중앙데일리

TechnoMart shaken by gym: Test

“12층서 태보했더니 38층 상하진동”

July 20,2011
A group of 23 middle-aged people did vigorous exercises at the 39-storey TechnoMart complex, and managed to make it shake.

They were part of a demonstration to prove the owner’s claim that ten minutes of intense shaking on July 5, which led to the evacuation of the complex near Gyangbyeon Station, in eastern Seoul, for two days, was caused by an exercise session in a fitness center on the 12th floor.

The demonstration was held at 3:30 p.m. yesterday, and people working on the complex’s upper floors said they were all shook up.

“I felt almost the same vibrations today as on July 5,” a worker on the 31st floor surnamed Lee said in an interview with the Korea JoongAng Daily.

Two Korea JoongAng Daily reporters also felt tremors on the 31st floor. The minor vertical vibration felt like riding a boat over light waves. If it had gone on for several minutes, it might have caused discomfort, dizziness or nausea.

On July 5, the upper 19 floors of the complex, which has apartments and a popular shopping mall, shook for 10 minutes. After having the complex inspected, owner Prime Group said the cause was 17 middle-aged people in a 12th floor fitness center performing Tae Bo exercises, a mixture of boxing and taekwondo that became popular in the 1990s.

“We tentatively concluded that the group exercise in the fitness center on the 12th floor of the building caused the shake,” Park Heung-soo, CEO of Prime Group said at the briefing yesterday. “After conducting more inspections to find other possible reasons, such as wind or machinery motion, we will make a final report on the cause of the shaking three months later.”

At yesterday’s demonstration, 23 middle-aged people were recruited to do the same Tae Bo exercise as done on July 5. They exercised to the same song, “The Power” by Snap.

The demonstration was done in three parts: two minutes of warm-up exercises, three minutes of light Tae bo and three minutes of vigorous Tae bo exercises.

Lee Dong-geun, professor at Sungkyunkwan University’s Architectural Engineering Department said at the briefing: “On July 5, the teacher was brand new. He started his or her job on that day and was teaching 17 people. Witnesses say the teacher was doing very hard motions, such as pounding the floor with feet, and the duration of the class was longer than usual, about 20 minutes.”

The teacher has disappeared because he doesn’t want media attention. The original 17 gym rats also refused to participate in the recreation of the July 5 episode.

“When the natural frequency of vibration of a building matches the frequency of people’s group activities, such as group stepping or jumping, it gives a big shock to the building and could even shake it,” Lee explained. “We call the phenomenon [mechanical] resonance.”

“The bigger the shock, the more it shakes the upper part of the building than the lower part,” Lee said. “That’s the reason why people working on lower floors felt very little vibration.”

In 1831, the Broughton Suspension Bridge in England collapsed due to mechanical resonance induced by troops marching over the bridge in unison. In June 2000, the Millennium Bridge in London shook when a massive group of people flocked to the newly-built bridge.

Hwang Hyeon-sun, a director of Prime Group who works on TechnoMart’s 38th floor, said the building usually swayed a little in heavy winds or storms.

“On July 5, the building shook more than usual,” Hwang said. “The shaking from the practice demonstration was similar to the shaking on the 5th.”

Experts also investigated the 4-D theatre on TechnoMart’s 10th floor, but said it wasn’t involved in the shaking.

Concerns remain, however, among workers in the building. About 50,000 people visit TechnoMart every day.

“People barely believe the building is safe, so I hope reputable companies hold press conferences for the sake of the 3,000 merchants,” said an employee of a restaurant on the 9th floor.


By Kim Hee-jin, Kim Hyun-jin [heejin@joongang.co.kr]

한글 관련 기사 [중앙일보]
“12층서 태보했더니 38층 상하진동”

테크노마트 태보수업 어제 시연
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일부 “20명 뛰었다고 … ” 의구심

19일 오후 3시30분 서울 구의동 테크노마트 12층 피트니스센터. 이곳에서 건물 흔들림 현상의 원인으로 지목된 ‘태보(태권도와 권투 동작을 결합한 에어로빅 댄스) 수업’ 공개 시연이 열렸다. 건물 고유의 진동수를 맞추기 위해 메트로놈(템포를 일정 간격으로 조정해주는 기계)을 진동 주파수 2.7㎐에 맞게 고정한 뒤 태보 시연자 23명이 흔들림 당시와 동일한 상황을 재연했다. 태보가 시작되고 8분 뒤 38층에 설치된 진동계측기에서 강한 진동 변화가 나타났다. 계측기에 나타난 가속도 크기는 7갤(진동 가속 정도를 나타내는 단위)로 평소 0.7갤보다 10배 가까운 수치였다. 계측기 뒤에 놓인 화분의 나뭇잎이 눈에 띄게 흔들렸다. 38층에서 근무 중이던 황연순(39)씨는 “5일 발생한 진동만큼은 아니지만 분명히 상하진동이 느껴졌다”고 말했다. 시연자 중 5일 당시 태보 운동에 참여했던 성순자(57)씨는 “당시 새로 온 강사의 첫 수업이었는데 평소보다 높은 강도로 몇 개의 동작을 수차례 반복시켰다”고 증언했다.

 정밀 안전진단을 맡은 대한건축학회 소속 성균관대 이동근 교수는 “실험 결과 태보 운동 당시 집단 뜀뛰기가 건물의 고유 진동수와 일치했다는 결론이 나왔다”며 “확률적으로 0.1%도 안 되는 경우”라고 말했다. 진단에 참여한 단국대 정란(건축학) 교수는 “수천 명이 동시에 집단운동을 했다면 몰라도 이 정도 상하진동으로는 건물 붕괴 위험은 없다”고 밝혔다.

 테크노마트 측의 공개 시연에 대한 의구심도 제기됐다. 이화여대 신영수(건축학) 교수는 “조금 더 격하게 운동했다고 진동이 발생했다는 설명은 이해하기 어렵다”며 “20여 명이 태보 수업을 했다고 건물이 흔들렸다면 안전을 보장할 수 없다는 증거”라고 지적했다.



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