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Reporting on science, technology and innovation in Arizona and the Southwest through a collaboration from Arizona NPR member stations. This project is funded in part by the Corporation for Public Broadcasting.Additional stories from the Arizona Science Desk are posted at our collaborating station, KJZZ: http://kjzz.org/science

Bridge Inspections: What 'Functionally Obsolete' Actually Means

Amanda Solliday-KAWC

Engineers classified I-10 bridge that washed out in California Sunday as “functionally obsolete.” But what does that really mean?

The Federal Highway Administration defines functionally obsolete as “does not meet current design standards (for criteria such as lane width), either because the volume of traffic carried by the bridge exceeds the level anticipated when the bridge was constructed and/or the relevant design standards have been revised.”

Dustin Krugel, a spokesperson for the Arizona Department of Transportation, explains it this way:

“It could mean it has narrow lanes or little to no shoulders,” Krugel said. “So it’s not how we would design the bridge today, but it may be structurally sound.”

The I-10 bridge in California, built in the 1960s, fell into this category because it required drivers to slow down before crossing.

RELATED: Interstate 10 In California Reopens After Bridge Collapse

Inspectors use another term to describe bridges that are considered deficient in some way — "structurally deficient." This means some part of the bridge is flagged for repair.

“While the terms structurally deficient and functionally obsolete may not inspire confidence,” Krugel said. “It’s a way for our bridge inspectors to determine a bridge’s overall condition. And it helps us prioritize where we need to invest our limited funds.”

Krugel adds these terms alone do not mean the bridge is unsafe.

Of Arizona’s 8,035 bridges, about 12 percent are considered functionally obsolete or structurally deficient, according to a tally by the Federal Highway Administration. Nationally, about one-quarter of bridges fall into one of these two categories.

ADOT manages a little more than half of these bridges within the state. The others are maintained by an individual city or county.