Signs of age are clear. The Douglass bridge, also known as the South Capitol Street Bridge, was designed to last 50 years. It's now 13 years past that. The district's transportation department has inserted so-called catcher beams underneath the bridge's main horizontal beams to keep the bridge from falling into the river, should a main component fail.
Alesia Tisdall, who drove over the bridge every day for 15 years but now crosses it only occasionally, said she found its "bounce" unnerving.
"You'd look at the person sitting next to you like, 'Did you feel that bounce?' And they'd be looking back at you like they were thinking the same thing," said Tisdall, a computer systems specialist at the Justice Department.
Peter Vanderzee, CEO of Lifespan Technologies of Alpharetta, Ga., which uses special sensors to monitor bridges for stress, said steel fatigue is a problem in the older bridges.
"Bridges aren't built to last forever," he said. He compared steel bridges to a paper clip that's opened and bent back and forth until it breaks.
"In a bridge system, it may take millions of cycles before it breaks. But many of these bridges have seen millions of cycles of loading and unloading."
That fatigue is evident in a steel truss bridge over Interstate 5 in Washington state — south of the similar steel truss that collapsed in May. The span that carries northbound drivers over the east fork of the Lewis River was built in 1936.
Because of age, corrosion and metal fatigue caused by vibration, the state has implemented weight restrictions on the bridge. Washington state Department of Transportation spokeswoman Heidi Sause said the bridge wasn't built for the kind of wear — bigger loads and more traffic — that is now common.
The biggest difference between the bridge over the Lewis River and the one over the Skagit River that collapsed May 23 is that the span still standing has actually been listed in worse condition. State officials hope to replace it in the next 10 to 15 years.
While the Skagit span was not structurally deficient, the I-35W bridge that collapsed in Minneapolis in 2007 had received that designation. The bridge fell during rush hour, killing 13 people and injuring more than 100. The National Transportation Safety Board concluded that the cause of the collapse was an error by the bridge's designers, not the deficiencies found by inspectors. A gusset plate, a fracture critical component of the bridge, was too thin.
There are wide gaps between states in historical bridge construction and their ongoing maintenance. While the numbers at the state level are in flux, Iowa, Nebraska, Missouri and Pennsylvania have all been listed recently in the national inventory as having more than 600 bridges both structurally deficient and fracture critical.
Pennsylvania has whittled down its backlog of structurally deficient bridges but still has many more to go, with an estimated 300 bridges in position to move onto the structurally deficient list every year if no maintenance is done.
Officials say northeastern states face particular challenges because the infrastructure there is older and the weather is more grueling, with dramatic and frequent freeze-thaw cycles that can put stress on roads and bridges.
Many Pennsylvania lawmakers have long sought to boost transportation funding, in part to address crumbling bridges. But this year's proposals, including Gov. Tom Corbett's $1.8 billion plan, stalled amid fights over details.
That's a common issue among infrastructure managers in other states, who say they don't have the money to replace all the bridges that need work. Instead, they continue to do patch fixes and temporary improvements.
Washington's Douglass bridge has been rehabilitated twice. Ronaldo Nicholson, the chief bridge engineer for the area, emphasized that if city officials feel the bridge is unsafe, they'll prohibit trucks from crossing or close the span entirely. Inspections have been stepped up to every six months instead of the usual two-year intervals for most bridges. In the meantime, officials are trying to stretch the bridge's life for another five years — the time they estimate it will take to build a replacement.