I think it happened right in that area, but I’m just not comfortable with a trigger failure being attributed a member under compression.
I’m probably prejudiced on this, because each member is engineered based on what type forces it is expected wot withstand, but my gut likes tension failures first, shear and torsion tied for second place, and compression failures last.
That said, the discoloration on the other side at that location tells me that something was going on in the bottom chord, or that something had gone on in the past at that location. Frankly, the transverse dark brown lines near the northern end of the western beam look like either joints or cracks. That’s not a great place for a joint, much less so for a crack, and I don’t see other joints in other members showing that color either.
We’re approaching the edge of granularity here, in what we can discern from the available imagery, but I’m reasonably confident we aren’t on the wrong end of the bridge. Feel free to ping me when more develops, whether that is later today or a year from now, and I’ll return the favor if I turn anything up. I’m very much interested in getting deeper into this, but recognize the likelihood we’ll have to wait a long time for definitive answers.
Let me throw in my two cents here. First off I supervise a large concrete batching facility in central Alaska, I also drive a large mixer truck on occasions. We had just recently an order to deliver concrete onto a similar two lane type of bridge spanning a large river 60 miles north of me, the length is similar, the bridge was built in the mid 60’s. the project was reinforcing the king post pillars against massive floes of ice and an earthquake improvement project of the area around the king posts as like the 35 bridge. One thing I definately noticed at one time and it literally had everybody looking around like it was an earthquake was when I had to re-mix my load of 44,000 lbs. of concrete and I had the drum rotating at high speed and it set up a sympathetic vibration through the bridge almost like the episode of Mythbusters where at a certain frequency the whole bridge shook and was bouncing big time and that is what was happening. I am really serious about this, the bridge in MI has two lanes of bumper to bumper slow traffic but there was also construction equipment working and there may have been a condition of a different type of natural frequency and set up a vibration that found a literal weak link. like breaking step in marching across a bridge.
I am betting on this scenario only because its been proven that you can set up a vibration if you find that certain resonance and I experiance one just this spring under nearly exact conditions.
I agree. I have seen a lot of fatigue failures (most not on bridges, thankfully), and every one of them has been in a tension member or in a tension area.
I have never seen a fatigue failure in a compression member. I have seen buckling in compression and seen all kinds of different joint failures (weld cracks, poor fusion, rivet shear, bolt shear — but more often shear failure in the plate the rivet or bolt goes through — bad metal, bad repairs, bad initial welding, but I have never seen a fatigue failure in compression.