Indiana Gov. Mitch Daniels last Friday “ordered the immediate closure of the Sherman Minton Bridge … pending further notice” after “the discovery of a concerning crack in the critical load-carrying element of the bridge on Thursday.”
The bridge runs over an interstate (I-64) that crosses the Ohio River between Indiana and Kentucky. It normally carries 80,000 commuters every day, so this is kind of a big deal.
The closure has caused traffic chaos — or, as Indiana puts it, “extraordinary traffic congestion — as two nearby interstates take on extra flow. The state DOT counsels motorists to “leave early and pack patience, carpool or use public transit where possible, and arrange telecommute or staggered shifts with your employer.”
A commenter at the Washington Monthly (9/12 10:35am) explains the problem more fully:
I am a structural engineer experienced with bridge design and inspections (over 20 years) including truss bridges such as the Sherman Minton bridge. Truss bridges are unique in the sense that they do not have load path redundancy and their primary load-carrying members are what we define as “fracture critical” members that carry tension and are very susceptible to fatigue. As these steel chords are repeatedly stressed and released in tension by traffic loads, the metal gradually weakens over time. Eventually cracks form, which means that the metal is very near the end of its effective service life.
If one of these fracture critical members were to fail, the lack of any redundancy would mean that the bridge will suddenly and catastrophically collapse, potentially killings hundreds of people on the bridge.
… From what I’ve been able to track down on this situation, the commuters in the Louisville area should all be thankful that the inspectors found these cracks and that the public officials had the courage to act quickly.
New York State has a bridge with the similar vulnerabilities: the Tappan Zee, with 130,000 daily vehicles.