OH CR4P!

From a reliability standpoint, would you be more inclined to trust a standard, familiar design or something daring and new?

While the old and familiar would seem to be the more tried and trusted, that's not always the case. In fact, the Minneapolis bridge collapse indicates that sometimes it is easier to become complacent and overlook the details when dealing with well-established designs and structures.

The Incident

On the evening of August 1, 2007, during rush hour traffic, the I-35W interstate bridge in Minneapolis collapsed. The entire span of metal and concrete was sent crashing into the Mississippi River, along with all the vehicles on it (the wreckage pictured left, Image Credit: Jeff Wheeler - The Star Tribune). The collapse killed 13 people and sent many more to the hospital.

This security tape and video of the event shows the collapse and the extent of the wreckage.

The Cause

The cause of the collapse, despite the ensuing politics relating poor maintenance to low taxes, was found to be due to a faulty design. The report filed by the National Transportation Safety Board (NTSB) claimed that certain gusset plates that connect bridge structural members were under-designed, specified with ½ inch thickness instead of a larger thickness. These plates were not sufficient to handle the bridge load, which had increased over time (from concrete additions) since its birth in 1967. The image to the right shows a photo of the bowed gusset plates, taken in 2003.

However, it should also be noted that the bridge had been labeled as "structurally deficient" and had scored low in safety ratings years prior to the incident. Multiple projects planned to reinforce the bridge were cancelled or postponed, and only weeks before the collapse did construction begin. The maintenance (which included joint work, and rail and concrete replacements) loaded the bridge with some 575,000 pounds of construction materials, which were on the structure during its collapse.

The Effect

After the bridge collapse, a new bridge was built at the same location, completed on September 18, 2008. State legislators were pressured to increase funding for statewide bridge maintenance. A fuel tax increase of $0.055 was the result.

Lessons Learned?

Interestingly enough, an American engineer by the name of Henry Petroski expected that a bridge collapse like this was coming. He supports the theory developed by Sibly and Walker which says that a major bridge collapse happens every 30 years. By then, the theory explains, a new generation of engineers has emerged to ignore the lessons of the past. The last major bridge collapse in 1970 (the West Gate Bridge disaster) meant the date was set for 2000, and the Minneapolis bridge was only 7 years late.

In an interview with Discover Magazine, Petroski talks about how standard, familiar designs are more likely to fail. He says "When you're trying to do something really new you tend to take a lot of care…. When these sort of standard bridges are built it's not the really high-level engineers that are doing the detail work on them."

The culprit is complacency. Petroski elaborates: "It appears that what causes the cycle is that engineers tend to get a little complacent. Engineers are human beings. It's part of human nature. As you get familiar with something and you think you've mastered it, you tend to get a little less careful. It happens to all of us. We're driving down the road at 70 miles per hour, and you think, 'Well, I'll just lean over to pick something up off the floor,' and there's an accident."

[Speaking of overconfidence leading to accidents, take a look at this video of the recent snowstorm in Utah. Keep in mind the cars were warned not to go down the hill…]

I think this theory makes a lot of sense. Complacency seems to go against the heart of engineering, which is to look for and fix problems. In a world where systems always proceed toward a state of failure, engineers should never be truly comfortable, even with a tried and true design. There should always be questions and always be improvement.

What's the solution to this supposed natural cycle of complacency? Can we hold on to an attitude of concern as new generations and leaders emerge, or are we destined to repeat past mistakes? I certainly don't want to wait another 30 years to find out.

Sources:

NY Times - Faulty Design Led to Minnesota Bridge Collapse, Inquiry Finds

NTSB Update on the Minneapolis Bridge Failure Analysis

Discover Magazine - The Man Who Predicted the Bridge Collapse. Kind Of.