[jeffers]

That gusset appears to have failed due to a moment. The far end of the member attached at that point was rotated about that gusset connection point, popped the bolts and pulled free.

A "possible gusset design flaw" is not much to go on. Much more useful would be the location of that gusset or gussets, as we still don't have the major components of the failure sequence in proper order yet.

Still focusing on the north end of span 5/pier 5, and/or pier six and the superstructure above pier 6 here. Both still look viable. Initial failure could have been anywhere from the midspan of span 5 through the south half of span 7 (center span), probably nearer the east side than the west.

The most interesting point on the bridge for me right now is just north of pier 6, east truss. Road deck south of pier 6, and presumably the span 6 east truss look prety good comparatively, but the span 7 east truss got hammered hard just north of pier 6.

I've isolated the SE kingpost (buckled under load) and the top chord, (folded down under moment at north end of span 6 road deck) and one other member, either a tension brace or the bottom chord, but that panel is so scattered that it's hard to piece it back together.



Blue arrow 1 indicates the SE kingpost, blue 2 is probably the tension brace angling north and down from the top of the SE kingpost, blue 3 is probably the east truss span 7 top chord, and blue 4 is probably the span 7 east truss bottom chord, although it may also be the diagonal tension brace if blue 2 is wrong.

Red arrows 1 and 2 seem to indicate that the top of the southwest kingpost rotated south, both bottom chords are bent consistent with this theory. So is the top chord of the pier 7 west truss, just slightly off the end of blue arrow 4.

Red arrow three indicates similar behavior of the span 7 east truss top chord.

If the center span broke free early on, its counterweight would be deprived to span 6 just south of pier 6, and would explain why the road deck was so compromised just north of pier 6.

However, the SE kingpost or other members closely associated with that panel of the east truss could easily have failed first, dropped the south end of the span 7 east truss, overloaded the south end of the span 7 west truss, and accomplished the same result.

I'm starting to consider a failure around the second or third panel of the east span seven truss, just north of the SE kingpost. That would be at the right end of the member marked with blue arrow 3. Supporting this is the position of the east truss span 7 bottom chord (blue arrow 4.), which does not appear to have leaned over eastward with the rest of the eastern truss at pier 6.

Initiating trigger or not, bad things happened there.

[jeffers]

Yes, the more i think about this, the more sense it makes. It resolves an issue that’s been bothering me since I first realized the trusses leaned east at pier 6.

THE SPAN 7 ROAD DECK DID NOT LEAN EAST WITH THE PIER 6 TRUSS PANELS. IT DROPPED STRAIGHT DOWN.

Since the trusses above pier 6 clearly leaned east, and since they did so without taking the south end of span 7 with them, span 7 MUST have seperated from BOTH main truss kingpost panels, PRIOR TO those panels leaning east.

It explains why the SW kingpost rotated, and why span 6 failed, it lost its center span cantilever counterweight.

The critical evidence is in the river, close to the south shore.

[TXnMA]

Thanks or posting that great overhead view of the south bank piers and associaated damage -- it is just what I was looking for.

To aid in visualization, I have overtinted the piers & footing in yellow, and have tinted the following elements in orange:

A. What I interpret as the western kingpost and its associated "triangle" -- with an arrow showing how it rotated clockwise (in the plane of the view) off of its shoe. (The shoe [possibly damaged] still remains atop the pier.)

B. The western edge of the cantilevered deck "tucked back in under" the remains of the deck where the top of the kingpost went under the deck. (Does this indicate that the western kingpost collapse was early in the collapse sequence?)

C. A secction of ?beam? bent into a curious arc shape. What does this say about the various moments in play here?

Also, this view confirms that the decking riverward of the piers is missing (and, presumably in the river). What does this say about the forces imposed on this system?