Quote:
Originally Posted by Mike in SW-MO
May have been an edit after you read the article, but this is telling:
Late Thursday, came the first definitive word from a government official about what was being done on the bridge when it fell. U.S. Sen. Marco Rubio, R-FL, said in a Twitter post that: “The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today.”
If the suspending cables pass through the concrete, makes me wonder if they snapped during the tests & the whole structure was tgen supported by just the concrete.
Great shades of the T-Dance skywalk collapse.
|
It doesn't look to be a support problem in the sense most think.
It's looking to be a problem of lay construction crew not realizing the consequence of their actions.
This is post-tensioned concrete, and while the tensioning steel was probably well-rated to carry the load asked of it under the construction plan, it had a brittle failure when the construction plan was altered in a way that placed the tensioning steel under unforeseen stress.
The best lay analogy I can think of if you tried to winch your friend's car out of a ditch. Suppose you use the appropriate winch and attach it at the approropriate points, but you don't notice that there is a curb at the edge of the ditch that the wheels have to travel over. Then, when the wheels hit the curb they snag, you're suddenly lifting the entire car off the ground, placing exponentially higher stress on the configuration than you would be simply pulling the car up an incline out of the ditch.
The original plans were for the moving supports to distribute the load differently, and the alteration created a high-stress area in the span while it was being placed. Then they noticed the flexion and sought to correct it by tightening the tension steel. This put the steel into failure range, but the gauges couldn't tell the worker's that, because as the tension steel approaches brittle failure, it stretches slightly so that additional tension doesn't create additional pressure, kind of like a rubber band at the end of its elastic range.
The failure to see the consequence of the change in construction plans is one thing, the decision to attempt to repair while suspended over active traffic is another entirely.