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.
The complete, total and uniform failure of the center span is difficult to reconcile. One might expect the span to fall to one side or to fall to one end. The center span sits on only four bearings. Being of non-redundant cantilever construction the other sections were not self supporting and also failed when the center span gave way.
The real time video clearly shows a nearly uniform failure of the entire center section. What is amazing is that both ends of the bridge detach at almost the same time.
http://www.youtube.com/watch?v=EjCic0YlJwQ
The critical points appear to be the four bearings that the center section sits on. If just one bearing moves or breaks a collapse will occur.
Here is my scenario
- Steel corrosion of the trusses and especially the bearings weakens the steel trusses and creates misalignment of the bearings.
- Cold weather reduces the ability of the steel to flex which induces cyclic fatigue stress cracks. (Hoan bridge failure in 2000)
- De-icing of the bridge deck with various corrosive agents further attacks the steel and the bearings.
- Recent spate of high temperatures and poor maintenance of the expansion joints results in the expansion joints being “locked up”.
- Lane closures, stationary traffic and repaving equipment creates an asymmetrical load on the bearings.
With some or all of the expansion joints clogged with dirt and debris a tremendous load is building across the top of the bridge. This unplanned top loading is creating excessive stress in some areas of the bridge and actually relieving stress on other components of the bridge. Asymmetrical loading due to the lane reductions is further contributing to the uneven loading of the bridges and misalignment of the bearings. Distribution of heavy trucks and construction vehicles on one end and or side of the bridge further contribute. Some or all bearings are not sitting in full contact in their pockets. At this point a bearing or truss support which has been weakened by corrosion and stress cracks has a catastrophic failure. Other trusses or bearings let go within a fraction of a second and the whole center section falls to the river in one piece.
Eye of Unk makes an interesting observation regarding the construction vehicles and harmonics. Very interesting.
Lastly, I also think we are going to hear a lot more about expansion joints.
So, given what you’re saying- the combination of the train going under the bridge and the jackhammer we keep hearing about- could have an effect...