Once again, the news intersects with our interests here at CatSynth, this time in a disastrous and tragic way. From AP:
MINNEAPOLIS – An interstate bridge jammed with rush-hour traffic suddenly broke into huge sections and collapsed into the Mississippi River Wednesday, pitching dozens of cars 60 feet into the water and killing at least seven people.
The eight-lane Interstate 35W bridge, a major Minneapolis artery, was in the midst of being repaired and two lanes in each direction were closed when the bridge buckled.
Here is what the bridge looked like for the 40 years it spanned the river:
And here's a piece of what's left after the collapse:
Between this last photo and the first, one can see that there was a total and complete structural failure, and tragically one that happened during a busy rush hour, killing several people and injuring more. It must have been incredibly frightening to watch the bridge collapse, or be on it as the road buckeled and cracked and suddenly one ends up 50 feet down on the river. We at CatSynth extend our sympathies to those who lost friends or family is this tragedy.
Of course, people are already beginning to ask what could cause a bridge like this to completely fall apart like this? The construction on the bridge comes to mind, but apparently that was just on the roadway and “None of it would be related to the structure” [AP]. As for the structure:
Gov. Tim Pawlenty said Wednesday night that no structural deficiencies were found during bridge inspections in 2005 and 2006. The bridge deck was scheduled to be replaced in 2020 at the earliest, Pawlenty said, and legislators offered a similar assessment.
But public reports on the bridge raised questions about its safety.
In 2005, inspectors from the Minnesota Department of Transportation deemed the bridge “structurally deficient,” in data submitted to the Federal Highway Administration's National Bridge Inventory.[Pioneer Press]
I did not know there was a National Bridge Inventory. Some more detailed structural engineering (from the same Pioneer Press article):
The I-35W bridge apparently is what state transportation officials consider a “fracture critical” bridge, meaning it has at least one critical tension member whose failure would be expected to result in a collapse of the bridge…
…engineers said the fatigue cracking was a serious issue due to the lack of redundancy in the main truss system. Only two planes supported eight lanes of traffic, they wrote.
“The truss is determinate and the joints are theoretically pinned,” the report states. “Therefore, if one member were severed by a fatigue crack, the plane of the main truss would, theoretically, collapse.”
At first, the description sounded to me like the classic resonance or self-excitation collapses, as described on this site. But it sounds like all that was needed was one well-placed beam to give, and the whole thing would fall apart. We'll see what info continues to come out in the following days…