Nightly News   |  May 24, 2013

How safe are our nation’s bridges?

The recent collapse of the Skagit River Bridge in Washington State raises questions about the structural integrity of other bridges around the country, many of which are considered vulnerable. NBC’s Ayman Mohyeldin reports.

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>>> a dramatic bridge collapse sent cars plunging into a river is complicating travel for some of us headed into the long memorial day weekend . the ntsb is investigating but it already appears a factor that may have to be considered affects one in nine bridges across our nation. for details on what happened we go to the pacific northwest where ayman mohyeldin joins us from mount vernon , washington. good evening.

>> reporter: good evening, ann. the skagit river bridge sits on the i-5. officials are scrambling to keep traffic flowing smoothly as they try to determine what happened here. it happened without warning.

>> there was a loud boom, a big puff of dust.

>> reporter: the north end of the stbridge sent two vehicles on a 25-foot plunge.

>> the bridge over i-5 just collapsed. we have vehicles in the water.

>> reporter: all three people in the cars survived including dan sly.

>> there was no hope. that's what i was thinking.

>> reporter: federal and state inspectors say a truck with an oversized load is to blame.

>> an oversized truck hit the girders and it failed.

>> reporter: the bridge was built in 1955 . inspected just last november it was listed as functionally obsolete by the federal highway administration which means the design is outdated but structurally sound.

>> the problem with the bridge is it was a fracture critical design which means if something goes wrong with the key structural element you may have a catastrophic result.

>> reporter: experts say bridges across the country are vulnerable. in 2007 dozens of cars plunged 60 feet into the mississippi when a bridge in minnesota collapsed, killing 13 people. and 14 died in 2002 when a 500-foot span of the i-40 bridge collapsed in eastern oklahoma . according to the american society of civil engineers , one in nine of the nation's bridges are rated a structurally deficient requiring repair or replacement. more than 200 million trips are taken daily across deficient bridges.

>> there are more structurally deficient bridges in our cities than mcdonald's nationwide.

>> reporter: the government will help rebuild the skagit river bridge . the governor says it will cost $15 million.

>> as far as time to restore the i-5 corridor, maybe it's weeks. if not, there are certainly months involved.

>> reporter: the driver of the oversized truck tested negative for drugs and alcohol. his wife said he was horrified as he watched the bridge collapse in his rear-view mirror but was relieved there was no loss of life. ann?

>> ayman mohyeldin tonight, thank you.