Experts: The footbridge collapsed probably due to the breakage of corroded cables

Prague - The cause of the footbridge collapse in Troja on Saturday was likely the rupture of the main load-bearing steel cables due to corrosion. Two experts from the Faculty of Civil Engineering at the Czech Technical University (CTU) in Prague stated this to the Czech News Agency (ČTK). They believe that the design of the project and the inspections carried out by the specialized company PONTEX were adequate. They do not know how it was possible that the monitoring system of the footbridge did not signal any problems just minutes before its collapse. The reason will only be clear after a detailed examination of all parts of the bridge, they agreed.

"In my opinion, the main load-bearing cables that supported the deck were compromised, and when several cables possibly broke, it all collapsed because the remaining cables were overloaded," said Vladislav Hrdoušek from CTU. According to him, it is strange that the monitoring system on the footbridge did not signal any problems and that the collapse occurred suddenly. However, he did not want to speculate on the reason.

According to Hrdoušek, the displaced position of the pillars after the footbridge collapsed indicates that damage occurred somewhere on the deck over the water. He pointed out that the suspended structure of the footbridge in Troja tends to move slightly, which causes slight opening of joints. "It is very difficult to seal it so that air humidity cannot get in," he said. This can lead to corrosion of the tensioned cables, which can result in their rupture and subsequent collapse of the structure, he added.

During inspections of suspended bridges and footbridges, according to Hrdoušek, it is appropriate to focus primarily on measuring the tension in the structure at selected cross-sections, which the company PONTEX reportedly did. His colleague from the Faculty of Civil Engineering at CTU, Tomáš Rotter, pointed out that PONTEX is a top company specializing in diagnostics.

According to Hrdoušek, it is not a fault that the company did not inspect the entire length of the cables. "It is not possible to assess the entire length of the cables, as this would mean infinitely many cross-sections," he stated. According to him, someone must always decide where the cross-sections will be made. "It depends on whether the person who comes there chooses the right place and measures exactly where a point occurs that leads to damage to the structure or even to its collapse," he explained.

Typically, according to Hrdoušek, such bridge structures are monitored at points of maximum stress, which is usually in the middle of the bridge or footbridge and above the supports. "Sometimes the structure may behave differently, and it is hard to predict," Hrdoušek noted.

Both experts do not see a fault in the design either. "The footbridge was designed by SHP, whose leading personality is Professor (Jiří) Stráský. He is a top world-renowned bridge engineer. I would not look for a fault there," defended the design from 1984 Rotter. Hrdoušek reminded that the quality of the structure was verified by dynamic tests. He sees the problem more in the durability of certain materials.

"The only thing I think should have been checked is the strength of the concrete, whether the concrete is crumbling and holding. And then, whether moisture is entering the structure, which is a matter of insulation. These things are hard to diagnose," Hrdoušek said.

According to Hrdoušek, there are nine similar bridges in the Czech Republic. "At one time, it was an exceptional structure," he pointed out. The first such footbridge was built in the late 1970s over the Sázava River in Hvězdonice, but it did not use concrete components like in Troja, he specified.