Posted on 08/09/2007 6:32:29 PM PDT by jim_trent
What did you think of my post #59 as a means of protecting against bearing failure?
> 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.
> What did you think of my post #59 as a means of protecting against bearing failure?
What I think is that it is more things to go bad. If frozen bearing were a primary cause of bridge failures, I haven’t heard of it. Every bridge I have ever inspected has bearings that are either well on their way to jamming up OR, are already there. This is not only old bridges. Some less than 10 years old are in poor shape.
I notice that some of the newest interstate bridges have a completely different manner of containing expansion and contraction. Basically, the use long wideflange beams pounded into the ground with the weak direction in the direction the bridge is supposed to move. The beams are surrounded by a corrugated metal tube. The top of the bridge rests on the beams. I believe the idea (although I have NOT talked to any designers about it) is that the beam is supposed to be able to move within the metal tubes. We will know in 30 or 40 years if this is any better.
> But if you can see the danger signs in the report, then others should have too.
It is not hard to see problem areas in something that has already fallen down. I have absolutely no doubt that the engineers who inspected this bridge did not think it would last the 10 to 15 more years that was planned (remember they were redecking the bridge, not just patching potholes or resurfacing it — that means they were not planning to replace it for many years).
The fault lays with the higher-ups in the MNDOT who bowed to political pressure to NOT spend money there so the politicians could spend it elsewhere. The MNDOT higher-ups will probably retire early or be publicly disgraced, but the politicians are probably distanced enough from the disaster to escape the voters wrath — if such a thing even exists.
Anybody want to speculate how many politicians will lose the next election over this stupidity? I take the number — zero.
With a big enough motor, low enough gearing, and proper maintenence, it should work, but those are not givens.
Six roller nests and at least 4 rocker bearings make at least ten such assemblies for this one bridge.
Bottom line always comes down to the client and how much they want to spend. The technology exists to build some very long lasting and competent structures, but often the design issues are resolved by a less than knowlegable client’s preferences and whims.
OTOH, the government also built this:
There's a big gap between the Lincoln Tunnel and Boston's "Big Dig", too.
So it's not only government vs. private sector at work - it's government's recent affection for lack of standards that's the real problem.
So was this:
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.
The Golden Gate Bridge gets short shifted, because of a simple limitation of photography. You rarely see a picture of the bridge from the side, straight on. There’s few good places to stand to take such a picture, and no lenses with wide enough viewing angles to encompass the entire bridge, except from extremely long range.
My son has a model train layout and inquired about modeling the Golden Gate Bridge. The simple calculations indicate just how little vertical structure supports that enormously long span.
In HO, the ratio of model to reality is 1:87. A one foot long model would be 87 feet long in reality.
The real world Golden Gate Bridge uses four vertical towers, approximately 750 feet tall, to support three spans of road deck, which is a total of almost 9,000 feet long.
These numbers are incomprehensible to the average person.
On my son’s model train track, a scale model of the bridge would use towers only 8.6 feet tall, roughly the heigth of your kitchen ceiling, to support a model bridge span that is 103 feet long, roughly twice as long as your whole house.
It is an incredible feat of engineering, one that few people truly appreciate.
My son now understands that he will not be modeling the Golden Gate Bridge in HO scale, at least, not in my house.
I’m not a civil engineer either, but there is one thing that may be of interest, with respect to the redecking that was in progress.
If jackhammering of the deck, for removal, was underway, the jackhammering could have provided the energy for initiating a catastrophic fracture of an already-existing corrosion-induced stress crack. If this was indeed the failure mechanism, it is likely that the jackhammering happened to hit a resonant frequency of a corrosion-weakened structural member.
Some of the early reports I heard seemed to indicate there was active jackhammering under way at the time of the bridge failure.
While there are multiple possible failure initiation modes, this is certainly one of them. Only time, and a really good failure analysis, will tell.
The government involvement concerned financing and right of way.
I have a problem with “resonant vibration” as a failure causation in the collapse of this bridge, be it vibration caused by jackhammers or cement mixers or railroad locomotives.
Stop in a traffic jam on nearly any highway bridge and you can easily feel the deck flex as trucks roll on and off the bridge. This is normal, and well within design limits.
When the Tacoma Narrows Bridge failed due to resonant vibration, it had been subject to oscillations up to 30 plus vertical feet in amplitude for several hours before ANY structural failure took place.
Bridges are expected to vibrate and engineers take steps to dampen high amplitude oscillations well before those oscillations can fail the bridge.
If any survivors or eyewitnesses were reporting continuing oscillations, even large enough to be visible or easily felt, then yes, resonant vibration, exceeding bridge design limits, would necessarily require consideration.
In the absence of any such reports, with the sole exception of one reporter claiming construction workers mentioned “wobble”, and refused to repeat these comments to MnDOT interviewers after the collapse, it’s very difficult to justify claims that resonant vibration failed the bridge.
Tesla well understood that the mechanical advantage offered by resonant vibration was its inherent ability to produce high amplitude oscillation is short periods of time, and with little applied energy added with each successive wavefront. It’s a mechanism which sums many low amplitude deflections into large amplitude oscillations. It’s not an effect that hides inside a structure’s molecular structure. If the vibration is so small as to be imperceptible, then it is probably well within normal design limits.
Loss of structural strength due to long exposure to small amplitude deflections falls under the heading of metal fatigue, not resonant vibration.
If a jackhammer’s impact causes a weld to crack or steel itself to crack, again, this isn’t resonant vibration as much as it is simple energy transfer through impact. The key element of resonant vibration is wavefronts which reflect and combine to create high amplitude oscillation.
It could be argued that ultrasonic waves can shatter brittle materials such as glass, or possibly even cast iron, but I don’t see this happening in comparatively ductile structural steel, and in any event, these types of oscillations are unlikely to be generated by trains, cement mixers, or jackhammers.
I won’t rule out resonant vibration in this failure, but it’s a long way away from the first place I’d look.
I’m saying more as a cause for initiation of the terminal failure event, not as a cause for the overall failure.
It’s pretty clear from the pics in that report, and a quick skim of the text, that this bridge has been in the process of failing for many years. I was just speculating on a not-unlikely cause for the trigger for the final catastrophic failure.
The flexing caused by trucks is likely to be at a very different frequency from a jackhammer. Even though the energy from the latter might be much lower than from trucks, if it, say, hit a resonant frequency in a structural member with a pre-existing crack, the jackhammering might be the final proximate reason for a catastrophic crack propagation and bridge failure.
Even if this speculation is correct, it would only be the metaphorical “blasting cap” added to the “large pile of dynamite” represented by years of maintenance neglect on the structure.
ping ,, interesting
save
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.
I don’t have to be a bridge expert, to know that having concrete columns spalling to the point where rebar is significantly corroding, is Not A Good Thing.
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