Andy's post on a mailing list today reminded me of something I meant to blog awhile ago, its a nicecogent engineering summary by Henry Petroski of the Minneapolis 35W bridge that fell (I drove across the day before it happened, and thought this is actually a really good shortcut, I should come this way more often) (emphasis added):
Engineers often look to examples of success and failure to guide their designs. Paradoxically, it is the failures that are the more reliable teachers. As the case of the Minneapolis bridge so clearly shows, a structure that stands successfully for decades is not necessarily a sound design. However, when a bridge fails, it provides invaluable lessons in what not to do.
No matter how carefully bridge designers anticipate failure on the drawing board (or computer screen), their structures will only be as reliable as how carefully built, maintained and inspected they are. Just because a bridge has given decades of successful service under adverse conditions of increasingly heavy traffic and neglect does not mean that it will continue to do so. It is the function of regular and careful inspections to catch what designers might not have anticipated.
In the wake of Minneapolis, there will no doubt be renewed vigilance. More careful inspections and more conservative interpretations of their findings may cause some immediate inconveniences, but they will also likely prevent some imminent failures.
In bridge design, as in all structural engineering, success can breed hubris and catastrophe, while failure nurtures humility and caution. Unfortunately, it does seem to take a collapse to re-sensitize inspectors and operators to the real dangers that lurk among rusting steel and cracking concrete. Let us hope that the lessons learned in Minneapolis are not forgotten once more.