This is complicated, but here goes. The earliest failure I can detect took place at U 10, east side.
Why?
Because it’s at the bottom of the pile. There’s one other piece of bridge, at the bottom of the pile too, in a different place, a short section of sidespan quite a bit south of the river, specifically the north end of the road deck that came to rest vertically, that the southbound semi crashed into, and that the southbound school bus almost crashed into.
Either could have been the initial trigger for the event. I prefer the mainspan trigger, because more bridge is involved, but from this distance, given the imagery available, I can’t say either way with any certainty.
For now, I consider the original trigger, one of the two specified above, to be moot.
The key to understanding the failure sequence, once U10 was not delivering in function, after it tore, is the understanding of the nature of the bridge itself, as designed.
At the highest level of simplicity, there were three bridges, from south to north, a standard beam and post highway overpass type bridge, a trussed span over the river, and another beam and post highway bridge at the north end.
At the next, more complex level, we look only at the trussed span. Strictly speaking, there were three trussed spans, supported by four double piers, from south to north, pier 5, 6, 7, and 8, but even this is misleading. The real meat of the nature of the bridge design comes now.
Instead of three trussed spans and four pairs of piers, think of four, T shaped piers, with very short sections of road deck connecting the horizontal ends of each T shaped pier.
The superstructure of the bridge was designed as a complex combination of a cantilever and a simple truss, such that the first few panels of truss, on either side of any given pier, are in balance with the oppossing first few panels on the opposite side of the same pier. In a perfect world, you could remove all but three panels of truss on either side of one pair of piers, and they would stand there in perfect balance, because that’s precisely what they were designed to do.
If, however, you remove one bolt from either side, the structure becomes unbalanced about the pier and will tilt off to the other side like an unbalanced teeter-totter on the playground.
With this in mind, we can now see the failure sequence.
1. U10 east fails, creating two significant problems for the entire east truss, basically everything south of the failure, and everything north of the failure. U10 is at the north end of the pier 6 “Tee”.
2. The south end of the east truss atop pier six now carries itself plus the short section of southern road deck, while the north end, separated by the U10 fracture, only carries itself. Like the unbalanced teeter-totter, the north end begins to rise, while the heavier south end drops, the whole rotating about the top of pier six.
3. The south end of the short section of road deck still attached to the pier seven “Tee”, is not designed to be self supporting, nor is it carried by pier 7, so it also begins to fall. Remember, so far, we are only looking at the east truss.
4. While the superstructure above pier 6 (east side) is rotating about pier 6 due to the unbalanced forces around pier 6, the short section of road deck still hanging from the pier 7 “Tee”, (actually a pretty long section of road deck, the full length of the mainspan minus 4 panels) sags enough to put most or all of its load on the west truss, centered at pier 6.
5. The west truss around pier six cannot sustain the additional load and begins to distort. The connections between the short section of mainspan road deck and pier 7 cannot hold the mainspan up either and shear off. Things are happening very quickly, and even a supercomputer might not get the sequence here exactly right.
6. The center span road deck is now in freefall two panels south of pier 7, in freefall on its east side two panels north of pier 6, and all or in great part, loading the west half of pier 6. The entire superstructure, the road deck and both trusses, begins to lean to the east, but cannot move laterally to the east, because it is still connected to pier 6 via the west truss. It twists instead.
7. This twist imposes a rotation onto the “tee” supported by pier 6, including both the east and west truss. The lateral crossbracing between the two trusses fails, and almost, but not quite simultaneausly, the U10 and L10 connections on the west truss let go.
8. The mainspan, now in complete freefall, drops straight down into the water, coming to rest where we see it in the post collapse imagery. The superstructure above pier 6, continues its sideways rotation, to collapse with the west truss atop the east truss, and it takes the north end of the pier 5-pier 6 span eastward with it.
9. The superstructure atop pier 7 doesn’t know it’s dead yet and remains standing for several seconds. But it too is unbalanced. Its south end carries nothing, while its north end carries the south end of the pier 7-pier 8 span. Slowly, it begins to rotate, both trusses in unison, river end up, shore end down, another unbalanced teeter-totter.
10. But there is a critical difference between the pier 6 assembly and the pier 7 assembly. The pier 6 superstructure rests atop a roller bearing nest, basically some very large steel rolling pins. This is done so the bridge can expand and contract as seasons change, sliding north and south atop pier 6. Pier 7 however, is fixed, pinned in place, the bearing plates are fixed, with no rollers, directly atop the concrete of pier 7.
11. Had this been simple beam and post construction, with a span depth miniscule compared to span distance, the pier 7-pier 8 span would have broken and fell, without greatly disturbing pier 7, even though there were no roller bearings at pier 7. But it wasn’t. The superstructure thickened atop the piers, and simple geometry, as the span sagged, pushed pier 7 riverward as if the earth holding it in place didn’t exist.
12. At some point in the sag of the north sidespan (Pier 7-pier 8), and the rotation of the top of pier 7, the pier 7 bearing plates disconnected. At that point, pier 7 itself stopped moving, and the superstructure slid atop pier 7, coming to rest as seen in the post collapse imagery.
The above is as close to a non-engineering failure sequence as I can manage. Jim Trent posted a must read opinion, from the point of view of an experienced inspector/designer, here:
http://www.freerepublic.com/focus/news/1879030/posts?q=1&;page=1#1
Since I’m not a professional engineer, and enjoy the professional freedom that allows without retribution, or penalty for error, I posted some deeper analysis and speculation, from the point of view of an engineering trained, longtime construction field supervisor, here:
http://www.freerepublic.com/focus/news/1884493/posts?page=1
The latter has imagery depicting the failure sequence, though the description may be more technical in nature.
If you still have questions, you can look deeper at the sources above, or ask them here and I will try to answer them.
Jeffers, your anaysis is very complete, and I thank you for providing it, as well as those which you have on the other threads last year.
I am very impressed by your ability the identify parts of the wreckage, and to sequence events in almost real time in a way that is understandable.
I know that ASCE is very interested in hearing input from their members on this bridge collapse in response to this very article.
Won’t you consider emailing your research to them? I think they would find it very helpful, as they are big into infrastructure improvements nationwide, working with Congress, etc. and this collapse is certainly the hot topic now as the poster child for all that ails the declining condition of the nations highways and bridges.
Your practical experience as a construction supervisor yields a viewpoint many engineers are lacking in, bridging(no pun intended) design and field erection. Sometimes the two don’t resemble each other.