Last Wednesday the I-35W bridge over the Mississippi River in Minneapolis collapsed for no obvious reason. The problems with disasters are the instant experts that make claims as to what happened. Much of this sounds like the black helicopter/alien abduction crowd. My position is let the investigation be completed in peace. Having said that I want to take a look at some of the assumptions already made.
The bridge was overloaded and could not hold the weight.
From what I understand the four-lane bridge was narrowed to one lane due to construction. Therefore, there were fewer vehicles and thus less weight on the span than when it is fully opened. As for the weight of the construction equipment their total weight was likely less than the total weight of the traffic when it is fully operational. The only question was the weight and concentration of the raw materials the construction company had on the bridge at that moment.
The vibrations of the traffic caused the bridge to collapse.
This one is easy. The traffic was at a standstill. Traffic induced vibrations were virtually nil. The bridge was also constructed so no part of it touched the river so the vibrations of the current could not be an effect.
Extreme temperature variations made the steel weak.
For forty years this bridge has gone through extreme temperature variations with no apparent damage. Sub-zero Fahrenheit temperatures in the Winter to nearly one hundred degree heat in the Summer. Yes, all metals contract when cooled and expand when warmed. However, the southwest deserts have Summer temperature variations that encompass a 70-80 degree range daily! It is not uncommon in southern California, Arizona, New Mexico, west Texas to have mild frost on the bushes in the pre-dawn darkness then be 110+ within twelve hours. I used to live in the Mohave Desert in southern California and experienced it. My point is that the temperatures in the nineties last week were extreme only for humans and other sentient beings. Considering how hot one has to get steel to soften it, the temps probably were ineffective.
The ground shifted.
I give credence to this one. The anchor points were in the river's embankments. Considering that the ground there is naturally soft, else the river would not be able to cut a channel through it, the original builders had to anchor it in bedrock from the side. If that bedrock shifted cracked or weakened, the anchors could have moved.
In spite of my admonishment about letting the investigators do their work, I do have one possible theory that should be investigated. In recent years many states have switched from salt to magnesium-chloride for ice and snow mitigation. Mag-chloride has been shown to corrode wires on vehicles. If the state or city used mag-chloride on that highway it may have corroded the steel enough to weaken it.
In closing, I just want to say that I have not been able to post recently due to computer issues. They are finally resolved and I can blather on about the issues of the day. Yippee! :-)
