Quote:
Originally Posted by Maksutov
Same weight but differences in vertical accelerations.
Kind of like running in worn out running shoes versus new ones. Same weight, but remarkably different impact on one's feet and legs. Translate that to the bridge structure.
I'd go with the old design and inadequate maintenance approach as the primary cause, rather than a poor design. A poor design and deferred maintenance as the cause are best exemplified by the Mianus River bridge collapse.
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We are agreed that it would be considered a poor design
today, because of no redundancy.
The question: should it have been considered a poor design in the sixties when it was built? Certainly it was known then that these type of truss bridges were among the weakest types of bridges. It was also among the cheapest. It's pretty clear that the intent was to get an inexpensive, easily built bridge in service quickly.
BTW, I just saw this NY Times article that had this alarming fact:
Engineers See Dangers in Aging Infrastructure.
By JOHN HOLUSHA and KENNETH CHANG
Published: August 2, 2007
"Still, a study by the Federal Highway Administration found that visual inspections, the primary method used by bridge inspectors, only rarely detect cracks from metal fatigue.
"In the study, completed in 2001, 49 bridge inspectors from across the country examined test bridges in Virginia and Pennsylvania.
Only 4 percent correctly identified a fatigue crack. Worse, many inspectors identified nonexistent problems, suggesting that bridges sometimes undergo unnecessary repairs while some serious conditions are not detected.
Inspectors now sometimes employ tools like ultrasound, but those add time and cost to their work."
http://www.nytimes.com/2007/08/02/us...=1&oref=slogin
Clearly, better methods need to be employed routinely in inspecting bridges.
I attached a blow-up of the image showing the concrete support of the I-35W bridge before the collapse.
I also attached the image accompanying the NY Times story. You can see the supports for both the I-35W bridge and 10th Avenue bridge in the image. (note: this image shows the 10th Avenue bridge actually has 4 lanes not 2.)
The I-35W supports look puny in comparison, and this is for a bridge that had twice as many lanes and did not have the extra supports in the middle of the river.
Bob Clark