The SourceCivil Engineering MagazineArizona train derailment and fire damage historical bridge
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Arizona train derailment and fire damage historical bridge

By Robert L. Reid

A UNION PACIFIC Railroad cargo train crossing a historical bridge in Tempe, Arizona, derailed and caught fire on July 29, causing a portion of the north–south bridge to collapse, sending several railcars toppling off the bridge and onto the ground below. No one was injured in the derailment, but one emergency responder was treated for smoke inhalation, according to a statement from Union Pacific, which owns and operates the century-old bridge and tracks. The train was a mixed-freight line carrying chemicals, lumber, grain, and other material. Although the bridge crosses the Tempe Town Lake—a reservoir created in 1999 along a segment of the Salt River—no railcars fell into the water. The three railcars that fell from the bridge were tankers, one carried a rubber material, and the other two carried cyclohexanone, a flammable industrial chemical used to create nylon and paint remover. One of those cars leaked an estimated 500 gal. of cyclohexanone into a storm drain that leads to a dry riverbed, according to a website about the accident maintained by the city of Tempe. 

A dozen cars derailed and as many as 10 caught fire, but the tanker cars were not exposed to the fire, Union Pacific said. Hazardous material teams covered the leaking car with a layer of foam to keep it cool and control any chemical vapors. Once the emergency response had ended, the Arizona Department of Environmental Quality (ADEQ) led the cleanup effort and said the air quality was good and there was no sign of any chemicals leaking into the lake. However, the ADEQ also said it was “too soon to know what, if any, impacts there are to the water in Tempe Town Lake,” the city website explained.

The chemicals from the damaged tank cars were transferred to other train cars so that the damaged cars could be disassembled and removed from the area. A 150 ft long portion of the bridge’s south end collapsed. That segment, along with two lumber cars that were still on the bridge, was demolished on August 2 using “small precision charges placed in precut locations around the base of a small portion of the bridge, so that the damaged section and cars fell in a controlled manner … onto a grassy area,” explained the city website. The rest of the bridge did not need to be removed. All other railcars had been removed by August 4, and reconstruction of the southern section of the bridge is under way, Union Pacific stated.

The cause of the derailment is under investigation by the National Transportation Safety Board, the Federal Railroad Administration, and the Arizona Corporation Commission. In June there had been a minor derailment at the same location that resulted in a small fire, according to the Tempe website.

Multiple railroad bridges have crossed the site of the Tempe Town Lake since 1887, most of them washed away by floods, the city noted. The current bridge is a nine-span Pratt truss-style bridge that opened in 1915.

This article first appeared in the September 2020 issue of Civil Engineering.

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