The southern pier structure moved quite a distance during the collapse, and wound up way out of plumb. The northern pier structure stayed in place, and stayed plumb.
The southern pier still had the bearing plates attached to the top of the piers. The northern pier had the concrete pier tops ripped away violently, tearing away the bearing plates off the top of the piers.
The southern pier was undermined, possibly by years of undetected scour, and moved to the north, putting extreme horizontal forces onto the northern piers. The bridge exploded off the northern piers, and twisted up and over the side laterally. This pulled the bridge south, causing the center span to snap and fall as a unit. The extreme bending in the center span as it collapsed(from the video) shows that tremendous horizontal force was in play. As the southern pier moved south, the southern half arch lost its bearing, and fell backward, but not before pulling the southern roadway toward the bridge, which pancaked the southern road sections. At the northern end the shearing of the bridge and the twisting collapse of the northern half arch pulled the northern roadway elevated spans off their supports and pancaked them.
This all happened in a matter of seconds.
For the southern pier to move as it did, and to wind up out of plumb so severely shows that it was the weakest link in the bridge.
The bridge was not experiencing anything near its design load that day, having one half of the traffic loading and no loading for snow and ice. There was no vertical loading that took away the safety factor already in the design.Heat was not a culprit, as cold affects the strength of steel more than a hot day would.
That the NTSB is talking about gusset plates at this point is meaningless. Gusset plates are overdesigned, and they held up the bridge for forty years.Gusset plates are used everywhere in structures—look around and you will see them. All the steel will have to be examined, but my hunch is that the problem with this bridge was not in the superstructure above the arch, but more foundational, such as a problem with the foundation under the southern pier, which was adjacent to the water.
Although the recent bridge report said that scour was not a problem, with the turbidity of the water, a diver would be hard pressed to see signs of scour when they can’t see six inches under the water.
This failure was catastrophic, and the way the center span fell, almost snapped at both ends, instead of failing at one end, and then pivoting down into the river, shows that extreme horizontal forces were involved.
That’s my take on it, just from looking at the pictures taken by photographers the evening of the bridge collapse and watching the video from the lock.
Examination of the steel remains to be done, but my opinion is just that, my opinion.
I have been reading your work on here before and find it very interesting. Thjank you for your insight. It has helped me to understand this.
From the video I saw, the center section just drops. First one side and soon the other. Once freed from the center section, the shore sections simply fall back on themselves pulling and twisting on the pilings on the shore on each side of the river. 1 of These pilings is only in the river when it is high.
My opinion only.
I can't fully tell exactly where in the center span all the trucks and rock we were discussing was, but IF it was all concentrated toward that south pier -- and the pier was possibly weakened by scouring -- is it plausible to suspect that the excessive weight concentrated in one spot may have placed stress on the pier in just the right angle or something to cause the pier to fail as you suspect -- or even if a gusset plate or a girder failed right there could that create the stress needed to cause the southern pier to shift and fail???
Are you sure you don’t have north & south reversed? (Hint: look at the sun angle and shadows...)
Thanks for the feedback.