The year 1967 was a watershed of sorts for bridge failure. The Silver Bridge in West Virginia collapsed unexpectedly that year, killing 46, and the disaster led engineers to place much more emphasis on what they called redundancy. Simply put, redundancy means that if one part fails, the whole bridge won't go down.
The lesson came too late, though, for the builders of the Interstate 35 bridge in Minneapolis, which was opened that same year, and which failed catastrophically during the Wednesday evening rush hour.
It turns out that bridge was on a federal list of structurally deficient spans - one of a startling 73,764 bridges across the country that are not up to grade. A further 90,226 are listed as functionally obsolete. These numbers are not quite as bad as they seem, because the categories cover a broad range of shortcomings - but they're bad enough.
Maryland has 410 bridges on the structurally deficient list, about 8 percent of all the bridges in the state. That percentage has been coming down, and it's better than the national average. But half of the bridges in Maryland were built before 1967, that important year, and their age is beginning to show. Most of those that are structurally deficient - 85 percent - are in this older group.
Most worrisome are the original interstate highway bridges, built in the late 1950s and early 1960s. Today they carry huge volumes of traffic, and some of them - on I-95 near Baltimore and on I-695 - make the deficient list. They were put up in an era when engineers favored all-steel bridges. But steel is susceptible to corrosion, and steel bridges are considerably more likely today to have structural problems than those using pre-stressed concrete or other materials.
What this means for Maryland: A crunch is coming. For too long, the roads and bridges in this state (and in the nation) have gotten less than adequate upkeep. In marked contrast to the way the great bridges of the 19th century were so thoroughly over-engineered, modern highway construction and maintenance have been done on the cheap, and the Minneapolis disaster is a reminder that the bill is starting to come due.
The American Society of Civil Engineers calculates it will require $9.4 billion a year for 20 years to eliminate all bridge deficiencies. It's a lot of money - but a crumbling infrastructure poses a clear and present danger to America's future prosperity.