Minneapolis I-35W Bridge Collapse Failure Analysis

opinion | 8-21-2007 | jeffers

[jeffers]

Your piers are labelled correctly, but not your connection points.

The mainspan (span 7) center vertical strut is member U1-L1. The next strut south of there is U2-L2. The next strut north of the center strut is U2’-L2’.

Your cantilevers and trusses are generally correct, but the two center cantilevers are drawn a little generous. In the proper labelling scheme, the pier 6 cantilever only extends out (towards midspan 7) to U6-L5.

I’ve looked very hard at a triggering failure in the southern approach. Over and over again, I have to rule it out, at least as the most likely sequence.

Primary reason, as you note, the pier 6 superstructure would have rotated, top towards the south, if span 6 sagged midspan. That would have lifted the south end of span 7 well up above the horizontal plane of the deck, and we simply do not see this in the video.

If it happened that way, the top chord fractured north of pier 6 almost immediately, before any (assumed) visible deflection took place in span 6, and before said deflection could induce (actual) visible elevation of the span 7 south end.

I won’t rule out a triggering failure at the pier 5 crossbeam/endbeam rocker assembly, given the history there.

But the video does not visibly support such a sequence, and therefore, it’s not the first sequence in my assessment of the likeliest possibilities.

[MinuteGal]

Hey, Bill O'Reilly.....get your sorry eyeballs over to this thread and tell your audience once again that Free Republic is a "hate site" populated by knuckle-dragging neanderthals.

Leni

[jeffers]

It’s not the best image I’ve seen but you’re looking at the south endbeam in this photo.

The junction between the south end of the main trusses and normal beam and post approach spans did not fall directly over pier 5. It was located south of pier 5, and was designed as a somewhat complex cantilever assembly.

The far end of the beams you see (resting on the ground) were between pier 4 (seen in the image) and pier 5, (not seen).

For illustration as to why the visible beams buckled to the near side of the visible pier 4, imagine pier 3, pier 4, no main trusses, and no pier 5.

In such a scenario, the visible beams sit with one end on pier 3, and are cantilevered out into free space past pier 4. Load the free end of the cantilever in excess of design limits, and yes, the beams will buckle just to the pier 3 side of pier 4, as you see in the image.

That is where the bending stresses reach maxima. The reason for this is due to the lifting reaction placed on the pier 3 end of the beams. If the only stress in play was the gravity load on the free end of the cantilever, then the beams would buckle over pier 4. But the dead load and connection assemblies at pier 3 resist the gravity load imposed on the free cantilever, and in effect, bend the beams over pier 4 like a fulcrum.

There’s more to this, involving the nature of the pier 4 beam connections and how they are permitted to move and restricted in movement, but this should outline the basic idea.

[BlueMondaySkipper]

bttt

[DrDavid]

I have seen claims that materials and heavy equipment were stored on the bridge, but I can’t say I’ve seen proof of this.

Would telling you that I saw the equipment sitting on the bridge a couple of times a day when I crossed it during June and July substantiate the claims?

[jeffers]

Yep.

Can you remember what you saw, and where it was?

Especially interested in heavy loads between mid span 7 and just south of pier 6.

[KC Burke]

In the deck repairs underway, does anyone know what was being done.

Removal could have had small cushman type debris removal staged to another point on the bridge and then hi-loadered into dump trucks. Cushman’s wouldn't’t be a big load nor would small piles of debris staged for reloading. Only one truck would have been filled at a time and then it would depart. Others waiting would have been empty, not full as some have speculated in discussing “trucks full of gravel”

In the repaving operation, if concrete, we could have seen ready-mix trucks lined up too far out on the structure and a paving screed. I haven’t heard there were ready-mix trucks in the collapse debris vehicles.

Conversely, if asphaltic paving or wear course was underway, a “paving train” would have consisted of a crawler driven small paving machine and trucks loaded with heavy asphalt lined up to dump in it. These heavy trucks could have been staged off the structure or they could have lined up on the closed lanes.

It appeared that small sections were being done in stages so I don’t really think that any of the foregoing were the case, but there has been so much talk of construction material loads that at least we should discuss what we might be looking for in the debris or observations.

I can imagine that MDOT and the contractor aren't talking about specifics at all until the forensics and formal investigations are done.

I can imagine that the contractor and his liability carrier as sweating blood.

[jeffers]

Something I noticed studying the images, was that the roughened deck surface visible after the collapse appeared to be confined to the SW corner of the bridge, maybe from pier 5 and south, in the southbound lanes.

I imagine the contractor had an overall schedule, and other parts of the contract work could have been underway elsewhere, but that corner seemed top be the primary focus of work in progress from what the post collapse imagery showed.

[KC Burke]

Then it seems to have appeared to have been a scarify and re-surface type of repair if roughened pavement is what was seen.

With that type of repair it can have a new surface in either concrete topping or asphaltic topping depending upon the substrate of the paving. Both operations can either be done in small segments or in large scale paving operations as previously described.

In either topping we are not looking for "trucks loaded with gravel" as some news accounts describe, but instead, we are looking for trucks loaded with hot asphalt or ready-mix trucks. The former would have been servicing a paving machine:

The machine and the trucks servicing it should have been smaller that what is shown above but this shows the operation and machinery type.

A concrete paving operation could have been a small vibrating screed with almost no weight or something of this type

Again, this is a larger unit that what would be used for bridge topping but it shows the sort of machine utilized and its appearance for those not familiar with this process.

[DrDavid]

Can you remember what you saw, and where it was?

From what I remember is they had what I think they were generators built on trailers, trucks that came and went and road cutting saws. There were several of each of these distributed the full length of the bridge on both sides. They took up a lot of the 2 outside lanes in both direction, where they were working.

This is kind of vague, but hopefully it is helpful.

[KC Burke]

I would think that the truck mounted items you saw were truck mounted air-compressors as opposed to generators.

Compressed air generators and the trucks are not heavy virbration or dead load.

Compressed air is used to blow off the light debris and dust after the larger chips are removed behind any scafication operation. That gives a clean surface that the topping / resurfacing can bond to as it is placed.

[mroper]

Very nicely done. You made one mistake, though, which confuses me as I read your analysis. The naming conventions you've adopted regarding the location of the panel points (U6, L7, U8, etc.) are not the "official" ones. I think you got confused by page 50 of the June 2006 Fracture-Critical Inspection report. That diagram is missing the legend that says the observer's point-of-view is upstream, i.e. to the west of the bridge looking east. In the diagram you posted, south is to the right, not the left as you assume. U8/L8 is still correct, above Pier 6 on the south end of the bridge. Most of the Pier 6 photos in your article were taken by photographers situated to the east of the bridge looking west toward Pier 6. This is nominally a "downstream" view.

So you've got it semi-backwards. What you refer to as U7/L7 and U6/L6 are actually U9/L9 and U10/L10 according to all the official documents available regarding the bridge. The panel point numbers increase as you head north from Pier 6 out over the river. The center point of the bridge is at U14/L14. The southernmost point of the main deck trusses is U0 (there is no L0 or L28 at the "points of the cantilevers), and the numbering increases from south to north. Since there is symmetry about the center there is also the convention of referring to the points north of U/L14 as U13' to U0', in descending order as you head north. Pier 5 (south of Pier 6) supports the south end of the structure at L1, and Pier 8 (north of Pier 7) supports the north end at L1' (also referred to as L27 in various documents). The main numbering sequence increases from south to north whereas the "prime" sequence increases from north to south.

This in no way invalidates your analysis since we're only talking about the labels of different points on the structure. It just confuses me greatly to have to make the extra transposition in my head when reading your article. Please have a look at page 48 in the June 2005 Fracture-Critical Inspection report and it should become clear to you. That is the correct orientation of the panel points when looking at the bridge (or what remains of it) from a position to the east (downstream). The diagram you used at the beginning is the upstream view. Things being symmetrical makes it easy to get confused. It took me a week to understand which panel points were which.

OK, on to another couple of points...

The guy on the bicycle who is seen in many pictures from 1 August is a real hero and yet his story is almost completely unknown. He just climbed up there on the roadway rubble pile and helped about 10 people to safety. Unsung doesn't begin to describe Dan Schueller's courage that day. He didn't waste any time and he's no spring chicken either. I wish there was more news coverage of what he did. I like to think I would have done the same thing, but man, that had to be terrifying to see that happen while you're out riding your bicycle. Schueller was maybe the first person on the scene who wasn't in the collapse event itself.

In response to those who suggest the Army Corps of Engineers' security videotape was edited, I think it's likely the camera works only when it detects motion in its field of view. That would explain a lot of the gaps in it. Notice how after the initial collapse it kicks in again after a gap when it "sees" span 8 on the north side begin to tip over backwards, and again when the woman with the cell phone runs by. At the start of the tape it probably activated just after it detected the initial motion of the center span. I doubt there was anything edited on the tape. IMO there was nothing recorded before then, although I have no way to know for sure. I do know for a fact that there are security video systems out there that use motion detectors for activation.

Speaking of that video, who was that woman, anyway? She must have been standing only a few yards from where the bridge came down. She's lucky it didn't shift to the north instead of the south. I'm sure the authorities found her and interviewed her as a key eyewitness.

Finally, I want to post a sample of a 3D rendering I did from a quick CAD model of the bridge I made (like you, jeffers, I'm probably spending way too much time on this). My goal is to recreate the field-of-view of the ACE security camera. So far I'm not pleased with the results. Being a security camera, it probably had a fisheye lens. My software is too primitive to handle that, but I'm still searching for a solution.



I can't figure out the "hump" on the roadway as you go north. Maybe that's why I want to believe the camera had a fisheye lens that distorted the image somewhat. The north end of the roadway around Pier 7 is several feet below the roadway at Pier 6 and I first thought that was why it looks curved. The model I made is pretty accurate so I don't think that explains it. I guess it's time to add the roadway to the model.

Like I said, good work, jeffers. I've been searching for somewhere on the web where people are discussing the 35W collapse and there aren't too many. There were a lot the first week after it happened, but most of the discussion has died down since then. I think there are alternative explanations for the failure sequence but yours is definitely on the short list of finalists. I'm like you: I'm too impatient to wait for the official NTSB report which we might not get for more than a year.

That's it for now.

[jeffers]

Regarding the official bridge labelling scheme, you are correct. The drawing I based my assumptions on is a poor one but the text, on deeper investigation, does indicate the correct labelling scheme, and I apologize for any confusion in this area.

I also note that an earlier poster’s scheme actually is correct.

I pity the efforts you had to go through to make sense of my analysis in this respect. It’s hard enough to understand when the numbers line up.

As you note, a different scheme doesn’t change the basic conclusions, though I would clearly state, it moves the suspected problem area to the U10 connection point of the east truss.

This opens the door for some new data, and I am still working on other areas of analysis, including construction traffic and further identification of structural members in post collapse imagery.

I doubt these avenues of analysis will lead to concrete conclusions, and have to decide whether to publish sheer speculation or let important, but unresolved, issues lie unmentioned.

I note with interest your assessment that other scenarios are possible. I have looked at some of these, and for the most part, found one or more conflicting datapoints that appear to rule them out, though U have not and will not do so categorically.

I’d be interested in your read of other failure mechanisms that remain possible, given the available pool of empirical evidence.

I believe that posting them, even as speculation, yields more benefit then harm, and opens the door for further discussion.

[mroper]

Thanks for responding. I was afraid everyone had finally left this discussion! I think it’s important to realize there is a line between what we know or can logically infer, and rank speculation. As long as we maintain an awareness of that distinction I think it’s reasonable to postulate and conjecture.

Your fine detective work on identifying the steel members of the structure post-collapse is 100% valid IMO. It may be too much to ask that we’ll see hi-res pictures of the scene as the rubble is removed. I would think they would remove the concrete and rebar first, exposing the steel underneath. There’s a lot under there we’ll never see.

Regarding alternative collapse sequences, my thoughts on that come from reading the URS bridge analysis report. They identified 52 critical beams that, if one of them failed, would cause others to fail like dominoes. They listed them in their report but I need to go back and read it again. IIRC they identified secondary failures in some members that were not in close proximity to the initial failed member. That stood out when I first read it and I must read it again. Their study, of course, is predicated on certain assumptions that may turn out to be invalid. Garbage in, garbage out. If there initial data was bad then the results are suspect.

My model is not a finite-element model. It’s visual only so right now I don’t envision plugging it into any stress analysis software to analyze. Speaking of the picture I posted above, it occurs to me that perhaps the deck had already begun to fall in the first frame of that video. That could potentially explain the “curvature” of the roadway that I cannot replicate with the intact model. And speaking of video, I read yesterday that someone thought the camera wasn’t a video camera at all, but rather one that snaps still pictures in sequence, either at timed intervals or triggered by motion detectors. I don’t want to speculate further on that without knowing more about the actual camera system they use.

Glad you’re still here.

[sgw1009]

Following are the individual frames of the Army Corps of Engineers video of the I-35W bridge collapse, which all may find of interest.

There are a total of 26 frames of video, and in the original (posted) sequence it lasts a total of about 12.4 seconds, meaning that the time between frames is about 0.477 seconds.

It is interesting to note that there is no bridge movement between the first and second frames, meaning that the video has captured the entire collapse from the start (which begins between frames 2 and 3, and is already underway at frame 3. It is also interesting to note that there is a very slight upward shift of the camera between frames 2 and 3 (coincident with the beginning of the collapse).

It is also noteworthy that the bridge segments closest to the camera (at least from initial observation) appear to be falling straight down.  It is difficult from this perspective to tell if the east (far) side of the bridge was falling ahead of the west (near) side.

Even though the bridge appears to have broken somewhere on the south end, out of camera view, these pictures can perhaps still provide some additional information and insight into the collapse.

 

Bridge00.gif

 

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[jeffers]

I downloaded those as soon as I got your e-mail.

Those and the other images you linked me to are very good ones, and have already allowed me to further the investigation.

Some things are becoming very clear, expect some minor changes in my opinion of where the earliest known failure occurred, but nothing major.

I’m going to take a few of those video images and manually extend the superstructure skeleton to the right to include pier 6 and you’ll see what I mean.

The focus is narrowing and the possibilities diminishing, and there are critical superstructure elements visible in both the older images, and to a better degree in the newer ones.

It will take time to put it all together, and I am obligated over the weekend elsewhere. Maybe mid next week?

In any event, thanks for your help with those images, your contribution to our informal analysis cannot be overstated.

[kwuntongchai]

Hi Everyone,

I’ve been away for a couple weeks, are there any current discussions on the collapse? How about current images from the debris cleanup?

Kwuntongchai

[Orion78]

Interstate 35W Bridge Photos
http://www.dot.state.mn.us/i35wbridge/photos/

[brityank]

From USA Today ^ is a report from the NTSB blaming the gusset plates; used 1/2-inch thick as against 1-inch thick. Seems they didn't fully account for the increase of road lanes from four to eight against the original design weight loading:

Design changes in 1977 and 1998 added additional pavement and concrete barriers that increased the weight of the Interstate 35W bridge in downtown Minneapolis.

Thank you for your info above.