Video: Caught on tape — the 35W bridge collapse; Update: Systemic? Update: E-mail from a bridge engineer; Update: Inevitably, the left starts to blame Bush
posted at 8:47 am on August 2, 2007 by Allahpundit
I figured this footage would turn up. We only get a glimpse but traffic doesn’t appear to be bumper to bumper, contrary to the reports yesterday. That may help explain why the cops have now revised the death toll down from nine to four, with the earlier number having been based on best estimates. They expect it to change throughout the day, though; the Strib, ominously, reports 20 people are missing. According to WCCO,
Firefighters swam car to car to look for survivors in the Mississippi River Wednesday evening. Clack said a lot of spaces around the collapsed bridge are hard to get into. Structural engineers will tell rescuers when it is safe to go into those area.
Still no word on a cause but the bridge was scheduled to be inspected … this fall. Standby for update as warranted.
Update: I mentioned the 2001 report in last night’s post but this is the first I’ve heard of earlier problems:
Reports issued by the Minnesota Department of Transportation over the past decade have detailed problems with the bridge. In 1997, the department noted problems with the approach spans on both ends, including “cracks . . . in the cross girder at the end of the approach spans.” In a 2001 report, department engineers said that the bridge’s deck truss “has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and floor truss system.”
But that report concluded that the bridge “should not have any problems with fatigue cracking in the foreseeable future.” As a result, they wrote, the department “does not need to prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project.”
Update: More warnings.
Two years ago, the U.S. Department of Transportation’s National Bridge Inventory database said the bridge was “structurally deficient.”
The Minneapolis Star Tribune quoted Jeanne Aamodt, a spokeswoman for the Minnesota Department of Transportation, as saying the department was aware of the 2005 assessment of the bridge.
The bridge received a rating of 4 on a scale of 1 to 10. A bridge receives a rating of 4 when there is “advanced section loss, deterioration, spalling, or scour.”
Update: Read this short but pointed assessment in Popular Mechanics about the scope of the problem. I wrote a few weeks ago about NYC’s antiquated power system. It’s all part of a theme:
Age and heavy use are by no means isolated conditions. According to a report card released in 2005 by the American Society of Civil Engineers (ASCE), 160,570 bridges, or just over one quarter of the nation’s 590,750 bridge inventory, were rated structurally deficient or functionally obsolete. The nation’s bridges are being called upon to serve a population that has grown from 200 million to over 300 million since the time the first vehicles rolled across the I-35W bridge. Predictably that has translated into lots more cars. American commuters now spend 3.5 billion hours a year stuck in traffic, at a cost to the economy of $63.2 billion a year…
The fact is that Americans have been squandering the infrastructure legacy bequeath to us by earlier generations. Like the spoiled offspring of well-off parents, we behave as though we have no idea what is required to sustain the quality of our daily lives. Our electricity comes to us via a decades-old system of power generators, transformers and transmission lines that has utility executives holding their breath on every hot day in July and August. Once we had a transportation system that was the envy of the world. Now we are better known for our congested highways, second-rate ports, third-rate passenger trains and a primitive air traffic control system. Many of the great public works project of the 20th century—dams and canal locks, bridges and tunnels, aquifers and aqueducts, and even the Eisenhower interstate highway system—are at or beyond their designed lifespan.
Update: Here’s an e-mail we just received from a bridge engineer in another state. All emphases mine:
I deal a lot with bridges in my job … and was out in the field inspecting bridges like this for 6 years before I moved into a more design oriented position.
The deck truss bridge is a fairly common design, especially in that era, before computer design. Trusses were the easiest way to span a large area, like the Mississippi River. The problem with all trusses is that they are fracture critical. The bottom chord (the bottom row of steel on the truss) is the main load bearing member, and there is only 1 on each side of the bridge. If anything happens to either one, there isn’t any redunadncy in the design, so everything goes down. The multiple girder row bridges you see in most shorter applications have some redundancy built in, so a failure usually isn’t catastrophic.
The inspection notes they have released sound pretty typical of a truss. All of them have details that are prone to fatigue cracking. Some of those panel points, where the different members are connected together, can have steel coming in from 6 or more different directions, and then you rivet, bolt or weld them together with plates. That means forces acting in all those directions, as well as rotation induced by movement in the bridge, all coming into a pretty stiff chunk of steel when they are all connected. All those forces on a stiff connection will find a weak point eventually. Usually, the cracks don’t occur in the main load bearing members, but in the beams going between the 2 trusses. However, with time and corrosion, the chance of failure keeps going up.
The thing that jumps out from the 2001 report that is online is that there were many Category E (the worst kind) details on the truss. Most of these are welded connections of members on the truss. The problem with welds is that they are hard to inspect, and a crack in a weld will not stop – if one steel member cracks at a weld, it can travel through the weld and propagate into the next steel member. I’m guessing they will find a certain connection detail that they blame, which will send everyone scrambilng to see what other bridges have them, so they can be repaired. The 2001 report also suggested that some of the easier to inspect welded details should be inspected every 6 months, since thay could be seen easily from the inspection catwalk on the bridge. We will see if this was being done or not.
I’m surprised they had certain members rated a “4” on a 0-9 scale, but they were seemingly only inspecting it every other year. The National Bridge Inspection Standards say they should do it yearly at a 4 or less. [Gov. Tim Pawlenty claims Minnesota’s DOT inspected the bridge in 2005 and 2006. — ed.] However, if it is under repair, inspections can be delayed until the work is done. I have a feeling this will come in to play as well. Most agencies don’t want to go out to inspect bridges when a contractor is working on a bridge, because it doesn’t want to impede their work, as well as not wanting to add to the traffic disruption.
Update: More from the Blotter on America’s systemic infrastructure problems. More than one in four bridges are structurally deficient (albeit not necessarily unsafe … yet).