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The successful contractor must raise money for the work, rebuild the bridges and maintain them for 25 years, during which the state makes annual payments under the contract.
"We are going to get a lot of them done -- fast," said Jeff Bridges, spokesman for the Missouri Department of Transportation.
Oklahoma and Iowa lead the nation in structurally deficient bridges. These states are criss-crossed by section-line roads, many of which have old bridges needing repair. "We have 1920s and 1930s bridges -- they were built for Model T's," said Walt Peters, Oklahoma's assistant bridge engineer for operations.
Last year, the Oklahoma Legislature approved an extra $100 million to repair or replace 137 bridges over six years. Even for a small bridge, a typical rehabilitation can cost $2 million.
Iowa, whose covered wooden bridges were made famous in the book and movie "The Bridges of Madison County," has more than 600 bridges of all types more than 100 years old, according to federal bridge data.
Most states, including Iowa, have done better maintaining bridges on major highways, where less than one in 12 is structurally deficient.
Yet many of the spans built during the bridge-building boom of the 1950s and 1960s soon will be reaching the end of their useful lifespans, even with repairs.
"They might be OK now, " said Gary Novey, Iowa's assistant bridge engineer. But what happens in 10 or 20 years? "That is what we have to be concerned about," he added. "If we get those coming at the same time, I don't know how we are going to deal with them."
No postcard bridges
Pittsburgh has long prided itself on its blue-collar ethos, and its bridges are no exception. Suspension bridges, truss bridges, arch bridges, they're all here, but none of them is the stuff of postcards. "There's no signature bridge in Pittsburgh," Harries said. "They're all just built for a purpose."
Virtually every major road goes over one of the city's 446 bridges, and the problems are in plain sight, like the Rankin Bridge's rusting rails and crumbling sidewalk. Underneath, Harries points to exposed, rusty reinforcing bars. Nearby, a disconnected downspout deposits water and debris at the foot of the bridge.
Two years ago, Pennsylvania had its first major bridge mishap in eight decades. A concrete beam broke and dropped off a bridge over Interstate 70 near Washington, Pa., narrowly missing vehicles passing beneath. One driver suffered a broken leg. The bridge's long exposure to corrosive road salt was partly to blame. Harries was PennDOT's principal investigator in that case.
Pennsylvania has a $1 billion statewide backlog of highway and bridge improvements. The planned solution, which still faces political hurdles, is to transform its portion of I-80, one of the busiest coast-to-coast highways, into a toll road and use the revenue to for needed repairs.
Until money becomes available for repairs, however, engineers have to resort to half measures like the "diaper" net that hangs beneath the Greenfield Road bridge over I-376 to catch falling concrete.
"We know what needs to be done, and we know how to do it," Harries said. "The problem is getting the resources to do it."