Pics of South Bridges
One was built in 1935 and is scheduled for re-decking next year. I am supposed to get the inspection report from last year soon.
There were news reports that bridge workers claimed the more deck they removed, the more the structure “wobbled”. MnDOT later said they interviewed every single one and could not confirm those stories. Hmmmm...
Empirical evidence shows imagery with perhaps two foot by one lane sections of the concrete removed before collapse. The evidence is anecdotal, there is no way of knowing how many of those instances occurred.
A trench of removed decking across the full width of the span would reduce the decks anti-sway capabilities to a series of antisway “islands”. Under each “island” the deck would act as a membrane and significantly impede sway, but each “island” would be able to sway independantly from the next.
Pre-collapse imagery shows vertical sway bracing between alternating vertical struts across the main trussed spans, but these would only keep the two trusses parallel, and would not act to keep the trusses linear. Obviously, stresses would rapidly accumulate if the truss deflected in the horizontal plane from a straight line.
One possible scenario has a significant trench cut in the road deck at an expansion joint. Some older expansion mechanisms had been replaced with newer designs. These needed cleaning, and the older ones were buckling and causing problems for snowplows, so either could have been candidates for replacement or repair.
Under that scenario, you have less resistance to sway, right where you’d expect accumulated stress from possible frozen bearings.
However, pre-collapse imagery does show horizontal anti-sway bracing in plane with the bottom chords of the main trusses, and also atop the deck trusses.
As you note though, all of that would be speculation and the possibilities are near infinite.
However, there are some things we “know”.
I hesitate to use that word, especially in the scientific or engineering sense, but with no lives on the line in this thread, “knowing” means that other conclusions are so unlikely as to be meaningless. With that in mind, we know that:
1. The bridge failed, probably (”probably” negates “know”) beginning with a few or even one member, and lack of redundancy allowed this to progress into complete collapse.
2. From the video, we know the main span dropped south to north, southern sections hit the water before northern end, and that the northern trussed sidespan (span 8) survived several seconds after the mainspan hit the water.
3. We know that the west truss at pier 6, and the north end of both span 6 road decks swayed and fell to the east at collapse, that the west truss panels immediately north and south of pier 6 survived largely intact, that both concrete columns at pier 6 are missing rocker plates and survived largely intact with no noticible deflection from vertical, and that the base of the pier 6 west truss kingpost came to rest slightly north of its original location atop pier 6.
4. We know that all other sections of the bridge fell straight down, within the limits of visual detection from post collapse imagery, suffering orders of magnitude less horizontal deflection in comparison with the at and just south of pier 6.
5. We know that the east truss panels adjacent to pier 6 are either badly distorted or no longer visible in post collapse imagery.
6. We know that this bridge had a long history of stress and fatigue cracking in critical members, cracked welds, significant pack rust and section loss, and significant corrosion, especially at and around drainage points.
7. We know that the most significant structural event in this bridge’s history was crossbeam and endbeam cracking, at both the north and south end of the trussed spans, that these areas carried significant loads from adjacent beam and post spans through cantilever design, and that these failures occurred coincident in space and time with a frozen rocker bearing at the east end of the south crossbeam.
“Page 19:
MAIN TRUSS (EAST TRUSS)
Crossbeam:
[1986] The SE rocker bearing froze, damaging the east end of the crossbeam, resulting in cracked
web stiffeners. The bridge was jacked
up. I-35W was closed to traffic. SE
rocker pin was replaced, cracks in two
stiffeners were welded and drilled out,
and bracing was added between the
crossbeam and beams #3 & 4.”
“In 1992, a crack was found in a crossbeam stiffener weld above the northeast rocker bearing,
which was drilled out. In 1997, at the same location, a weld between a vertical & horizontal
stiffener was found cracked through entirely. Cracks were also discovered at the end of
horizontal stiffeners near the northeast & southwest rocker bearings. Strain gauges were
installed to analyze stresses, crack ends were drilled out, and installing bracing between the
crossbeam and 2 stringers reinforced the northeast connection.”
8. We know that the 2006 inspection report was internally inconsistent with respect to the condition of the south crossbeam west rocker bearing:
Page 12:
“Crossbeams & Rocker Bearings: The two “cross-beams” are welded plate girders each
one is supported by two “rocker” bearings attached to the cantilever ends of the main truss.
These rocker bearings are built into the crossbeam web except the southeast rocker, which,
due to the bridge super-elevation, connects to the bottom flange of the crossbeam. The
crossbeams & rocker bearings were re-painted in 1998/1999. The faces exposed to the
finger joints have extensive surface pitting with some areas of severe section loss with holes
at the base of stiffeners. The rocker bearings are measured & checked for movement during
each annual inspection.
****All four bearings appear to be functioning. They show obvious signs
of movement.”****
Page 30:
MAIN TRUSS SPAN (WEST TRUSS)
...SW rocker
bearing has no movement.”
9. We know that the engineers inspecting the bridge believed that the frozen hinge pin on span 2 caused pier 1 to tilt northward out of plumb.
“Page 9
The hinge joint in span #2 is locked in full expansion several beam-ends are
contacting, and the hinge bearings are “frozen” and no longer functioning.
Consequently, pier #1 has tipped slightly to the north, and the south abutment
bearings are in full contraction. This area should be thoroughly inspected.”
10. We know that the 2006 inspectors were concerned that the east pier 6 roller nest had frozen.
“Page 22:
Pier #6 (Downtown, West Bank of Mississippi):
Pier consists of two concrete columns with a pier wall at the base, two “rollernest” bearing
assemblies. [1997] Bearings have surface rust, moderate
corrosion and show no signs of movement.”
11. We know from post collapse imagery that the pier 7 rocker bearings survived intact, and that these were fixed in the horizontal plane, not roller nests.
There’s more, but these cover many of the more significant issues. Before going any further, it would be helpful to know whether or not you agree that we can reasonably assume these eleven points are correct. Big difference between disagreement on givens, and disagreement on conclusions drawn from givens.
In my experience, major failures occur because of multiple, additive causes, and that very often, these causes are known well in advance of failure.
Turning that around the other way, it is likely that some or all of the contributory causes of this failure were known prior to collapse, but were not “connected” or “summed”, because no-one believed collapse was imminent.
Turning it all the way around, I’d be uncomfortable with any explanation of the failure process that didn’t include some or all of the bridge’s known history and problems.
Succinctly, these were cracked members and welds, frozen bearings, and significant rust and corrosion.
If an explanation ties these three together, I’m more likely to accept it, than if an explanation tries to claim a cause of failure which has no historical or anecdotal support.
“Knowns” 2, 3, and 4, additionally, imply a conclusion that’s difficult to overstate.
I am unable to conceive of a collapse progression in which span 7, the main river span, did not seperate just north of pier 6 before collapsing downward. When viewed from above, the road deck at pier six is so far east of the road deck in the river just north of pier 6, that I “know” these two were no longer connected before gravity pulled them down. The alternative has span 7 dropping as a unit in a vertical line, breaking apart just north of pier 6 on ground impact, and the section south of the break “bouncing” 60 to 70 feet east. I can’t swallow that.
Whatever else happened, in any other section of the bridge, span 7 separated into two pieces, largely before gravity began to act on those two pieces.
Add all this together, and in my opinion, the most likely failure sequence occurred as detailed in my earlier post.
Like you, I have a hard time devising a mechanism where frozen bearings lead to stresses that precipitate the collapse we know took place.
I also agree with you that frozen bridge bearings generally do not cause significant damage, but in this case, frozen bearings caused historically documented damage in at least two instances, significant cracking in and around the south crossbeam/endbeam/rocker bearing assembly, and vertical tipping at pier 1.
As you note, we can’t “know” anything for sure, and unfortunately, the structural components critical towards proving or disproving my opinion fell into the deepest part of the river, into the navigation channel just east of the entrance to the lock.
We won’t “know” anything until the final report, if then, but until then, the best we have is the confluence of cracks, frozen bearings and rust, and post collapse imagery. The “most likely” explanation is a statistical tool, not a certainty. But for now, it’s the best we have.
I have not laid out the drawings and all the after photographs in order to agree or disagree with some of what you said. Specifically:
1) Obvious. Another report I have been reading identifies 52 different members between the two support piers (26 on each side of centerline) that would cause complete collapse if any ONE of them broke.
2) Don’t know for sure. I have not studied or laid out the available information in enough detail to agree or disagree.
3) Don’t know for sure.
4) Don’t know for sure.
5) Don’t know for sure.
6) Yes. Straight from the report.
7) Yes.
8) Yes.
9) Yes.
10) Yes.
11) Don’t know for sure.
> “In my experience, major failures occur because of multiple, additive causes, and that very often, these causes are known well in advance of failure.”
That is also my experience.
I have just about exhausted what I can say on this. I made several general statements that I believe will hold up in the final report. I did not get specific enough to identify exactly which structural member failed first and will not do so. To speculate to that detail without being there and physically inspecting the debris that comes out of the river and studying EVERY photograph taken would be irresponsible on my part and could be cause for someone to come after my PE.
It is sufficient to say that this bridge was an accident waiting to happen. The trigger that caused one of the 52 critical members to fall was probably the sum of several things. However, the mere fact that it was being redecked means that someone in MNDOT thought that that bridge would last another 10 or 15 years. How that conclusion was reached must be identified and corrected. I think that conclusion was totally irresponsible. If it did not collapse last week, there is absolutely no question in my mind that it would have collapsed before the new deck wore out.
BTW, I saw a news article in the local paper they weekend that tells me we are not going to get a truthful report. The head of the MNDOT was identified as a Republican several times. Case closed.
Pics of South Bridges
One was built in 1935 and is scheduled for re-decking next year. I am supposed to get the inspection report from last year soon.
Well, here’s something I’d like to ask you. Where is the main truss that was sitting on the east side of pier 6 ? We can see a large section of intact truss that sat on the west side of pier 6 ( evidently ) but there is no sign of the corresponding east side. Where is it? I would have to guess it is under the roadway which is collapsed to the ground east of the eastern pier. I would love to be there when they raise the concrete roadway, just to see what’s under it.
BTW, isn’t it a little weird that they want to reassemble the bridge? All the parts are readily identifiable, I think, and the question is where did they end up, not where did they start out. I think you indicated the same thing when you talked about divers identifying structural members.