By Yun Suh-young

A form of aerobic exercise performed by dozens of people caused the vertical vibration felt in certain floors at Techno-Mart, a 39-story shopping mall in Gwangjin-gu, Seoul, two weeks ago, an inspection committee tentatively concluded Tuesday.

A simulation of the exercise was carried out on the 12th floor of the fitness center of the building Tuesday by inspectors to prove that the group exercise caused the building to vibrate. The same number of people, 23, as was there at the time of the shaking on July 5, participated in the simulation exercise.

A professor from the Architectural Institute of Korea who participated in the inspection of the building told a radio program that the most likely cause of the shaking at the mall was from vibrations caused by “taebo,” an aerobic sport which is a mixture of taekwondo and boxing.

“The same kind of vibrations were felt at the higher stories of the building when we performed a simulation of the exercise at the fitness center Sunday,” said Chung Lan, professor of Architectural Engineering at Dankook University at Jukjeon campus.
Vibrations measured on a vibrometer installed at the 38th floor of the building were ten times greater than usual.

The tentative conclusion was the consensus among the six professors from the architectural institute and vibration measurement experts who participated in the building’s inspection, he said.

“We used a laser Doppler vibrometer while performing the same kind of aerobic exercise that was performed at the time of the shaking which occurred on July 5. We noticed that the shaking was felt in the upper floors while the exercise was being performed but no other place showed signs of vibrations,” said Chung.

Mechanical resonance is the most likely explanation for the vibrations, according to the inspectors.

“It just happens to be that the vibration set up by the “taebo” exercises coincided with the resonance frequency unique to the building,” the professor said. When an external vibration hits the resonance frequency of a certain object, the vibration is amplified and causes excess shaking even from slight movement.

“The new fitness instructor apparently carried out the exercise twice as hard as usual. That must have been the reason,” said Chung.

However, the conclusion drew public criticism. Many bloggers and Twitter users posted messages critical of the inspection outcome.
“If the real cause of the vibration was the exercise, then shouldn’t the people who built the shoddy building be held accountable?” tweeter Stefanet quipped.

Another tweeter honeypooh37 said, “It’s really ridiculous that a building shook due to an exercise at a gym.”
The final results of the detailed safety inspection will be released in two months by the Architectural Institute of Korea.