I suspect a resonance was being built into the structure of the bridge with concrete mixers on one side of the 4 lane while the other side was gridlocked in traffic, I drive concrete mixers and they are just like a side type of washing machine that has say a huge wet sleeping bag in it and makes a thump thump thump except we are turning over 20 tons ( the mixer I drive is actually rated at 12 cubic yards @ about 4,000 lbs per yard)and I know for a fact on a bridge improvement project I was on this spring in Alaska my sole mixer induced a harmonic wave that set up in the span and made it start bouncing several inches enough to be distinctly noticeable much more than normal, the roadwork on the I35 bridge had several from what I saw on video much larger front discharge concrete mixers. I hope enough people with more background in wave resonance and such look at my theory. Perhaps the structure started to have side to side stresses never engineered at the time it was built couple with wear at key points.
Excellent points! I never thought of a concrete truck in that way before, but the washing machine analogy makes perfect sense. I’m with you on the vibration...
The center span (the portion actually over the water) looks like it could easily have a resonance introduced into it from any number of sources, because it looks to be mechanically isolated and damped by the shore structures and the piers.
If the construction workers inadvertantly hit just the right frequesncy of vibration, from whatever source or combination of sources, or even if the shock waves induced into the structure reflected back onto each other (and multiplied the forces at whatever point they met) the results would eventually be devastating. Do it right, and you can in fact turn that bridge into a giant, self-destructing, 1100 foot long tuning fork using only construction equipment.
I’ve seen what big chunks of metal can do when you hit them exactly hard enough, with exactly the right force, in exactly the right spot. Metal will fail in the most spectacular ways if you fatigue and stress it properly...
I’m standing by my thoughts on “inferior steel” as well, at least until we hear from the NTSB. I know they will do a metallurgical analysis of the steel, and if they rule it out, I will stand corrected. Ultimately that may be a contributing factor to this as well; there likely will not be a single cause to this, rather a combination of things.
Excellent points!