Thanks! Looks like U-10 is on the south side of the bridge, near Pier 6 and over the water.
I wonder where the construction equipment and material was located at the time?
Are there any before and after pictures of Pier 6?
I am interested in after pictures that show the entire structure of the pier.
Jeffers has again done an outstanding job of documenting the point of failure of the center span. His ability to document with pictures and expalanations are unmatched.
My gut instinct tells me that the failure of the center span at Gusset U10 was an event that had its beginning in a shift of Pier 6 towards the river due to scour or failure of the soil beneath the pier footings. The horizontal shift popped the entire bridge off of Pier 7, after the strain had built up to an intolerable level. Thus the approach to Pier 7 fell and a portion of the center span wound up on the side of the pier, rather than just failing downward. The horizontal shift towards Pier 7 also pulled the approach span off of Pier 6 as well, causing it to fall.
The bridge failed under a situation when it was nowhere near its design capacity—no ice, no snow, no wind, no eight lanes of bumper to bumper traffic, and no lessening of its factor of safety, and it was standing for 40 years.
The bridge fell so violently and so quickly that something foundationally was wrong. The result shows up in how the piers looked after the collapse.
I don’t doubt that Plate U10 failed, as the failure of any one plate would have brought the center span down. There were also fifteen other plates that showed a failed condition, no doubt a result of the collapse itself.
I may be all wet, but I recall a post collapse picture taken the evening of August 1st that showed that Pier 6 had moved. Pier 7 did not move at all, as it was locked in by concrete, whereas Pier 6 was in the water of the river.
I’m not a structural engineer like Jeffers, but something about the gusset plate theory doesn’t ring right to me as the only initiator of the entire event.