Posted on 08/01/2007 4:28:27 PM PDT by ButThreeLeftsDo
Just turned on the news. 35W bridge collapsed in the Mississippi River. Cars, trucks, semis.....
Fires burning, tanker trucks, at least one school bus, more than ten cars......
Just now breaking.......
Yeah, just heard that on the top of the hour news from Fox News Radio.
Just eight missing. The intel must be better, now.
Minnesota nice!
Thank her from a Texas lady for her service to our country!
I believe you did! That’s good news...very promising!
Once the king post buckled a little, that expansion joint above would tear loose, popping pins across the joint, from SE to SE very fast. That would result in the fall seen in the video and reported as "the center falling, or dropping out". So the fatigue was in the bottom chord, or the elements of the chord attachment at the SE pad.
Here's the before for ref.
That's SE to SW...
Good evening C_F! Prayers for all in the area affected by this tragedy.
When the center span came down it was tipped slightly side to side, as you suggest, but it was also tipped end to end, south end first, by the video. As far as the main span goes, it’s pretty clear that whatever failed affected the south set of cantilevers first. You can see the north cantilevers flex and then snap, just south of the north piers, and the northern approach span sits there several seconds before rocking back shoreward and collapsing.
However, it’s hard to see in the photo, but the southbound semi truck that burned did so because he went head on into a near vertical section of road deck, supported on one end, tilting down to ground on the river end. You can see it in the photo I posted, right next to the school bus.
From a different angle, it’s clear that the north end of that vertical slab sits under the approach span that the bus and semi sit on. The flat slab the semi is on rests on the hood of an SUV which was northbound, and another northbound vehicle smashed head on into the end of the slab the semi and bus rest on.
Probability says that the triggering failure occurred in the first span to fall, in other words, the bottom of the debris stack.
There are two main bridge components at this level. One is the southeast cantilever superstructure, the other is the north end of the approach span referred to above. The failure of a single member in either of these assemblies could have initiated the collapse progression.
However, if the trigger had been the approach span, it left the intervening approach span suspended in place while the south end of the center span beat it to the ground. The same is true if the southeast cantilever of the center span failed first. The “bottom of the stack” sits both north and south of a span which is not at the bottom of the stack. The slab the semi and bus are on probably failed laterin the sequence.
Given this, it makes sense for the larger structure to fail first, because it would send bigger ripples across the intervening approach span to drop the next adjacent span than if the sequence occurred in the opposite direction. If the approach span failed first, the tail would be wagging the dog. Further still, half the tail would be wagging a very large dog, because the south end of the near vertical span appears to rest on its original supports. Obviously this is not conclusive, just playing the odds here. The failure of a big section of bridge is more likely to shake loose a much much smaller section of bridge, after jumping over a section which failed later in the sequence, than the other way around.
From that point, just about any member of the southeast cantilever could have initiated the collapse, but very rapidly, the failure transferred considerable load to the kingpost of the southeast cantilever, which was unable to maintain integrity and buckled. That’s what initiated the major collapse sequence, because as you have noted earlier, there are considerable point loads at each of those four main piers, and of the four, the imagery suggests that the southeast kingpost failed first. Because the southwest cantilever kingpost is largely intact and visually straight, the lateral stringers and diagonal braces between the two had to let go early on to allow the southeast kingpost to crumple.
Because the center span fell without significant deflection, no major curves in the road surface visible in the video, it is safe to assume that whatever failed was out of sight to the right of the camera’s view, in other words, towards the middle or the southern end of the southeast cantilever structure. I mean, it’s possible the connecting pin between the two eastern cantilevers failed, or any other point on the east side, and the north kingpost was better able to resist the additional loading that its southern counterpart, but much less likely since the load transfers would have been progressive and as yoi p[osted, there wasn’t redundancy in the structure. If a tension member had failed out in the mainspan, I’d expect to see it, then adjacent panels fail progressively towards both ends, but the video doesn’t support this.
Putting it all together reduces the possibilities to two main groups.
1. One or more members in the approach span two south of the main south piers failed, triggering collapse of the north end of that span, inducing through shock, vibration or load transfer, greater than design loads on the southeast cantilever, which failed, bringing down most of the rest of the bridge.
2. One or more members failed very close to the southeast pier, either in the main cantilever assembly, or in the lateral bracing system. The video indicates that the triggering failure propbably wasn’t very close to midspan, That leaves the river side close to the southeast pier, and the entirety of the counterbalancing shore side of the cantilever assembly.
Of these two, I favor the second, because of the “wag the dog by remote control” discussion above. Naturally, any part of this chain of observations could be wrong. Sometimes things bounce when they hit the ground, and end up on top of other things which fell later in the sequence. But the odds and imagery generally support this conclusion, and until they fine tooth comb every piece of steel from the debris, that’s about all we have to work with.
Eye of
Unk wrote:
I wonder what would happen if it was found to be cutting torch marks at the critical stress points,
anybody can buy a portable cutting torch.
***********************
While it’s true that anyone can rent a torch, cutting steel isn’t a low profile operation, and you don’t just whisk through three quarter or inch thick plates like a knife through butter either.
It takes a good while to get a puddle of molten steel going, and then mishandling of the jet or residual water in adjacent concrete can blow that puddle in all directions like a Fourth of July sparkling fountain.
Except once, when I was headfirst down a Sonitube light standard concrete form cutting rebar, I don’t think I’ve ever cut steel that the whole jobsite didn’t know about. It’s not something you’re likely to get away with on the sly.
Excellent catch from that first pic. I had seen that, but before one i posted earlier, and did not catch the much better view of the buckled kingpost.
Good eye!
Important to note, the bottom chord of a cantilever usually isn’t in tension. Think teeter-totter. Raise the seat board with a vertical strut (kingpost), then run a diagonal from the fulcrum to each end of the seat board (topchord). The top chord should remain in static tension, while the kingpost and both diagonal braces are in compression.
I’m sure you alread know this, but for others, a cantilever is basically a balancing act. From the center of one span to the center of the next is just an upside down triangle, balanced on the piers in its middle. Sometimes, the shore end of a cantilever is weighted or fixed by a large concrete counterweight. Sometimes, each end is just attached to the floatinjg end of another cantilever.
Something’s been bugging me about that first pic, and I just realized what it is. The road deck shoreward of the pier is intact, while towards centerspan, it’s vaporized. Starting to speculate here, that the trigger could well have been right in that area, just north of the southeast pier.
Take a look at the bottom chord and bridge foot assembly on the west side cantilever in your second image.
You see what I see? Corrosion, new paint, patchwork steel, something, almost all the way out to the next strut? I’d expect the cantileverassewmblies to weather and age in pairs. If the other side looked like that before the collapse...
Having had 24 hours to look at the photos and video it seems almost certain that the bridge failed in the section in the image at #2464. More specifically I think it has to be that left kingpost.
While the investigators might find subtle differences from what has been suggested here I doubt that we are far off.
I think it happened right in that area, but I’m just not comfortable with a trigger failure being attributed a member under compression.
I’m probably prejudiced on this, because each member is engineered based on what type forces it is expected wot withstand, but my gut likes tension failures first, shear and torsion tied for second place, and compression failures last.
That said, the discoloration on the other side at that location tells me that something was going on in the bottom chord, or that something had gone on in the past at that location. Frankly, the transverse dark brown lines near the northern end of the western beam look like either joints or cracks. That’s not a great place for a joint, much less so for a crack, and I don’t see other joints in other members showing that color either.
We’re approaching the edge of granularity here, in what we can discern from the available imagery, but I’m reasonably confident we aren’t on the wrong end of the bridge. Feel free to ping me when more develops, whether that is later today or a year from now, and I’ll return the favor if I turn anything up. I’m very much interested in getting deeper into this, but recognize the likelihood we’ll have to wait a long time for definitive answers.
Nice annotations and analysis.
Let me throw in my two cents here. First off I supervise a large concrete batching facility in central Alaska, I also drive a large mixer truck on occasions. We had just recently an order to deliver concrete onto a similar two lane type of bridge spanning a large river 60 miles north of me, the length is similar, the bridge was built in the mid 60’s. the project was reinforcing the king post pillars against massive floes of ice and an earthquake improvement project of the area around the king posts as like the 35 bridge. One thing I definately noticed at one time and it literally had everybody looking around like it was an earthquake was when I had to re-mix my load of 44,000 lbs. of concrete and I had the drum rotating at high speed and it set up a sympathetic vibration through the bridge almost like the episode of Mythbusters where at a certain frequency the whole bridge shook and was bouncing big time and that is what was happening. I am really serious about this, the bridge in MI has two lanes of bumper to bumper slow traffic but there was also construction equipment working and there may have been a condition of a different type of natural frequency and set up a vibration that found a literal weak link. like breaking step in marching across a bridge.
I am betting on this scenario only because its been proven that you can set up a vibration if you find that certain resonance and I experiance one just this spring under nearly exact conditions.
My final cliche of the day: time will tell...
I will have to find some complete “before” pictures too look at, but it looked like a conventional truss type bridge to me. The only thing that is unusual about it is that the road deck was on top rather than along the bottom like normal. I am unsure what your use of the word “cantilever” means in this case.
Anyway, I have no expertise in this particular bridge, just engineering observations. After the final report on the collapse, we may all be wrong. BTW, I do agree that the failure was probably near, but not at, one of the supports.
Bridge piers generally don’t float. River beds are by definition “bedrock”.
They can have my 1978 Camaro when they pry it from my cold dead hands....
That works - the video makes it appear that the top chord failed first but that could have been secondary to the pin failure.
BTW - lots of corrosion on the bottom chord at the pier in the before picture.
The complete, total and uniform failure of the center span is difficult to reconcile. One might expect the span to fall to one side or to fall to one end. The center span sits on only four bearings. Being of non-redundant cantilever construction the other sections were not self supporting and also failed when the center span gave way.
The real time video clearly shows a nearly uniform failure of the entire center section. What is amazing is that both ends of the bridge detach at almost the same time.
http://www.youtube.com/watch?v=EjCic0YlJwQ
The critical points appear to be the four bearings that the center section sits on. If just one bearing moves or breaks a collapse will occur.
Here is my scenario
- Steel corrosion of the trusses and especially the bearings weakens the steel trusses and creates misalignment of the bearings.
- Cold weather reduces the ability of the steel to flex which induces cyclic fatigue stress cracks. (Hoan bridge failure in 2000)
- De-icing of the bridge deck with various corrosive agents further attacks the steel and the bearings.
- Recent spate of high temperatures and poor maintenance of the expansion joints results in the expansion joints being “locked up”.
- Lane closures, stationary traffic and repaving equipment creates an asymmetrical load on the bearings.
With some or all of the expansion joints clogged with dirt and debris a tremendous load is building across the top of the bridge. This unplanned top loading is creating excessive stress in some areas of the bridge and actually relieving stress on other components of the bridge. Asymmetrical loading due to the lane reductions is further contributing to the uneven loading of the bridges and misalignment of the bearings. Distribution of heavy trucks and construction vehicles on one end and or side of the bridge further contribute. Some or all bearings are not sitting in full contact in their pockets. At this point a bearing or truss support which has been weakened by corrosion and stress cracks has a catastrophic failure. Other trusses or bearings let go within a fraction of a second and the whole center section falls to the river in one piece.
Eye of Unk makes an interesting observation regarding the construction vehicles and harmonics. Very interesting.
Lastly, I also think we are going to hear a lot more about expansion joints.
Jury selection begins in Elzahabi trial.
This dude was arrested in 2004.
Just don't want to be too quick with the "terrorism" impossibility.
There are a few articles on this guy that are worth reading and pondering.
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