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VerticalShaking Accident and Cause Investigation of 39-story Office Building

LanChung1, Taewon Park*2 and Sung Sik Woo3¹

Professor,Department of Architectural Engineering, Dankook University, Korea²

AssistantProfessor, Department of Architectural Engineering, Dankook University, Korea³

Manager,Technical Services Group/Structural & Civil Engineering Team, HyundaiArchitects & Engineers Associates, Korea

Abstract

At around 10a.m. on July 5, 2011, a building shaking occurred at Techno Mart 21 located inGuui-dong, Seoul, for 10 to 15 minutes, resulting in emergency evacuation ofthe residents. Because this building is a 39-story building, evacuation fromthe upper floors took a long time. In the initial phase of the accident, thebuilding structure was determined to be safe through an emergency safetyevaluation conducted by the Korea Infrastructure Safety Corporation operated bythe Korean government. Such social anxiety was further amplified by speculativereports released to the press regarding footing settlement, wind-inducedvibration and climate-induced vibration prior to a review of the vibrationsource by an expert group. Thus, the need to discover the vibration source andto present a solution was suggested.

The purposeof the study was to conduct an initial investigation on the vibration source,identify the exact cause of the vibration through an investigation into thepredicted vibration source and structural analysis, and establish theappropriate measures to prevent recurrence of such accidents.

1.Introduction

1.1Summary of Accident

At around 10a.m. on July 5, 2011, a building shaking occurred1) at Techno Mart 21 locatedin Guui-dong, Seoul, for 10 to 15 minutes, resulting in emergency evacuation ofthe tenants. Because this building is a 39-story building, evacuation from the upperfloors took a long time. It has been reported that it took over an hour forpeople on the upper floors to evacuate during the 9/11 terrorist attack, andthus, the issue of risks related to evacuating from high-rise buildings is asensitive matter for tenants. Also, due to the history of approximately 500deaths during the collapse2) of Sampoong Department Store in 1995, Koreansreact more sensitively toward building accidents and this has emerged as amajor social issue. In the initial phase of the accident, the building structurewas determined to be safe through an emergency safety evaluation conducted bythe Korea Infrastructure Safety Corporation operated by the Korean government.However, the fact that the vibration source remains unknown has led to greateranxiety and concerns among building tenants and users, resulting in a reducednumber of people shopping at the shopping mall and subsequent decline in therevenue of the merchants. Such social anxiety was further amplified byspeculative reports released to the press regarding footing settlement,wind-induced vibration and climate-induced vibration prior to a review of thevibration source by an expert group. Thus, the need to discover the vibrationsource and to present a solution was suggested.

The purposeof the study was to conduct an initial investigation of the vibration source,identify the exact cause of the vibration through an investigation into thepredicted vibration source and structural analysis, and establish theappropriate measures to prevent recurrence of such accidents.

1.2Summary of Building

An overviewof the building at which the abnormal vibration accident occurred is shown inTable 1.

Table 1. Overview of theBuilding

Category	Description
Building  Scale	6  under and 39 ground floors,
Structure	Steel  structure + steel-reinforced concrete structure
Construction	August  18, 1998
Elapsed  Years	Approx.  13 years
Main  Uses	Office,  shopping, performing arts and sports facilities
Max.  Height	188.7m

 2.Investigation into the Vibration Accident

2.1Vibration Accident and Description

This sectiondescribes the accident and the actions taken between the date of vibrationoccurrence, July 5, and August 11, 2011. Details are given on the 1st vibrationaccident, on which the initial report was made to the National EmergencyManagement Agency, the actions taken by the local government, prevention centerand building operation department, the sudden collapse of the ceiling, and the2nd vibration accident that occurred approximately one month following the initialreport.

1)1st Vibration Accident

A constantvibration occurred at Techno Mart building situated in Guui-dong, Gwangjin-gu,Seoul, for 10 to 15 minutes from 10:10 a.m. on July 5, 2011, and a report wasreceived by the National Emergency Management Agency. The Agency conducted an emergencyevacuation of the building tenants at 2 p.m., and convened experts inarchitectural structures for a meeting to establish countermeasures. At the meeting,they resolved to perform an emergency safety evaluation of the building, whichwas commissioned to the Korea Infrastructure Safety Corporation, which specializesin the safety evaluation of large-scale structures.

The resultsof the safety evaluation showed that the structural members and foundation of the building did not cause theabnormal vibration and based on these results, the relevant experts initiallyconcluded that it was a resonance effect caused by abnormal vibration. However,the cause of the abnormal vibration remained unknown, and this led to the decisionto perform an investigation to trace the precise cause.

2)Ceiling Collapse

After the 1stvibration accident, there was a sudden drop of the ceiling finishing materialon the 10th floor of the commercial tower on August 3, 2011, and the event was reportedto the National Emergency Management Agency by the tenants. The relevant administrativedepartment convened the experts who took part in the investigation into the 1stvibration accident to perform a field survey. Based on the results of the fieldsurvey, it was confirmed that a regular safety inspection had been performed toinspect the ceiling interior prior to this incident. In particular, the footprintsof safety inspectors and traces of damage were found on the ceiling, which hadfallen, and from this, it was confirmed that this incident was not caused byvibration.

3)2nd Vibration Accident

Around thetime of conclusion of the 1st vibration accident investigation, vibrationsimilar to the 1st vibration accident occurred for 1 to 2 minutes at around 11a.m. on August 11. This incident was again reported by the tenants to theNational Emergency Management Agency. The experts partaking in the 1st vibrationaccident investigation were summoned for a meeting. This incident occurred at aphase where the experts were identifying the cause of the initial accident andplanning a demonstration of the accident, and it increased their confidence inthe results of the initial investigation.

2.2Speculated Source of Vibration

Researcherscould not accurately understand the situation at which the vibration accidentoccurred. Thus, they presented possible vibration sources to be analyzed andreproduced to deduce the vibration source. The possible vibration sources presentedby the researchers included wind load, air conditioning control room mechanicalvibration, footing settlement and structural crack; the details are shown inFig.1.

1) Wind load:occurrence of vertical vibration caused by factors such as eddy resulting from windblowing against the structure

2) Airconditioning control room mechanical vibration: resonance phenomenon due to mechanicalvibration caused by air conditioning equipment in the air conditioning controlroom situated on each floor of the office tower of Techno Mart

3) Fitnesscenter on the 12th floor of the office tower: structural vibration caused bythe resonance phenomenon of vibration due to spinning, treadmill, grouprhythmic exercise, etc., at the fitness center

4) 4Dtheaters of CGV on the 9th floor of the commercial tower: structural vibrationcaused by the resonance

Fig.1. Speculated ofBuilding Shaking Sources

In addition,issues of footing settlement and loss of foundation caused by heavy rainfallthe previous day were suggested, but during a field visit, it was determinedthat such phenomena did not occur and cannot cause such vibration. Thus, ananalysis of the four possible causes stated above was conducted.

3.Review of the Design Documents

3.1General Details

Designdocuments of Techno-mart 21 building must contain clear information necessaryfor production and installation of structures such as the size and location of eachmember, center of column, dimensions of the ribbed parts, size and number ofbolts, welding size, and other details. Thus, in order to understand thecurrent status and information regarding the building, determine whether it wasdesigned in accordance with the design documents, and use the information inthe safety evaluation, this section describes the results of the review of thedesign and structural calculation documents.

3.2Main Contents of Structural Drawing

1) Steel wasused to manufacture all the beams and columns of the office tower. Moment framewas designed for the long span direction and braced frame for the short spandirection, with the core wall being a shear truss wall. They were designed as adual frame system, resistant to wind and seismic loads.

2) The steelform deck plate slab system was applied to the 2nd floor and above, whereas thereinforced concrete slab system was applied to the 1st floor and below.

3) The beamsand columns of the underground floors of the office tower were all made of steel.  

4) As for theunderground floors of the shopping mall, SRC was used for the column, steel forthe beams of the 1st and 2nd underground floors, and reinforced concrete forthe beams of the 3rd to 5th underground floors.

5) Areinforced concrete structure was applied to the exterior retaining wall of theunderground floors.

6) As for thefoundation, individual footing was used for the shopping mall and matfoundation for the office tower.

7) Granitewas in-filled with a 600 mm slab on the top, and was designed to have thepredicted inflow of water yard-drained at the sump-pit.

8) The steelcolumns of the office tower were designed as box columns up to the 13th ground floor.The external columns in the long span direction (excluding the core) on the14th floor and above were designed as box columns, while other columns weredesigned as the H-type.

9) The spanof the office tower frame structure (excluding the core) was mainly 10.2 m and7.5 m in the short span direction and 7.2 m in the long span direction.

3.3Review of Design Documents

The resultsof the review of the building design documents can be summarized as follows:

1) Theresults of the review of the design drawing and reports confirmed that allinformation was accurately provided in the design documents.

2) It wasconfirmed that the structural design met the standards of KBC-19963).

3) The memberand joint details described in the structural calculation sheet and the memberand details indicated in the design drawing were found to be consistent.

4) The reviewof the design documents confirmed that the design of the building satisfied thestructural design standards, and the results of the structural analysis showedthat there were no problems in terms of stress.

4.Prediction of Vibration Source

4.1General Details

On-sitemeasurement and demonstration were performed as shown in Fig.2. on thespeculated vibration sources, which include wind load, mechanical vibrationfrom the air conditioning control room, fitness center and 4D theaters, inorder to identify the vibration source. The details are as follows:

Fig.2. VibrationMeasurement and Speculated Vibration Sources

4.2Prediction and Reproduction of Vibration Source

4.2.1Vibration Caused by Wind Load

The KoreaMeteorological Administration4) provides the public with constant measurementsof wind speed at a unit of cell. According to the data, the wind strength atthe time of the vibration accident was gentler than usual, based on which itwas determined that there was no reason for any larger vibrations caused by thewind load than usual. However, vibration caused by wind load is most common andthere were many experts who predicted that the vibration accident was caused bywind load. Thus, a decision was made to measure the vibration of the buildingin the event of high-strength wind such as a typhoon near the building.

High-strengthwind caused by typhoon Muifa was expected 40 days after the accident, duringwhich the building vibration was measured. The results showed that no one feltany vibration at this time, and the upper part of the building was not shown tobe amplified due to vibration.

4.2.2Mechanical Vibration of 4D-Theater

The buildingcontains 4D theaters, which produce mechanical vibration during the movement ofthe seats

topresent a more realistic experience to the audience during movie screenings.However, the vibration

accidentoccurred prior to the screening of 4D movies, and the vibration of the seatsoccurred 30 minutes after the start of the movie. Based on these results, itwas determined that the mechanical vibration of the 4D theaters did not have animpact on the vibration, which occurred at around 10 am.

Table2. Timetable of the 4-D Theater

Screened  movie	Transformers  3
1st  screening time	10:30
2nd  screening time	13:00

4.2.3Group Rhythmic Exercise at Gym

There is awide range of exercise programs available at the fitness center concerned, and diverse forms of vibration canoccur depending on the location of the people inside. Thus, it was checkedwhether the size of acceleration and vibration caused by the group rhythmicexercise (Taebo), etc., that can periodically produce strong or constantvibration on the floor could be transmitted to the 22nd, 26th, 30th and 34thfloors. In particular, because the 26th floor contains empty offices and has noreason for unusual vibrations, it is where the resonance phenomenon has thehighest possibility of occurrence in case the external load matches the naturalfrequency of the building.

1) Experimenton Vibration Excitation by Humans In order to perform verification with respectto the 2.7Hz frequency, which was slightly amplified with the operation of thetreadmills, a vibration excitation experiment was carried out in the G.X. roomusing a metronome and the vibration transmission and amplification werechecked.

1Vibration Excitation andMeasurement Plan

Fig.3. andFig.4. show the view of the G.X. Room of the fitness center on the 12th floorwhere the vibration excitation experiment was carried out.

Fig.3. 12th Floor Plan(Fitness Center)

Fig.4. VibrationExcitation by Humans in the G.X. Room of the Fitness Center on the 12th Floor

Fig.5. Time History andFFT Analysis of Vibration Excitation on the 12th Floor

Verticalvibration was performed at the 2.7Hz frequency, which was determined topotentially cause vertical vibration, using a metronome at regular intervals,with 14 persons and the vibration excitation time of 3 minutes. Themeasurements of the acceleration responses of the floor slabs were taken on the12th floor as shown in Fig.5.

2)Results of Experimental Measurement

Theexperimental results of the time history analysis of acceleration and frequencyanalysis conducted at the upper part of the building are shown in Fig.6.

As shown inFig.6., acceleration increased after a certain period of time, unlike the twotypes of measured Taebo exercise programs, and this tendency was shown on allmeasured floors. Also, the amplification of the acceleration response was more prominenton the upper floors, confirming that the frequency of the vibration excitation(2.7Hz) was a frequency that amplified the vibration in the vertical direction.

Fig.6. FFT Analysis of34th and 36th Floor from Taebo

Theamplification of the acceleration response is more clearly shown in Fig.7. and Fig.8., which are the result of thefrequency analysis. As shown in the figure, the amplification ratio of aparticular number of vibrations seen in the result of the frequency analysis ofspinning and treadmill was greater during vibration excitation by humans, andit was confirmed that the higher the floor, the higher the amplification ratio.

Fig.7. Time History Analysisof Acceleration Following Vibration Excitation by Humans

Fig.8. Results ofFrequency Analysis Following Vibration Excitation by Humans

Table 3. andTable 4. show a comparison of the maximum values and RMS of the acceleration responseof the 12th and 34th floors. As shown in Table 3., in the case of the RMSresponse, the acceleration of the floor on which vibration excitation wasperformed was high, with low distribution of the response on the 34th floor buthigher than the response caused by spinning or treadmill operation.

Table 3. Comparison ofthe Accelerations During Group Exercise (RMS)

Table 4. Comparison ofthe Accelerations Using Group Exercise (Peak)

4.2.4 Vibration Causedby the Mechanical Room and the Wind

The existingequipment in the air conditioning control room is in continuous operation.Thus, there was no reason to think of it as a new vibration source. The variousmachines in the mechanical room produce vibrations at a significantly higherfrequency than the natural vibration frequency of the building, and thus it wasdetermined that there was no relation between the mechanical vibration producedby the air conditioning control room and the vibration accident. Also, the windload at the time of the vibration accident was confirmed to be low as shown inthe table from the meteorological data provided by the Korea MeteorologicalAdministration.

4.3 Resonance Phenomenon

All matterhas its own unique movement, and the characteristics of the movement aremathematically noted as natural frequency and natural period. The resonancephenomenon occurs when the natural vibration frequency of a matter equals thevibration frequency of an external force. In the case of general vibration,reduction of vibration occurs away from the epicenter, whereas in the event ofresonance, vibration becomes stronger away from the epicenter. Such phenomenoncan be understood based on the dynamic amplification factor that occurs in thestructure as shown in [Eq. 1].

[Eq. 1]represents the displacement amplification factor according to the vibrationfrequency of the external force and the vibration frequency of the structure,and the vibration frequency ratio based on this is shown in Fig.9. Thisequation shows that when the natural vibration frequencies of the externalforce and the structure match (fp/fn=1), the displacement response is amplifiedalmost infinitely, and such amplification effect is referred to as theresonance phenomenon.

Fig.9. Concept ofResonation Phenomenon

However, dueto the damping effect that interferes with the movement in actual structuresand objects, the ratio of amplification decreases depending on the strength asa solid line in Fig.8. (damping 1.0%) and infinite amplification does not occurin actuality. The collapses of the Angers Bridge in France (1850), TacomaBridge in Washington in the U.S. (1940), and the Millennium Bridge are famousfor being caused by the resonance phenomenon; these accidents occurred due tothe amplified displacement of the structure resulting from the vibrationfrequency of pedestrians or the wind load matching the natural vibrationfrequency of the structure.

The case ofthis building is very unusual in that resonance occurred in the longitudinaldirection of the structure due to a group rhythmic exercise of humans, and theamplification of displacement was too small to have an impact on the safety ofthe building, yet it caused discomfort to the users of the building.

The resonancephenomenon does not only have negative impacts, but has been very useful inreal life. For instance, switching to a certain radio or TV channel concentratesthe external frequency signal concerned to cause resonance with the receiverand allow users to receive the broadcast.

5.Conclusion

Measurement/demonstrationof the predicted vibration sources, which include the fitness center, 4Dtheaters, wind load and mechanical vibration, were performed and thewind-induced vibration was measured. The acceleration responses of each of thevibration sources were reviewed based on the measurements, and the results areas follows:

(1) As forthe fitness center on the 12th floor of the office tower, the on-sitemeasurements and demonstration with respect to the group rhythmic exercise,spinning and running on the treadmill, which were regarded as the possiblesources of vibration at the time of the vibration accident, showed thatspinning and running on the treadmill did not have an impact on the vibrationon the upper floors of the building. Also, exercises such as Taebo did havefrequency characteristics transmitted to the upper floors, but did not amplifythe vibration.

(2) In thecase of repetitive rhythmic exercise at 2.7Hz, which is the vertical vibrationmode of the building, the frequency characteristics are transmitted to theupper floors, with the amplification of acceleration increasing at higherfloors. Repetitive rhythmic exercise performed by a group can cause resonanceand increase the vibration level, and was determined to be the source of thevibration that occurred on July 5, 2011.

Acknowledgment

The presentresearch was conducted by the research fund of Dankook University in 2014.

References

1) Newsreport, http://www/news/nation/2011/07/1190308.html

2) Park TW,Inspection of collapse cause of Sampoong Department Store, Forensic ScienceInternational, 2012; 217: pp.119-126.

3) Ministryof Construction & Transportation, Korea building code–1996.

4)http://www.kma.go.kr/weather.obsrvation/pastcal.jsp, website of  Korea Meteorological Administration.

5) Min, KW,Lee, SH, Kim, JK, (2004) Vibration tests of 5-storey  steel frame with viscoelastic dampers,Engineering Structures, Volume 26, Issue 6, pp.831-839.