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Weak Support Structure Likely Caused Fatal Accident at Tokyo Construction Site; Calculation Error Said to Be at Fault

Yomiuri Shimbun file photo
A construction site in Tokyo’s Yaesu district where steel beams fell and killed two workers on Sept. 19, 2023.

An accident that left two dead at a construction site in Tokyo’s Yaesu district in September last year was allegedly caused by an error in calculating how strong a temporary support structure needed to be, according to investigative sources.

The Metropolitan Police Department is investigating the accident on suspicion of professional negligence resulting in injury and death. The department believes that the temporary structure might not have been strong enough to hold up the steel beams it was supporting.

The accident occurred around 9:20 a.m. on Sept. 19 last year during construction work, when a steel beam was being hoisted by a large crane to fix the beam to the seventh floor of the frame for the building.

Five beams, each 13 to 18 meters long, weighing a total of about 48 tons, fell to the third floor, about 20 meters below the seventh floor. Of the five workers who were on one beam, two were killed and three were seriously injured.

According to the sources, two of the beams that fell were supported by the temporary structure, which was made from steel pipes and other materials.

How strong a temporary support needs to be is calculated mainly based on the weight of the beams to be supported and how the support will be used.

General contractor Obayashi Corp. told the police in a voluntary interview that the company had made a mistake in calculating the strength needed, causing the temporary support to be too weak, the sources said. A subcontractor based in Saitama Prefecture built the temporary support based on calculations by Obayashi.

A joint venture between Obayashi and Taisei Corp. has been constructing the seven-floor building as part of a plan for a commercial complex that will have 51 floors aboveground and four underground. Construction was suspended after the accident, but was resumed on Jan. 16.

Obayashi told The Yomiuri Shimbun that the company would not comment on the accident because it is being investigated by the police.