Sennaya Square, Sennaya Sq., Saint Petersburg, Russia
The "Sennaya Ploshchad" pavilion was built in 1963 — at that time the station was called "Ploshchad Mira." Previously, there was the Church of the Assumption of the Blessed Virgin Mary on this site, which was blown up in 1961. The architects of the pavilion were Aron Getskin and Vera Shuvalova, the authors of other standard metro stations — "Park Pobedy," "Elektrosila," "Frunzenskaya," and "Gorkovskaya." In the 1960s, the use of cantilevered reinforced concrete structures in construction was fashionable. It was believed that a protective canopy several meters long without supports made the building stand out. Such structures were used not only for ordinary five-story buildings but also for social and cultural facilities. At that time, reinforced concrete cantilever structures were actively used in building construction; it was thought that a protective canopy several meters long, held without supports, highlighted the building and gave it a special feature. Cantilevered concrete slabs "decorated" not only social and cultural facilities but also ordinary residential five-story buildings.
On June 10, 1999, a disaster occurred — the concrete canopy of the above-ground vestibule broke off. Seven people died, and twelve were injured. The tragedy was caused by cracks in the canopy. The worst part is that these cracks were known. A resident of one of the houses on Sennaya Ploshchad, whose windows faced the vestibule, saw the cracks but did not know what to do. Nowadays, it is clear what should be done in such cases: take photos, send them to the metro management, as well as post them on social media and in the press. The eyewitness in the 1990s did not have a camera... He called the metro police hotline. They promised to pass the signal to the appropriate authorities but did not. Probably, they also did not know whom to contact.
Moreover, metro employees inspected the canopy for strength two months before the tragedy. But they only examined it from below; it was impossible to reach the top. For 36 years, the 20-ton concrete canopy existed amid constant vibration, temperature fluctuations, and so on, until it finally gave way. When the collapse occurred, the builders and designers were blamed. The metro station was being completed at an accelerated pace, rushing to open by November 7, and the canopy was a temporary structure. Later, the station was planned to be integrated into a hotel building (like at "Ploshchad Alexandra Nevskogo"). It was assumed that the hotel’s load-bearing walls would first reinforce those parts of the cantilever adjacent to the vestibule roof. The canopy, covering an area of 130 square meters, was attached to beams at least five meters long. According to the laws of physics, such beams should have a counterweight. There was one, but the length of the fastening arm was only one and a half meters. Alexander Trofimov, an associate professor at the Department of Reinforced Concrete and Masonry Structures at the State Architectural and Construction University, who remembers this story well, explained what the examination revealed. "The main problem was hidden in the upper covering of the canopy, which is the most dangerous zone. Under the influence of external factors, the reinforcement rusted, reducing its cross-section due to corrosion. There was also insufficient anchoring (embedding behind the support), all of which led to the collapse," he says.
The tragedy at "Sennaya" occurred on June 10 at 19:40. That June Thursday in 1999, there were not many people — rush hour had already passed. On the stairs leading to the foyer, a group of ten teenagers was standing. People were habitually ascending the stairs to the doors when suddenly a loud crack was heard (as later found out, the load-bearing reinforcement broke), and immediately after, the 24-ton concrete canopy hanging over the entrance collapsed onto the upper landing of the stairs. Seconds later, it literally pressed the passengers present there into the pavilion’s glazing... Under the debris of the fallen 24-ton canopy, five people died; two more died in the hospital. Twelve were injured and maimed. Rescuers later said that if the canopy had fallen flat, the number of victims could have been several times higher: the structure was 32 meters long and 3.5 meters wide.
Several technical errors were immediately found. The main one: the steel rods holding the slab were embedded 15 centimeters deep, not 60 as required. However, the criminal case was closed in December of the same year. The designers were found guilty, but by then they were no longer alive.
The debris clearing continued late into the night, then the fragments were cut into two- to three-meter pieces and taken to the metro depot in Avtovo. By order of the governor of St. Petersburg, Vladimir Yakovlev, an expert-technical commission was created in the city, which was to, first, determine the causes of the accident at "Sennaya," and second, inspect the canopies at 12 other stations of the St. Petersburg metro.
The commission included specialists in construction, engineering structures, and load-bearing constructions from leading institutes in St. Petersburg (LIISHT, LISI, the 26th Design Institute, Technical University). In addition, the city prosecutor’s office initiated a criminal case for negligence and violation of safety rules during construction work (Articles 216 Part 2 and 293 Part 2 of the Criminal Code of the Russian Federation).
The debris laid out in a line consisted of pieces of concrete slab with wire reinforcement. It turned out to be the front part of the canopy, which consisted of two slabs. The fracture edge was even, as if cut with a razor. An explanation was found quite quickly. It turned out that the slabs were not connected to each other — each had its own reinforcement. Experts believe that if the frame had been continuous, the slab would not have broken off so sharply but would have simply hung on the reinforcement.
The slab closest to the station building was held on cantilevers and side walls. The front one, which collapsed, simply rested on five concrete cantilevers, each of which was attached to a load-bearing transverse beam by two steel rods 30 mm in diameter. At the same time, the anchoring zone (the depth to which the load-bearing rods were embedded in the beam) was only 15 cm — the norm being 60 cm. Seeing such a structure, experts were horrified: it was unclear how the canopy could have hung for 36 years since the station’s opening — its only support was the reinforcement.
Besides these purely structural errors, the expert commission also identified violations in the operation by St. Petersburg metro employees responsible for the safety of capital structures. In particular, it was established that the canopy had not been inspected since the station’s construction, despite the fact that the structure was constantly exposed to negative factors: from the vibration of escalators and trams passing near "Sennaya" to flows of warm air causing concrete deterioration.
If metro workers had bothered to inspect the canopy, they would inevitably have seen cracks and signs of corrosion on the external reinforcement. Then the tragedy could have been avoided. The immediate cause of the collapse, according to experts, was most likely overheating of the concrete and reinforcement — on June 10, the city experienced a terrible heatwave. As for other St. Petersburg metro stations with canopies, all were also inspected. The expert commission’s conclusions were far from reassuring: at least four of the twelve stations — "Gorkovskaya," "Park Pobedy," "Vasileostrovskaya," and "Frunzenskaya" — have similar structures in critical condition. The commission members recommended dismantling the canopies at the first three stations, and "Lenmetrogiprotrans" has already been tasked with preparing the corresponding project. Meanwhile, temporary supports have been installed at all these stations to ensure the safety of the canopies.
By the way, after the accident, the condition of canopies in the Moscow metro was checked, and according to the authorities, their condition does not cause concern.
Sources:
https://spbdnevnik.ru/news/2019-06-10/tragediya-na-sennoy-dvadtsat-let-spustya
