By Kim Ji-su
Days after the vertical tremor incident at Techno-Mart in Seoul, there exists only speculation over the cause even among experts. However, some of this speculation is too broad to provide a convincing argument to ordinary people. Moreover, in my opinion, it is scientifically incorrect.
Some speculators say the building has a characteristic vibrational (resonance) frequency, and that for whatever reason if the building happens to be shaken by this resonance frequency, it will undergo enormous stress and eventually collapse.
They argue that in such a case, even an unusually minute force like wind, people’s movement at a fitness center, or cinematic sound, can provide a source for this catastrophic vibrations. They even cite two rather famous examples ― the Tacoma Bridge in the U.S. and the Millennium Bridge in the U.K.
However, the two examples are all wrong. The collapse of the Tacoma Bridge in 1940 was not caused by pure mechanical resonance and wind blowing over the bridge as often misleadingly described in some physics books, but by more complicated interactions involving vortex-induced vibration and rigid structures.
Abrupt vibrations on the Millennium Bridge in 2000 were not caused by this characteristic vibration. The vibration was caused by some 2,000 Millennium celebrators crossing the bridge at any time of the day, and their synchronized walking. The cause of the latter phenomenon touches boundaries of human psychology and behavioral science outside the scope of this work.
There is a fundamental difference between bridge-crossers at the Millennium Bridge and people at the fitness center of Techno-Mart. The bridge-crossers made moves that worsened the sideway vibration of the bridge. This can be analogous to swing riders. People just know how to ride a swing by instinct.
However, coordinated exercising moves by people at a fitness center cannot possibly be a cause for vibration. They did not move in such a way as to worsen the up-and-down movements of the Techno-Mart building. It was random exercise so experts should find other reasons for the vibration. Sound from cinema also cannot be a cause for vibrations of this magnitude.
Without looking into details of the architectural design of the Techno-Mart, it would be wrong to make any assumptions as to the cause of the vibration. Three possible causes can be immediately suggested as indicated in the book, “Why Buildings Fall Down: Why Structures Fail” written by Matthys Levy and Mario Salvadori.
Firstly, structural fatigue has to be taken into account. The Amoco Tower in Chicago, formerly called Standard Oil of Indiana Building completed in 1974, had to go through pricey replacements of its marble veneer with granular ones in 1989 due to concerns over building safety. Earlier concerns on oxidation of the marble veneer by the acidic winds in Chicago were ignored to save construction costs, believed to be only 1 percent of the total.
Instead, these later replacements cost the construction company 50 times more, not to mention additional fatigue accumulated in the structure during the oxidation process. The Point Pleasant Bridge Failure in West Virginia in 1967 is another classic example of structural fatigue. The collapse of the bridge cost the lives of 46 people. A three-year investigation concluded pinhole cracks in the I-bars in the bridge’s structure, caused the collapse.
Secondly, liquefaction in the summer monsoon season needs to be considered. Unlike the Sampoong Department Store collapse in June 1995, structural failure and illegal live-loads increase, or extensions were not discovered in this case. However, the building is located right next to the river and so has to be highly prone to underground water attack even during the dry season. Adding suspects, the site where the building stands turns out to be a landfill.
Water intrusion can always push the entire structure up and down. This is similar to a ship launching in water. The building is 189 meters tall so wind sway would last around five seconds. Witnesses confirmed that this cannot be a cause. The vibrations were vertical rather than horizontal. Periods are short and non-periodic.
Thirdly and finally, slab-connection failure can be the cause of this incident. Most witnesses resided on the 18th floors and upwards. This means localization of the incident. It was a 10-minute-long vertical vibration violent enough to have caused sickness for some of the residents.
Without looking into the details of the slab connection, the case of L’Ambiance Plaza collapse in Bridgeport, Conn., can be cautiously presented here. The second worst incident in U.S. construction history cost 28 lives. Slabs in the nine-storey building collapsed in just five seconds due to failures of shear heads connecting them to main rods.
It might be one of the above, or mixture of these factors which worsened by happening simultaneously. As Professor Hong Sung-gul, the president of the Korea Seismological Engineering, says these arguments should have been addressed at the beginning. We hold a great architectural tradition from our ancestors who built the World Heritages.
It is a pity that modern buildings do not contain the spirits we used to have in ancient times. Would it be too much if we ask construction companies and local authorities to build modern equivalents of Seokguram Grotto and Bulguksa Temple? As E.H. Carr puts it, history is an unending dialogue between the past and present.
The writer worked for LG Chem as a senior scientist for six years before joining CLSA Korea as an analyst after studying at the London School of Economics. He obtained his MSci/Ph.D. in physics/chemistry from Bristol/Cambridge University and co-authored the book, “Digital Native Story.” He can be reached at dr.jisukim@gmail.com.