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Engineers Study Bridge Failure

Monday, August 29, 2016

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Members of the civil engineering faculty at the University of Illinois at Urbana-Champaign have published a new evaluation of the events that led to a 2013 bridge collapse in Washington state, the school announced Wednesday (Aug. 24).

With bridge design, permitting and human error as the focus of their examination, the team not only seeks to explain how the catastrophic failure occurred, but also to make recommendations of ways transportation departments can avoid future accidents on bridges of similar design.

Skagit bridge collapse
By Martha T / CC BY 2.0 via Wikimedia Commons

A team of civil engineering professors at have published their analysis of a 2013 Washington state bridge collapse in an effort to prevent future accidents and collapses like this one, increase safety and prevent costly repairs, they said.

“The bridge repair costs exceeded $15 million, and that doesn’t account for the economic losses that the area felt because they and visitors no longer had access to the interstate,” Professor Tim Stark said of the team’s analysis.

“Even though this accident occurred three years ago, it’s still very important because many bridges have this same design, not only in Washington but in other states.”

Civil engineering professors Stark, Ray Benekohal, Larry Fahnestock and Jim LaFave published their analysis in the American Society of Civil Engineers’ Journal of Performance of Constructed Facilities.

Summary of Events

Opened in 1955, the Interstate 5 bridge, located about 60 miles north of Seattle, is a major commercial route between Washington and Canada, normally carrying about 71,000 vehicles a day. As previously reported, about 11 percent of the vehicles are commercial trucks transporting goods between the two countries.

The four-lane I-5 bridge collapsed into the Skagit River on May 23, 2013, after an 18-wheeler carrying an oversized load rammed the span's overhead structure.

No fatalities were reported, although two cars and three people fell into the frigid Skagit River; the tractor-trailer did not go into the water. The driver, William Scott, reportedly worked for Mullen Trucking in Alberta, Canada, and had a state-issued permit to carry the oversized load across the bridge.

The steel truss bridge was listed as "functionally obsolete" at the time, meaning its design was outdated. Although it wasn’t initially clear whether the bridge collapsed on its own, Washington State Patrol Chief John Batiste told reporters at the time that the size of the load on the truck appeared to create a problem, causing the driver to strike the bridge.

Skagit River bridge collapse
By kdingo from Seattle, US / CC BY-SA 2.0 via Wikimedia Commons

The four-lane I-5 bridge collapsed into the Skagit River on May 23, 2013, after an 18-wheeler carrying an oversized load rammed the span's overhead structure.

The Washington Department of Transportation (WSDOT) ultimately filed a lawsuit—against Scott and his employer; the pilot-car driver and her employer; and the owner of the structure being transported—to recover the funds used to rebuild the I-5 bridge.

Regulatory, Structural and Communication Factors

The professors’ analysis identified several factors contributing to the accident:

  • Regulatory: the truck had a permit to cross the bridge;
  • Human: Miscommunication between the drivers of the truck hauling the oversized load and its pilot car; and
  • Structural: a minor impact caused a chain reaction that collapsed the bridge.

Some of their findings are similar to those issued by the National Transportation Safety Board in July 2014 following its investigation.

Permitting Error, Measurement Policy

Stark posited that inaccurate database records allowed an oversized vehicle to be authorized to cross a bridge with lower clearance than its height.

The curved design of the I-5 bridge meant its vertical bridge clearance varied—clearance over the outside lanes was lower than that over the center lanes. However, according to the analysis, WSDOT only keeps the maximum clearance in the bridge database used to issue permits for oversized vehicles.

“Many bridges have a square opening, so the clearance is the same across all lanes. The problem with this bridge was that it curved down over the edge lanes,” Stark said.

“The oversized trailer was 15 feet, 9 inches tall. The database said the bridge was 17 feet 3 inches, which was in the center—almost two feet higher than the edges, which is where the oversized trailer was traveling.”

I-5 bridge pre-collapse
By Denver Gingerich (Ossguy) / CC BY-SA 2.0 via Wikimedia Commons

The curved design of the I-5 bridge meant its vertical bridge clearance varied—clearance over the outside lanes was lower than that over the center lanes. However, WSDOT only records the maximum clearance in the database it uses to issue permits to overheight vehicles, the team found.

The Illinois Department of Transportation reports the lowest vertical clearance for a bridge, not the highest, Benekohal noted. He recommended that other states adopt this policy, as well as its periodic use of LIDAR to verify clearance, which can change with repavement, snow or other factors.

Driver Error

The team attributed the pilot car as the source of human error in the event.

The pilot car, assigned to guide oversized vehicles in order to avoid incidents like the one on the I-5, has an antenna in place to alert the driver if it makes contact with a structure. When struck, it indicates the clearance is too low for the overheight vehicle behind it. The pilot driver is supposed to call the truck driver so he or she can adjust course.

However, on the I-5, the pilot car either did not hit the bridge or the driver didn’t hear the strike and didn’t notify the truck driver, Stark noted “so that part of the safety mechanism failed.”

Moreover, he added, the pilot car antenna was not straight, so it was not able to register the full height correctly.

Stark suggested the use of a sensor on top of the antenna that automatically contacts the truck driver if it hits something, eliminating the need for drivers to communicate quickly about course changes.

Structural Role

The professors’ structural analysis revealed that the impact to the second cross-frame, rather than the first, caused so much damage because it twisted and pulled down the top of the bridge truss, they said. This caused the entire structure to fail.

truck that hit I-5 bridge
By US NTSB / Public domain via Wikimedia Commons

Despite a pilot car equipped with an antenna to indicate low clearance for the overheight truck following it, the safety measure failed because the antenna either didn't connect with the structure or the car's driver didn't hear the impact in order to notify the truck's driver.

“I think one of the interesting things about this failure is that the initial damage of where the truck hit was not a primary support, it was a cross beam, and the damage cascaded, causing the entire collapse,” LaFave said.

“We then looked at ways to reinforce bridges with this design, to increase the capacity and reduce the chance of this kind of failure occurring,” LaFave continued.

“We can selectively add supports so there are ways to redistribute the impact load, so the structure can remain stable and stay standing even if there’s damage to a particular area.”

While structural failures are to be avoided, the team noted, the silver lining in the case of the I-5 is that it provided an opportunity to learn from the failure.

“We want to understand what happened so that we can be a part of preventing something like this from happening in the future, and thus provide a safer and more reliable infrastructure,” Fahnestock said.


Tagged categories: Accidents; American Society of Civil Engineers (ASCE); Asia Pacific; Bridges; Department of Transportation (DOT); EMEA (Europe, Middle East and Africa); Health and safety; Infrastructure; Latin America; North America; NTSB (National Transportation Safety Board); Program/Project Management; Safety

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