The collapse at the Ulsan thermal power plant boiler tower occurred on the 6th during preliminary work to demolish the tower. The 61-meter-tall tower was to be demolished in a way similar to felling a large tree. The plan was to first carry out a "pre-weakening" process to weaken the lower structural members, then use blasting and other means to topple the entire tower to one side in a "tilt-down" operation.

In the afternoon on the 6th, a structure inside the Ulsan Thermal Power Plant on Yongjam-ro, Nam-gu, Ulsan is collapsed. Marks in red indicate traces of a "weakening operation" that makes the lower part of the structure fragile to facilitate blasting work. /Courtesy of Ulsan Fire Headquarters

At the Ulsan thermal power plant, boiler tower units 4, 5, and 6 were subject to pre-weakening. Looking at unit 4, which completed pre-weakening first, only the columns remained at the bottom of the tower and other structures had been removed. The upper section remained in its original state. The accident occurred while work was underway on unit 5. Work has not yet begun on unit 6.

In the industry, the pre-weakening process is called "notching the base." It works on the same principle as chopping one side of a tree's base with an axe so the tree falls to the side.

However, notching the base is a task that involves considerable risk. If the location, material, and size of the portion to be removed first are not calculated precisely in relation to the entire structure, a collapse can occur during the work. It is similar to how a tree can fall in an unintended direction and cause a major accident if the base is notched incorrectly.

Choi Myeong-gi, a professor with the Korea Industrial Field Professors Corps, said, "It is possible the accident structure was over-weakened," and noted, "Various loads act on structures, and errors in load calculations may have led to mistakes in the stability review."

There is also a need, some say, to support the structure with a crane while carrying out the weakening process to guard against accidents. Choi said, "It would have been better if measures such as using a crane to hold the structure from above or installing braces had been in place to prevent a weakened structure from toppling."

Meanwhile, the Ministry of Employment and Labor (MOEL) plans to investigate whether the business sites violated the Occupational Safety and Health Act and the Serious Accidents Punishment Act as soon as the fire authorities complete rescue operations. When dismantling buildings 31 meters or higher above ground, a harmful and risk prevention plan must be submitted and reviewed and confirmed by the Minister of the MOEL.

※ This article has been translated by AI. Share your feedback here.