The Union Pacific Salt River Bridge, a technological marvel when it was built in 1912, has survived more than a century of flooding and weather on the Salt River, in a domain now known as Tempe Town Lake. The record ended with an exercise derailment and a fire on Wednesday, July 29.
There were 95 cars in the exercise, 3 derailed and landed in the empty park under the bridge. Two of the derailed cars contained cyclohexane, one flowing towards the dry floor, into the lake. The third car shot contained rubber equipment.
A smoke-inhaled user and a firefighter treated for dehydration at 111 degrees of heat. No other injuries were reported.
Three old rail bridges lattice on the site, built between 1887 and 1912, were swept away by flooding.
The Arizona Eastern Railroad, a component of the Southern Pacific Railroad, built the metal design known as the Lattice Bridge, with nine Pratt-style spans stretching 1,291 feet from one bench to another in 1912. It is a long-term solution for destructive flooding. that hindered the progress of the railway in the early 20th century.
After derailment, trained teachers from ASU’s School of Sustainable Engineering and Built Environment at Ira A. Fulton Schools of Engineering provide data on the history, design and long-term history of the Union Pacific Salt River Bridge.
Question: What is a lattice bridge?
Barzin Mobasher: A lattice bridge is composed of interconnected triangular structures made directly of steel or wood parts that cannot be deformed by tension. The Salt River Bridge is a Pratt-style farmhouse with nine steel spans resting on concrete pillars.
Each section, or segment of the bridge, consists of two farms: right and left. Each farm has a horizontal top rope, which absorbs the compression force, and a shrinking rope, which carries the bending forces as the exercise passes. The two farms are connected to the most sensitive through diagonal X-shaped elements, called suspenders, which are used to hold in place.
Each lattice design rests on concrete pillars at the end. Photo courtesy of davidpinter/CC via https://creativecommons.org/licenses/through/3.0
The stress force can straighten the design, while the compression force can cause the design to sway and turn on, with a voltage breakout most likely when two elements are connected. The vertical and diagonal parts between the strings charge strength and carry the weight of a moving exercise to the batteries so that the voltage connections do not break and the compression parts do not deform under the load.
Proper structural research is to ensure the balance between tension and compression. If we need to design the precise farm today, many structural research teams that were developed in the 19th century are still valid and applicable.
Q: Since the bridge is owned by the railway, is it free of national structural inspections?
Samuel Ariaratnam: Railroads are regulated through the federal government because they have interaction in interstate transportation. The Federal Railroad Administration (FRA) is a component of the Transportation Decomposer and has established federal needs for rail bridges. The FRA calls on personal railway owners to put in place bridge control systems that come with annual bridge inspections and know the load capacity of their bridges. These are implemented through 49 C.F.R. Part 213 “Track Safety Standards” and 49 C.F.R. Part 237 “Bridge Safety Standards” and Supply Regulations Required by Congress. Railway bridge design and testing can be discovered in the American Railroad Engineering Manual of the American Railroad Engineering and Track Maintenance Association (AREMA).
Question: He reported that the railway inspected the bridge in early July, shortly after a fire derailment on 26 June. What are the reporting regulations for these inspections?
Anthony Lamanna: Rail bridges are inspected regularly. When a bridge score begins to decrease, indicating a bridge deterioration, inspection periods are shortened. In addition to repairs, other answers can be taken, such as a slow order to restrict the speed of exercise on the bridge.
It is to remind others that the exterior of a bridge does not necessarily reflect its structural strength. For example, the Burro Creek Canyon Bridge in western Arizona is built with cut metal and is designed to rust. Oxide formed through this special metal creates a water-resistant coating that protects the underlying metal. So, even if it looks rusty, it’s structurally sound.
Q: What are the reasons for the bridge collapse?
Mobasher: Because the bridge is 108 years old, possible reasons come with degradation, overload vibrations, single connection failure, reinforcement failure, cracking of a voltage detail and lack of redundancy in structural design.
Although it’s still clear, derailment of exercise can result in an impact.
It is not yet transparent if structural tracking tools have been implemented, adding fatigue crack tracking, to assess the existing condition, especially after the June 26 fire/derailment. Although it is indicated that an inspection was carried out earlier, the effect on the result would be the removal of indispensable structural components, resulting in buckling and rupture.
Q: What kind of long-term structural damage can be expected after this type of chimney and derailment on a 1912 lattice bridge? Due to its age, is it worth looking to fix the existing bridge, even temporarily, the structure of a new bridge?
Narayanan Neithalath: The chimney will reduce the structural strength of the metal by approximately six hundred degrees F; The initial damage can therefore be exacerbated through the chimney. Deformation and buckling occur before the total failure. Of course, this may also have depended on the initial condition of the bridge, it was inspected on July 9, so we do not know the actual cause. It appears that the initial damage was similar to an effect on, not materials, such as fatigue, although the effect of long-term vibrations on the structural integrity of the bridge cannot be ruled out.
The derailment of the exercise, the cause of which has not yet been determined, probably led to the partial collapse of the bridge and the resulting chimney can cause additional damage to the bridge. The wooded plots burned for a long time and more than part of the lattice bridge subjected to a primary fireplace.
Chances are that a significant part of the bridge will require reconstruction, as well as a detailed evaluation and reinforcement of the concrete structures on the south side where the chimney burned.
Three cars derailed in the July 29 incident and landed on the bridge. Photo via Deanna Dent / ASU
A segment of the bridge is over the Salt River, now Lake Tempe. The moment segment crosses Rio Salado Drive – possibly the structure of Rio Salado Drive would have been changed.
This is the segment that is absolutely broken and where the cars fell off the bridge. I suspect that the derailment of the cars in this segment caused the collapse and the next fire of the cars in any of the segments of the bridge. Older bridge designs, while respecting design code, have sufficient design redundancy. A recent example of the slow sinking of metal bridges is the sinking of I-35 W in Minneapolis which, in retrospect, had sufficient design redundancy.
There are plenty of metal bridges in the United States that have a history of durability and durability if they are kept well.
However, just as building codes do not stipulate that high-altitude vessels have to deal with the effect on the loading of an aircraft in flight, bridge design codes do not stipulate that bridges will have to deal with catastrophic events.
This loss of stiffness makes the buckling of many elements almost imminent. It should be noted that when the total weight of an exercise car is on a single beam, the failure of a single component can result in a structural failure.
Neithalath: Steel in structures such as bridges suffers significant deterioration of the houses below the chimney site, even if the combustion temperature of the wood cannot melt the metal. If temperatures above three hundred degrees C are imposed over an even shorter period of time, there may be a loss of strength and structural stiffness and an increase in material deformability. However, the type of metal used in this bridge (since it was built a century ago and rebuilt several times) is vital in determining the extent of the damage. Exposure to the chimney site would cause the bridge elements to deform and twist, causing structural stability. The chimney site also adjusts the metal microstructure, depending on the composition of the metal.
Mobasher: Like the World Trade Center, the long age of chimney locations produces relief in the rigidity and strength of steel. The place of the quality wood fireplace in a car takes some time, so the parts can be broken down sequentially.
Q: Did all farms fail at the same time, or is it possible that there was a sequential time-lapse failure?
Q: Could the chimney have a later impact?
Mobasher: The impulse of a moving exercise is high enough to cause significant damage. What happened first is not known now. The origin of the chimney will also have to be determined.
Q: Are there new types of fast bridges that would likely be used in Tempe Town Lake?
Lamanna: If the bridge wants to be replaced, the batteries will most likely be able to reuse the batteries. This means they can use more modern technologies to drive the structure of a new bridge. For example, they can simply use prefabricated concrete beams that are manufactured offsite, transported, and installed.
Neithalath: Modern high-speed rail systems use box beams or arc bridges, metal arc bridges or composite metal and concrete bridges. These new structural systems can also be used, but not as undeniable as metal lattice bridges, in research and design. Given the greater number to which these bridges are subjected, compared to what they were designed, newer and more effective types of bridges can be considered.
Q: From a transport perspective, what effect will the loss of this bridge have on rail trade? Are there other routes that can be adapted until they are rebuilt/replaced?
Ram Pendyala: A bridge is a main lifeguard in a transportation system. When cut, this can cause a significant interruption in the movement of goods. Therefore, the effect of the loss of this bridge can be quite significant, especially if the trains want to change routes substantially. Although there are other routes that can be adapted until the bridge is rebuilt, the change in route of trains will most likely generate abundant prices and travel times, as well as logistical challenges.
If there is a lot of capacity available on other routes, the outage may be less severe; However, if available capacity is limited, the movement of the load may also be particularly disturbed, which would have a negative effect on trade. In the midst of a pandemic that has already particularly disrupted the chains of origin, the loss of this bridge is likely to cause the problem.
Lamanna: Since the incident, trains are already taking a direction of choice; this direction will result in increased rail traffic and vehicle delays at junctions.
Remember, the trains arrived first. Tracks do not cross roads, roads cross tracks, which take precedence. In Arizona, trains can legally block a junction for 15 minutes at a time (Ariz. Rev. Stat. 40-852), which is longer than we see in general condition.
Top photo via Deanna Dent / ASU
The Union Pacific Salt River In Tempe, completed in 1912.
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