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And, as to what happened? This photo http://www.flickr.com/photos/s4xton/981290582/in/set-72157601157770382/
might explain something. Why is the location the girders buckled **behind** the pier? You’d think they’d have bent right on top of the pier. It’s not exactly normal for a girder to bend a couple feet out into the span.

It looks like a compression buckle. They got shoved, and bent out of shape? What shoved them, why? Did the whole south landside cantilever arm (U8 to U1) bust loose off Pier 5 because of expansion joint issues and crush the approach span?

There needs to be some explanation of this, curious as to what anyone thinks. This is the spot directly “in front” of the burning Tastee Truck and schoolbus.

One more bit, here's a diagram of how a cantilever with a suspended span works.

The mens' arms are the tension members, the sticks are the compression members. In my diagrams above, red are tension, blue are compression, and green function as either compression or tension.

So, the failure mode I'm thinking is that, say, the guy on the right looses his grip on his stick with his left hand. What happens? the weight in the middle will make him tip inward, causing a complete failure. How will the middle section fail? The right guy tips inward, everything pivots where left guy and right guy's hands meet middle guy. Depending on the strength of middle guy's connection to left guy, he will either hang and dangle from left guy and pull left guy into the river, or (in the 35W case) bust loose from left guy and drop more or less straight down.

Meanwhile middle guy's falling has town loose left guy's right hand loose from the stick, and the stick drops, then left guy tips over to our left.

(Also, the buckle in the girders on the approach span, now that I look at it, appears to be too uniform from left to right to have been caused by the twisting south span... Must be some artifact of how a continuous girder bridge buckles over an intermediate pier if it loses one abutment)

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.