C+S September 2020 Vol. 6 Issue 9 (web)

While this is not yet clear, the train’s derailment could result in an impact event. It also is not yet clear if any structural health monitoring tools, in- cluding fatigue crack monitoring, were done to assess the existing condition — especially after the June 26 fire/derailment. While it is stated that there was an inspection done earlier, impact would take out essential structural components leading to buckling and failure. Q: What types of long-term structural damage can be expected after this type of fire and derailment on a 1912 truss bridge? Because of its age, is it worth trying to repair the existing bridge, even temporarily, while a new bridge is constructed? Narayanan Neithalath: The fire will reduce the structural strength of steel starting at around 600 degrees F; so the initial damage could be exacerbated by the fire. Deformation and buckling happens before complete failure. This of course, could have depended on the initial bridge condition also — it was inspected as recently as July 9 — so we don’t know the actual cause. It looks like the initial damage was impact-related, and it is not materials-related, such as fatigue, even though the impact of long-term vibrations on the structural integrity of the bridge cannot be discounted. The train derailment, the cause of which still needs to be determined, likely led to the partial bridge collapse, and the fire that resulted could cause further damage to the bridge. The lumber cars burned for a significant length of time, and more than half the truss bridge was subjected to significant fire. Tempe Railway Bridge History (cont.) • A temporary wooden trestle bridge was built in 1903 by Southern Pacific’s rival, the Atchison, Topeka and Santa Fe as part of a new competition in Phoenix. A permanent steel bridge was completed in 1904. Despite a concrete foundation and massive concrete piers on which the steel bridge rested, a flood in 1905 washed out two of the north spans. Additional floods over the next year washed out replacement spans and the bridge was abandoned. • The Maricopa and Phoenix Railroad ordered a steel bridge, formerly used in Texas, to replace the one continually damaged by floods. At this time, the alignment was changed on the north end. The bridge began to arrive in segments in 1904, but wasn’t built on the Salt River until 1905 — the worst year for floods on record. The rails and ties were immediately washed out. The sturdy concrete pilons withstood the flood waters in March and April, and in August, a train went over the bridge for the first time. • This bridge continued to serve until the new bridge was completed in 1912. • The “new” bridge, known as the Union Pacific Salt River Bridge and the site of the July 29 derailment and fire, was built by the Arizona Easter Railroad, part of the Southern Pacific, with nine “Pratt-style” spans stretching 1,291 feet from bank to bank in 1912. While the new bridge replaced the steel Maricopa and Phoenix Bridge, it reused the tough, well-tested concrete piers which had survived numerous floods since 1905.

Proper structural analysis is critical to ensuring the balance of tension and compression. If we want to design the exact truss today, many of the structural analysis tools that were developed in the 19th century are still valid and applicable. Q: Since the bridge is owned by the railroad, is it exempt from local and national structural inspections? Samuel Ariaratnam: Railroads are regulated by the federal govern- ment because they engage in interstate transportation. The Federal Railroad Administration (FRA) is part of the Department of Trans - portation, and has established federal safety requirements for railroad bridges. FRA requires private railroad owners to implement bridge management programs that include annual inspections of bridges and to know the safe load capacity of their bridges. These are enforced through 49 C.F.R. Part 213 “Track Safety Standards” and 49 C.F.R. Part 237 “Bridge Safety Standards” and provide Congress mandated regulations. Design and ratings of railroad bridges can be found in the American Railway Engineering and Maintenance-of-Way Association (AREMA) “Manual for Railway Engineering.” Question: It has been reported that the railroad inspected the bridge in early July, shortly after a June 26 fire-derailment event. What are the reporting regulations for these inspections? Anthony Lamanna: Railroad bridges are inspected regularly. When a bridge rating starts to go down, indicating deterioration of the bridge, the inspection periods get shorter. In addition to repairs, other remedies may be taken, such as a slow order to restrict the train speed over the bridge. It’s important to remind people that the outward appearance of a bridge doesn’t necessarily reflect its structural soundness. For example, the Burro Creek Canyon Bridge in Western Arizona is built of corten weathering steel and is designed to rust. The rust formed by this special steel creates a tight coating that protects the underlying steel. So while it looks rusted, it is structurally sound. Q: What are the potential causes of the bridge collapse? Mobasher: Because the bridge is 108 years old, potential causes in - clude degradation overload vibration, failure of a single connection, gusset plate failure, cracking in a tension member and lack of redun- dancy in the structural design. Three cars derailed during the July 29 incident and landed on dry land below the bridge. Photo: Deanna Dent, ASU


september 2020


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