It is likely that a significant portion of the bridge would need rebuild - ing, along with detailed assessments and strengthening of the concrete support structures on the south side where the fire burned. Subramaniam Rajan: Steel is a very durable structural material and has been the material of choice for railroad bridges built before 1970s. One segment of the bridge is over the Salt River, now Tempe Lake. The second segment runs over the Rio Salado Parkway — it may have been modified when the Rio Salado Parkway was constructed. This is the section that is completely damaged and where the rail cars have fallen from the bridge. I suspect that the derailment of the rail cars in this section triggered the collapse and subsequent fire of the rail cars in both sections of the bridge. Older bridge designs, while meeting the design code, do not have enough redundancy in the design. A recent example of progressive collapse of steel bridges is the collapse that occurred at I-35 W in Minneapolis that, in retrospect, did not have enough redundancy in the design. There are hundreds of steel bridges in the United States which have a very good history of being durable and long lasting if they have been properly maintained. However, similar to building codes that do not stipulate that tall build - ings must withstand impact loading from a flying aircraft, bridge design codes do not stipulate that the bridges must withstand catastrophic events. Mobasher: A timeline history will tell us what the exact sequence of events was. One would need about 30 to 60 minutes of fire for the steel to lose stiffness — the point at which spans which were easily carrying the traffic load, will buckle. At 500 degrees C (930 F) , steel is unable to carry its own weight and both strength and stiffness drop by 50%. That loss of stiffness makes buckling of many elements practically im- minent. It is important to note that when the full weight of a train rail car is on a single truss, the failure of a one component can result in the structural failure. From an environmental point of view, wood fire, like the burning lumber cars in the recent derailment, is less damaging than hazardous chemicals. However, the duration of the wood fire burn in this incident ensures that high enough temperatures were reached. Q: What does fire do to steel structures? Neithalath: Steel in structures such as bridges undergoes significant property deterioration under fire, even though the temperature of lumber burning cannot melt the steel. If temperatures greater than 300 degrees C are imposed for even a shorter time duration, there could be loss of structural strength and stiffness, and increase in deformability of the material. However, the type of steel used in this bridge (since it was constructed a century ago, and rebuilt multiple times) is important in determining the extent of damage. The exposure to fire would cause warping and twisting of members in the bridge, which will interfere with structural stability. Fire also changes the microstructure of steel, depending on the composition of steel. Mobasher: Much like the World Trade Center, the extended period of
fire results in the reduction of stiffness and strength of steel. The fire of the construction grade wood on a rail car takes an extended amount of time, so components can fail sequentially. Q: Did all of trusses fail at the same time, or there could have been a sequential time lapse failure? Mobasher: Both could be possible given the nature of fire and the railcars. Q: Could the fire have started after the impact? Mobasher: The momentum of a moving train is sufficiently high to cause significant damage. Which came first is not known now. The origin of the fire needs to be determined as well. Q: Are there specific, new types of bridges that would likely be used over Tempe Town Lake? Lamanna: If the bridge has to be replaced, it’s likely they can reuse the piers. This means they might be able to use more modern technolo- gies to speed up the construction of a new bridge. For example, they could use precast concrete girders that are made off-site, trucked in, and lifted into place. Neithalath: Modern high speed rail systems use concrete box girder or arch bridges, steel arch bridges or steel-concrete composite bridges. These novel structural systems could also be used, though not as simple as steel truss bridges in analysis and design. Considering the increased loadings that these bridges are subjected to, as compared to what these were designed for, newer and more efficient bridge types could be considered. Q: From a transportation perspective, how much impact will the loss of this bridge have on railway cargo commerce? Are there other routes that can be adapted until it is rebuilt/replaced? Ram Pendyala: A bridge is always a major lifeline in a transportation network. When it gets severed, it can lead to significant disruptions in the movement of freight. So, the impact of the loss of this bridge could be quite significant, especially if trains have to be rerouted substan - tially. Even if there are other routes that can be adapted until the bridge is rebuilt, it is likely that rerouting trains will add considerable cost and travel time, as well logistical challenges. If there is plenty of spare capacity on other routes, then the disrup- tion may be less severe; but if spare capacity is limited, movement of freight could be quite significantly disrupted — leading to an adverse impact on commerce. In the midst of a pandemic that has already dis- rupted supply chains considerably, the loss of this bridge is likely to compound the problem. Lamanna: Since the incident, trains are already using an alternative route; this route will have increased train traffic and vehicle delays at crossings. Remember, trains were here first. The tracks don’t cross roads, the roads cross the tracks, which have the right-of-way. In Arizona, trains can legally block a crossing for up to 15 minutes at a time (Ariz. Rev. Stat. §40-852), which is typically longer than we see under nor - mal conditions.
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csengineermag.com
september 2020
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