The New York Times reports that an engineering firm hired by the Minnesota Department of Transportation to investigate the I-35W bridge collapse has identified a design defect. I'm not sure I understand the article, either because of my inadequate understanding of engineering or because it's confusingly written, or both; but here are some extracts:
Investigators have found what may be a design flaw in the bridge that collapsed here a week ago, in the steel parts that connect girders, raising safety concerns for other bridges around the country, federal officials said today.
If those who designed the bridge in 1964 miscalculated the loads and used metal parts that were too weak for the job, it would recast the national debate that has emerged since the collapse a week ago, about whether enough attention has been paid to maintenance, and raises the possibility that the bridge was structurally deficient from the day it opened. It does not explain, however, why the bridge stood for 40 years before collapsing.
This suggests that I might have been on the right track when I advanced the theory in a previous blogpost that a design mistake made more than 40 years ago was responsible for the collapse of the bridge. A number of writers have made the point that bridge designers from the end of World War II until the late 1960s were more confident of their designs and built in less redundancy; in the trade-off between expense and safety, they were more willing to cut expense than engineers have been since then. The America that emerged from World War II was proud and confident of its ability to churn out machines; we took the view that we had mastered mass production and engineering. This turned out to be an overconfident view. American automakers started to encounter problems in the late 1960s that they have not yet solved; bridge engineers in the late 1960s decided that they had to change the balance between expense and safety from the one they'd used in the two decades after the war. The observation I quoted from Henry Petroski's Engineers of Dreams, that major bridge collapses came about every 30 years—the Tay Bridge in 1879, the Quebec bridge in 1907, the Tacoma Narrows bridge in 1940—may turn out to be right, with this caveat: The I-35W bridge collapse in 2007 may turn out to have been caused by mistakes made in 1967.
The implications for our infrastructure, should this turn out to be the case, may be massive. I'm sure we've still got a lot of bridges built between 1945 and 1967. One final note: The MnDOT investigation will not be the final word. That will come from the National Transportation Safety Board. This is one of our excellent government bureaucracies. It's independent of the bureaucracies responsible for constructing and inspecting and regulating bridges, highways, airports, etc. Its role is purely investigative and advisory, and it has no stake in defending previous decisions or actions taken by other bureaucracies. Presidents of both parties have appointed very able chairmen, who in turn have a high respect for the agency's professionals. We can be pretty sure that the NTSB will do a first-rate job on this.