it could be a deck stringer.
The image shows it subjected to either bending moment or compressive overload. If it’s part of the sway brace assembly, it probably indicates that the kingposts did not fail in unison. The truss tops could have leaned towards each other, or one could have buckled first, either mechanism could produce the deflection show in the image.
If it’s a chord from a deck truss, it might have folded during failure when something else hit it on the way down, on final impact, or, it could indicate that the deck truss itself failed, allowing deflection between the main trusses.
If it’s a deck stringer, that span of deck, between deck trusses, could have failed from point loading, (unlikely in my opinion) or impacts during the collapse could have affected it.
Speculation, it’s a sway brace crossmember, and was all or part of the trigger event in the failure, or else it merely indicates that the south kingpost buckledor leaned east before the west kingpost rotated off the pier.
Best I have, sorry it isn’t more.
Interesting note...the member you indicate at yellow arrow B is almost certainly the top chord from the west main truss, from just south of pier 6. Your dashed lines are either improperly drawn, or else something significant happened at its connection with the west pier 6 kingpost. I suspect the former.
There is visible deflection just above and left of the yellow arrowhead. I suspect this trend continues underneath the road deck, and that this member remains attached to the top of the SW pier 6 kingpost, further “down” in the image.
This would indicate even more deflection that what is visible, further supporting my contention that the west pier 6 kingpost rotated, base northward, at or during main collapse, as previously noted by red arrows 1 and 2.
Could have happened two ways:
1. South end of span 7 shears off and drops, relieving cantilever counterbalance weight to the main trusses just south of pier 6, such that span six deflected in the center, rotating one or both main truss assemblies at pier 6, just as we see on the north side of the river at pier 7.
2. Crossbeam/endbeam/rocker bearing failure at pier 5 relieves span six of its southern cantilever counterbalance, allowing span 6 to deflect midspan, rotating one or both main truss assemblies at pier 6, and severing same from the southern end of span 7.
There are probably other sequences to account for what we see, but few that also incorporate the bridge’s significant history, namely chronic and severe problems at the expansion bearing assemblies.
Of the two, I prefer sequence one, though I have no strong evidence to substantiate that preference. That evidence would be underwater. However, the main span would necessarily be subject to greater stresses, and in my opinion, subjected to greater chance of member failure due to increases in loading brought about by frozen bearing assemblies.
Important to note, either span 7 truss, east or west, could have failed first. If the east truss let go at river’s edge, most of span 7’s southern gravity loading would have applied to the west truss and could have severed it. Same applies if the SE kingpost buckled first.
On the other hand, if the west truss sheared off first at revier’s edge, the span seven loads would be applied to the east truss, possibly failing it, or concentrating load on the east pier 6 kingpost buckling it.
My preference has the failure on the east side first. Failure of the west truss would have tended to rotate the east truss kingpost to the west, not the east as we see in the imagery. East truss severed, one or two struts north of pier 6, east pier 6 kingpost buckled, or sway bracking between east and west kingposts let go, or a combination of all three was the first major components to fail, in my opinion.
Something small could have failed elsewhere, concentrasting loads in this area, but this was the first major piece of the bridge to fail, in my opinion.
If not, look back around pier 5, but that area appears to be a distant second place to me.
Main failure, east truss or sway bracing, at pier 6 or within two panels north of pier 6.
Best I have for now, I’ll stay with that educated guess till I see something new to make it unlikely.
Thanks very much for your efforts in posting these analysis details for me -- and for all FR posterity!
My interpretation of "B" was that it was the edge of the western deck. I am going back to Canvas to annotate what I believe to be the probable resting position of the top chord on that side.
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Meanwhile...I don't know if you are following all the (four -- at least) FR threads where failure analysis comments are being posted, but in the thread,
Enginers puzzle over bridge collapse
Ronzo has posted some very significant photos of the rollerplate mechanisms atop the piers that I think you will find to be very informative:
1) http://www.freerepublic.com/focus/f-news/1876107/posts?page=105#105 has a superb "before" closeup of the western rollerplate assembly and kingpost base -- from ca 2002 -- showing significant corrosion there. The detail on that assembly atop the northwestern (riverside) pier is outstanding -- and informative!
2) In http://www.freerepublic.com/focus/f-news/1876107/posts?page=128#128", Ronzo has identified what appears to be the "shoe"/"rockerplate"/rollerplate" (missing from the northeastern pier top) -- lying on the ground!!.
3) In http://www.freerepublic.com/focus/f-news/1876107/posts?page=131#131, Ronzo posted a 'before" photo of the northeastern piertop and rollerplate assembly that appears to show significant corrosion there -- and the likely cause. He shows a drainpipe coming down that pier, and, just above the rollerplate, the drainpipe is disconnected and offset, allowing deck drainage to fall directly on that piertop assembly!!
By Jove, Ronzo I think you may be onto something!!! ;-)
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I'm actually on a trip (with my wife in El Dorado, AR -- doing genealogy research) and am sending this from our motel room. Next time we "land" somewhere, I'll FTP up to my webspace some more annotated photos and comments on what else I see in this critical north shore area of collapse.
jeffers -- I have done some more differential video frame analysis on the south pier collapse, and will have graphics to send soon. do either of you have a source URL for that (by the locks) surveillance video that includes frames of the bridge before the collapse began? The only ones I've found start with the collapse already in progress... :-(
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Again, thanks for all the great engineering analysis -- and sharp eyes! I'm convinced that FR folks will have the likely cause for failure posted in the FR archives -- well before anyone else reaches the same conclusion!!