The Interstate 5 bridge going over the Skagit River was built in 1955 with a steel truss design. When the truck hit a bracing bar running across the top of the bridge Thursday evening, the whole section came down.
UC Berkeley engineering professor and bridge expert Abolhassan Astaneh says that's because the design is what's known as "fracture critical" -- if any one element fails, the whole thing goes. Back-up systems were harder to design without computers.
He thinks there are a couple hundred fracture critical bridges in California, including the San Rafael and the old Carquinez bridges.
"Fracture critical doesn't mean it's going to collapse tomorrow, what it means is eventually we have to budget as a nation to replace those bridges," Astaneh said.
What about the original eastern span of the Bay Bridge, also a steel truss design?
"Even the old Bay Bridge is extremely resilient when it comes to collisions or allisions; the issues with the original Bay Bridge are more based in seismic performance," infrastructure analyst Bart Ney said. "So I don't think you'd see that same type of problem here."
But could the self-anchored-suspension span of the new bridge have fracture critical features?
Astaneh think that's a possibility, in part because the self-anchored suspension span is not anchored into land, but holds itself up with a single cable looping around the structure.
"Even if bolts are good, the bridge is fracture critical," Astaneh said.
"We have more bolts, more tendons, more everything in strategic places, so that if sections do take damage during an earthquake, there's a safety net built in," Bay Bridge spokesperson Andrew Gordon said.
But many of us cross bridges everyday throughout the Bay Area and some of the spans, like the Golden Gate Bridge, are considered senior citizens.
The Golden Gate and the Dumbarton bridges are considered "functionally obsolete" by federal standards. The eastbound Carquinez and the Richmond-San Rafael bridges are considered "structurally deficient." By definition, functionally obsolete means old design features not built to current standards; a functionally obsolete bridge is not considered deficient in any way.
The Golden Gate Bridge design is functionally obsolete because there are no shoulders and lanes are narrow.
Structurally deficient means it needs to be replaced of monitored. A structurally deficient bridge is not necessarily unsafe, but it does require significant maintenance, rehabilitation or replacement.
The Richmond-San Rafael Bridge is deficient because its decking has potholes and there is corrosion.
Engineer Vijay Saraf is an engineer with Exponent in Menlo Park, a company which studies why structures fail.
"Just because a bridge is structurally deficient doesn't mean it's going to collapse," Saraf said.
The American Society of Civil Engineers estimates that if the U.S. were to do all of the required retrofitting on America's infrastructure by 2020, it would cost $3.6 trillion.
Caltrans has done a thorough inspection of all bridges in California and assures that they are all safe. Bridges in California are inspected every two years.