ASU Engineers Offer Insight on Tempe Railway Bridge Collapse
Structure age, collision event, and fire could all be contributors to collapse
This article originally appeared in ASU Now. The Union Pacific Salt River Bridge, a technological marvel at the time it was built in 1912, has survived more than a century of floods and weather events on the Salt River, in an area now known as Tempe Town Lake. That record ended with a train derailment and fire on Wednesday, July 29, 2020. There were 95 cars on the train — three were derailed and landed in the empty park below the bridge. Two of the derailed cars contained cyclohexane, with one leaking into the dry ground — not into the lake. The third downed car contained a rubber material. One person was treated for smoke inhalation and a firefighter was treated for dehydration in the 111 degree heat. There were no other reported injuries. Three previous railway truss bridges on the site, built between 1887 and 1912, were each washed away by floods. The Arizona Eastern Railroad, part of the Southern Pacific Railroad, built the steel structure known as a truss bridge, with nine, Pratt-style spans stretching 1,291 feet from bank to bank in 1912. It provided a long-term solution to the destructive floods that impeded railway progress in the early part of the 20th century. In the wake of the derailment, expert faculty members from ASU’s School of Sustainable Engineering and the Built Environment in the Ira A. Fulton Schools of Engineering provide insights on the history, structure, and future of the Union Pacific Salt River Bridge. • Samuel Ariaratnam, professor. • Anthony Lamanna, associate professor. • Barzin Mobasher, professor. • Narayanan Neithalath, professor. • Ram Pendyala, school director, professor. Barzin Mobasher: A truss bridge consists of multiple, interconnect- ing triangular structures fabricated with straight pieces of metal or timber which cannot be distorted by stress. The Salt River Bridge is a Pratt-style truss and is comprised of nine metal spans which rest on concrete piers. • Subramaniam Rajan, professor. Question: What is a truss bridge?
The Union Pacific Salt River Bridge in Tempe as completed in 1912.
Each span, or segment of the bridge, consists of two trusses – right and left. Each truss has a horizontal top chord, which absorbs the compres - sion force, and a bottom chord, which carries the bending forces as a train passes through. The two trusses are connected at the top by diagonal, X-shaped members, called braces, which serve to hold each other in place. Tension force can straighten the structure, while compression force can cause the structure to sway and buckle – with tension failure most likely where two elements are connected. Vertical and diagonal com- ponents between the chords add strength and help carry the weight of a moving train to the piers so that the tension connections do not fracture and the compression components don’t buckle under the load. Tempe Railway Bridge History • Maricopa and Phoenix Railroad Bridge: February 1887. Three “spans” of 150 feet each and a trestle. Damaged in 1890 when flood waters in the Salt River washed away the west end trestle. Repairs were made and it was back in use by May. • Disaster of 1891: A year later, in February 1891, the river rose and driftwood piling up around the trestles, weakening the bridge and causing parts of it to fail. Additional debris destroyed the remains of the bridge. • A new permanent bridge, with nine spans and trestle approaches, was completed in August 1891. Like the original, it was constructed with heavy wooden timbers. Unlike the original, it had nine spans that totaled 1,200 feet. Although much sturdier, floods in 1902 led to a "dramatic accident,” though it continued to serve until a new steel bridge was built in 1905. • For two years, the 1891 bridge remained next to the steel replacement until it was dismantled in 1907. • Disaster of 1902: “Spectacular mishap” on regular run from Phoenix to Tucson, Oct. 29, 1902. As a train crossed the river pulling three freight cars, a first class Pullman car and two coach cars, the bridge collapsed, sending the locomotive and fright cars to the dry riverbed. The Pullman tore free and was left dangling from the remains of the bridge, held by the coach cars attached behind. None of the 40 paying passengers were seriously injured and only five casualties were reported. A person “catching a free ride" on top of the train and four cattle. The track was repaired and trains were running within a week.
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