Question

On May 23, 2013, a 160-foot span of the Skagit River Bridge on I-5 north of Seattle collapsed moments after upper bridge supports were struck by a tractor trailer with an oversized load. The truck made it safely across, but two other vehicles fell into the water 24 feet below. Three people were rescued without major injuries. The bridge was constructed in 1955 and designed for an expected life of 50 years.

The Skagit River Bridge is rated by the Federal Highway Administration (FHWA) as functionally obsolete—it is not designed to today’s standards, but it is not necessarily unsafe. The steel element could cause collapse. There are about 18,000 fracture critical bridges throughout the United States, build mostly between the mid-1950s and late 1970s. Modern construction methods are much more resilient to damage.

In 2007, the I-35W bridge carrying traffic over the Mississippi River between Minneapolis and St. Paul collapsed suddenly during rush hour, killing 13 people and injuring 145. The Minnesota bridge, completed in 1967, was also a fracture critical bridge and was classified as structurally deficient by the FHWA. Structural deficiency indicates that the bridge has one or more defects in its support structure or deck and therefore requires maintenance, repair, and eventual rehabilitation or replacement.

The nation’s 611,845 bridges have an average age of 43 years, and almost 23% are rated as either structurally deficient, functionally obsolete, or both. The FHWA calculates that more than 30% of U.S. bridges exceed their 50-year design life. The required fiscal investment for reconstruction and renovation poses a significant challenge for federal, state, and local governments—but some progress is being made. Decisions on how to allocate funding to upgrade and replace deficient bridges are influenced by both economic and non-economic factors.

• The Skagit River Bridge carries an estimated 71,000 vehicles a day and is a main commercial route between the United States and Canada. How would you calculate the economic impact of the catastrophic failure of the bridge? Compare the economic impact to commuters versus commercial traffic.
• What factors should be considered when engineers determine whether to either rehabilitate or replace a deficient bridge?
• The Federal Highway Administration released $1 million in federal emergency funding the state of Washington the day after the I-5 bridge collapse and almost a month later allocated $15.6 million in federal funding to help rebuild the bridge. Discuss the ethical dilemma of state and local governments that have aging infrastructure to repair before tragedy strikes, but insufficient funding to make the repairs.
• Both of the bridges described here were routinely inspected and deemed safe for use. Discuss how this inability to predict structural failures complicates the job of transportation officials.

Answer 1