Floorless Fiasco: The 2021 Mexico City (Mexico) Railway Overpass Collapse
Background
Mexico City is the capital of the North American country Mexico and holds a population of 9.2 Million people (as of 2020). The city is located in central Mexico, 94km/59mi west-northwest of Puebla and 62km/39km east of Toluca (both measurements in linear distance).
Mexico City is home to a 200km/124mi metro system, connecting 195 stations by means of 12 different Metro lines. The newest among them, line 12, was opened to the public in October 2012. Also called the golden line from it’s color on the maps the line serves 20 stations on 24km/15mi of standard gauge track, changing between underground, ground level and elevated installations. Unusually for a metro line the trains are fed electricity from an overhead catenary, similar to normal railways. In 2019 the line counted 369590 passengers per day.
The train involved
Services on Line 12 are provided by the CAF FE-10 electric multiple-unit, which was acquired specifically for Line 12 and started service in October 2012. 30 units of seven cars each were made, each one measuring 90.39m/297ft in length at a service-ready weight of 126 metric tons. The trains have a capacity for 1471 people each and can reach 90kph/56mph.
The accident
On the third of May 2021 at 10:20pm a metro train is travelling on an elevated section of track ahead of Olivos Station. Later estimates set the train at 75 percent capacity, meaning it carried approximately 1100 people, maybe slightly less. The 640m/2100ft section between Tezonco and Olivios Station is built in a double-track configuration on top of T-shaped supports, placing the tracks approximately 5m/16ft above ground. The track runs on the median of a large road with lanes on either side.
Suddenly, at 10:22pm, the bridge between support columns 12 and 13 disconnects from the columns, causing the tracks to drop out below the eastbound train. The rear two cars of the train fall into the gap that opened beneath them, ending in a V-shape as the rear of the seventh car remains on the edge of the bridge, if barely. Flying debris from the collapse strikes a passing car, killing the driver. Aboard the train 14 people are killed as the bridge collapses, with dozens more being injured. The driver later testifies that he felt a sudden jolt before the train informed him of a power failure, which led him to trigger an emergency stop. Coming to a halt just outside Olivos station he then saw smoke rise from the back of the train and passengers leaving the forward cars before being informed of the collapse.
Aftermath
Witnesses from passing cars and locals awoken by the noise were the first outsiders to tend to the survivors, joining passengers in climbing all over the wreckage trying to evacuate those who couldn’t leave the train themselves. Car 6 had struck the ground as its back dropped down, with car 7 crashing into the roof of it, causing severe structural damage to both cars. With professional responders starting to arrive a few minutes after the accident an adjacent mall’s parking lot was turned into a makeshift control post, coordinating the rescue and centralizing evaluation of injured survivors before they were taken to hospital. After a few hours all responders were ordered off the wreckage, with those in charge fearing that the train cars or parts of the bridge could come down completely, endangering responders. Cranes were brought in the day after the accident, securing the train cars to allow the search for victims and survivors to continue. The accident would end up claiming 26 lives and leaving 98 people injured severely enough to require hospitalization. One victim and one injured survivor where driving past the site at the time of the collapse and had their car struck by debris, everyone else was in the rear three cars of the train. Had the train run above lines of traffic the outcome could have certainly been worse.
Investigators shared the site with responders the day after the accident, and since it was clear pretty quickly that the train’s condition played no part in the accident the train cars were removed from the site on the same day. Service on the entire Line 12 was suspended and replaced with buses, who struggled to offer sufficient capacity even as the fleet grew past 500 vehicles. The accident was Mexico’s deadliest incident in almost fifty years, leading to 3 days of national mourning being declared. Apart from the Attorney general a Norwegian risk management firm, DNV, was also appointed to investigate the collapse.
As it soon turned out the collapse did, in fact, not happen without warning. Line 12 had had a history of issues with the elevated section dating back to before service even started, with pre-operational tests of the new line showing the track having “sunken” above several support columns. Drivers recorded excessive vibrations pretty much as soon as trains started running on the line in the area that would later suffer the collapse, which got so bad that over 10 thousand train wheels had to be replaced early due to excessive wear before the line was even officially open. The operators of the Metro brought in the German TÜV, an engineering and technical inspection company, who produced a report saying the train wheels were “mismatched” for the track, among other engineering flaws on the trains, and recommended replacing the trains, which was shot down. Instead, operations continued with speeds dropping to walking pace in some places.
In 2014 the elevated section was shut down for almost two years to repair technical and structural faults in the supports. A new investigation concluded that the line suffered from “mediocre quality” construction and engineering, and that the ongoing repairs would just constantly postpone the issues, not fix them. However, they also said that operations were still “within the accepted limits of safety”, at least for the time being. These findings were backed up by a French engineering firm who inspected the line post-repairs in 2016 and did not report any concerning findings.
Ahead of the 2017 Puebla Earthquake residents near Olivos station documented visible cracks in both the supports and the bridge itself, which were repaired when additional damage from the earthquake was fixed. Experts suggesting that the line needed thorough inspection and reconstruction rather than quick repairs were ignored, likely to ensure a quick return to full operation. By 2018 cracks appeared on another support column and were repaired, again without looking further into their cause. Further reports from locals about damaged infrastructure and “wavy” tracks were shot down in late 2019, when a local engineering firm claimed that there were no structural issues with the bridge, while head of the metro’s operator claimed that reports of wavy track were simply untrue.
In June 2021 DNV published the first part of their report, outlining the immediate causes of the accident. They found that the collapsed section showed poor welding on the steel connectors within the support columns, lacking connection between steel pieces where the track mounted to the supports, an overall lack of steel reinforcements as a whole and unfinished or improperly executed welding in various places. On top of that they also found that different types of concrete had been mixed for the bridge-sections between the columns for no apparent reason. Their main complaint was that the bridge-section only carried 10 reinforcement bars, in accordance with a last-minute instruction by supervisors during construction, rather than the intended 20.
Publishing the second part of the report, which further explained the immediate cause, in September 2021 DNV explained that a lack of reinforcements, especially Nelson-Stud connectors in the bridge-section had caused lengthwise girders to buckle and move more independently than they should, aided by excessive vibrations from poorly engineered and mismatched trains. In simplified terms, the lack of reinforcements as well as poor construction had turned the points where the bridge mounted to the support-pillars into hinges, bending up and down slightly each time a train-wheel went over them. If you hold up a ruler and push down repeatedly on the far end of it, you get a similar effect.
This defect caused distortion to the main lengthwise frame and extensive fatigue in the support, since forces couldn’t be distributed through the structure as intended. The cracks in the concrete were a warning sign of the metal trying to move within it, forced by the massive leverage of the bridge-sections. Similar to the paper-clip comparison to the 1998 Eschede Disaster (which was previously featured on this blog) the train involved in the accident simply bent the steel structure down one too many times, and it gave way. This motion broke the girders going left to right between the lengthwise beams, completely taking away the concrete’s support. And without the steel reinforcements the concrete simply couldn’t stay in place.
The third part of the report was delivered by DNV in February 2022, but the city’s government rejected the findings and chose not to publish it. They explained that the report was “deficient, poorly produced and […] false”, supposedly presenting questionable supposed technical issues that served the country’s opposition parties rather than being an objective paper. However, in May 2022 a Spanish newspaper published parts of the report, allowing the public to learn that the root cause was noncompliance with “basic bridge construction”, a lack of independent certification and poorly timed changes to the design of the rail line without sufficient adaption of the engineering.
Public pressure following this release led the city government to release a commented version of the report after all, pointing out how, among other things, the DNV had used Google StreetView to acquire pre-collapse images of the section. The mayor proceeded to announce that the city was to hire a new set of engineers to “present the whole truth”. She had similarly criticized an article by the New York Times, claiming their explanation that poor design/construction wasn’t the cause of the collapse but rather lacking maintenance. How lacking maintenance makes solid objects disappear was not explained.
The College for Civil Engineers of Mexico (CICM) conducted an independent study of the collapsed section of the line, observing that the elevated track uses two different bridge-designs. One section uses concrete girders and a slim support design, while the other, which was where the collapse took place, uses steel girders and a different, wide support design. They also found deficiencies and flaws of minor severity on 68% of the elevated section, while the remaining 32%, which were entirely built in the steel girder design, presented vulnerabilities that required additional analysis. Among the issues listed were poor welds, improper spacing between structural components as well as cracking in various spots. The two designs exist because the sections were constructed by two different companies, who apparently didn’t fully cooperate in the planning stages.
In December 2021 the project director for Line 12 and nine former officials and supervisors were formally charged with manslaughter, negligent cause of bodily injury and property damage in several cases. Several of the defendants were previously banished from holding any public role in Mexico after a separate investigation prior to the collapse. The line had undergone last-minute changes prior to and during construction, such as changing from a rubber-wheeled system to conventional steel wheel trains and changing the routing from entirely underground to partially elevated above ground. Budget- and scheduling-issues accompanied the project throughout its construction and it is suspected that certain processes were rushed, prioritizing timely completion over necessary care. This would explain the odd mixture of partially faulty concrete and the lacking reinforcements. A report by the public prosecutor claimed after the accident that the collapsed span lacked 35% of the required bolts meant to tie the bridge to the support columns. This caused the hinge-motion mentioned above, which worsened over time until, on the night of the accident, the bridge fatally shifted and sheared off the support in just 1.9 seconds.
By June 2022 approximately 90% of the survivors and victims’ relatives have received damage payments between 22600 USD/22000 Euros and 290000 USD/280000 Euros in return for a promise to not pursue legal action or demand further payments in the future. The remaining 10 percent chose to pursue legal action instead. Mister Ebrard, to whose time as Mayor the Line was a “Halo Project”, and Miss Sheinbaum, who was the mayor for 2 years by the time of the accident, were both considered front runners for the 2024 presidential election, but neither is expected to win now, especially not Ebrard who chose to go to exile in France in 2015. His image in particular was hurt when speculations arose that he urged the project to be rushed so it would be finished before the end of his time as the Mayor. Meanwhile Sheinbaum lost a lot of support due to her comments after the accident as well as the revealed inaction as problems surfaced ahead of the collapse.
Reconstruction of the collapsed section started in February 2022, along with the demolition and reconstruction of a faulty support column 200m/656ft from the site. In total, 6.7km/4.2mi of the elevated line will be reinforced before it is expected to reopen. The trains are expected to remain unchanged as the contract with the manufacturer doesn’t expire until 2026.
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