Vibrations of the TAIPEI 101 skyscraper caused by the 2011 Tohoku earthquake, Japan
The strong-motion recordings generated by the 11 March 2011, Tohoku, Japan earthquake recorded by the building seismic array in the TAIPEI 101, the second tallest building in the world, are more complicated than those at the free-field stations. Fundamental and higher-mode vibrations can be clearly seen on the spectra. The fundamental-mode frequency is about 0.15 Hz, which is the natural frequency of the skyscraper. Spectral ratios of ground accelerations at the building to those at a nearby borehole station increase from unity at the fifth floor underground to factors of 110 and 146, respectively, on the 74th and 90th floors above ground for the fundamental-mode vibrations. The frequency content of accelerograms, recorded on the fifth floor below ground from the great 2011 Tohoku-Oki earthquake, appears mainly in a frequency band of 0.015–0.1 Hz, leading to a fact that the ground motions did not cause high excitation in the TAIPEI 101 skyscraper.
Key wordsBuilding array TAIPEI 101 vibration Tohoku earthquake
The great Tohoku-Oki earthquake (Mw 9.0) of 11 March, 2011 occurred off the northeast coast of Japan, resulting from slip on a roughly 500-km-long and 200-km-wide megathrust fault (e.g., Lee et al., 2011; Suzuki et al., 2011). This earthquake caused tremendous damage in northern Japan due to extensive tsunami as well as strong shaking. This earthquake resulted in 15,550 fatalities, 5,344 missing, 5,688 injured, 224,798 houses collapsed and more than 460,000 houses damaged (Tsuru and Murakami, 2011).
Strong ground motions generated from the 2011 Tohoku earthquake were well recorded by nearly 1200 K-NET and KiK-net stations across Japan, with a peak ground acceleration as large as 2933 gal at MYG004 and more than 1 g at 10 sites (Aoi et al., 2011). From snapshots of seismic wave propagation of the strong ground motion during the earthquake, Furumura et al. (2011) observed the amplified and prolonged ground shaking in some populated cities due to the resonance of long-period ground motions within sedimentary basins. They also found that the 2011 Tohoku earthquake generated a large response in a relatively wide period ranging from 0.5 to 30 sec, resulting in a significant impact on many wooden-frame houses to high-rise buildings. Several high-rise buildings in Tokyo, about 370 km from the epicenter, swept but did not have any significant damage during the great 2011 Tohoku-Oki earthquake. However, the tip of Tokyo Tower was bent by the strong shaking of the earthquake.
2. Data Collection
The TBA consists of four stations: two pairs of force balance accelerometer and R-1 rotational seismometer em-placed, respectively, in the southwest (T1S3 and T1S3R) and northeast corners of the 90th floor (T1S4 and T1S4R); and another two pairs of force balance accelerometer and broadband velocity sensor emplaced in the southwest corner of 74th floor and the fifth floor below ground (T1S2 and T1S1), respectively. The frequency range of the ac-celerometer is from DC to 200 Hz and has the ability to record shaking level as high as 2 g. The broadband velocity sensor is capable of measuring ground-motion up to 20 cm/sec with a bandwidth from 0.008 Hz to 70 Hz. All of the stations record signals continuously at 100 and 20 samples per second, respectively. Time synchronization is maintained in the main processor based on time received according to the NTP (network time protocol) through communication with an internet time server. The R-1 rotational seismometer has a resolution of 1.2 × 10−7 rad/sec (eentec, 2007). The sensitivities of the velocity and acceleration sensors are 2.384 × 10−5 cm/sec/count (Hutt et al., 2008) and 2.337 × 10−4 cm/sec2/count (Kinemetrics, 2002), respectively. The downhole broadband station (101B) is located at a distance of about 500 m from the TAIPEI 101 skyscraper and the sensor is installed at a depth of 100 m (Huang et al., 2010).
3. Vibration of the Taipei 101 Skyscraper
Figure 2(b) shows the N-S component seismograms recorded at T1S1, T1S2, T1S3, T1S4, and 101B. The maximum values of peak ground accelerations (PGA) in the N–S direction on the fifth floor below ground and the 74th floor were 0.253 and 0.74 cm/sec2, respectively. The PGA values on the 90th floor were about four times of those on the fifth floor below ground. It is noted that seismograms recorded at the TBA are more complicated than those at station 101B. The main building’s TMD with a gross weight of 660 metric tons did not sway during the earthquake.
To retrieve significant information recorded by the TBA, DC-offset is first eliminated by subtracting the mean amplitude from raw data. The Fourier amplitude spectra were calculated and smoothed using a 3-point running Hanning average. Fifty consecutive smoothing processes were applied to the raw spectra. This number was chosen empirically considering its visual effect on the spectral shape. In order to obtain the same scale, velocity spectra at 101B were multiplied by the angular frequency to produce the acceleration spectra. Figure 2(c) shows the acceleration spectra along the N–S component on the 90th floor and the station 101B. Spectral amplitudes on the 90th floor show several local peak amplitudes at different frequencies, while those at station 101B gradually decrease with increasing frequency.
The values of f0, fi (i = 0, 1, 2, …, 6), and the ratios of AZi, AEWi, and ANSi for the Z-, EW-, and NS-components.
In Fig. 3, several major vibration modes, associated with respective local peak amplitudes, can be delineated. The frequencies with local peak amplitudes may be closely associated with the vibrations of the TAIPEI 101 skyscraper caused by the earthquake. The maximum spectral value appears at f0 = 0.15 Hz, which is the frequency of fundamental mode and also the natural frequency (or the natural period of 6.8 sec) of the building TMD (Haskett et al., 2003), for the three components at T1S2, T1S3, and T1S4. The spectral amplitudes along the three components at T1S1 are much smaller than those at T1S2, T1S3, and T1S4. This indicates that ground motions in the high-rise building are much larger than those on the basement. The horizontal spectral amplitudes are larger than the vertical ones when f < 1 Hz. This might be due to a fact that the loading forces on the building caused by the earthquake are mainly along horizontal direction, especially in the lower frequency regime. For the vertical component, the spectral amplitudes at T1S3 and T1S4 are much larger than those at T1S2, T1S1, and 101B when f > 2 Hz. The frequency content of accelerograms recorded on the fifth floor below ground appears mainly in a frequency band of 0.015–0.1 Hz (see Fig. 3). This observation explains why the ground motions did not cause much response in the TAIPEI 101 skyscraper.
The Taipei basin is a triangular-shaped alluvium basin filled with the Quaternary unconsolidated sediments overlying the Tertiary basement (Wang-Lee and Lin, 1987; Chang et al., 1998). Lin (2001) observed that the sediments thicken northwestward from a thin basin margin in the southeast to about 700-m thick sediments in the northwest. The topmost part of sediments is a soft layer, composed of unconsolidated sand, silt and clay with a thickness varying from 50 m in the southeastern part to 120 m in northwestern part. Previous studies of site effect in the Taipei basin (e.g., Wen and Peng, 1998; Wang, 2008) showed that high amplifications at low frequencies (0.2–1 Hz) and at high frequencies (1–3 Hz) could be correlated, respectively, with the areas of deepest sediments and those near the basin edges, except for a steep basin edge in the west. The TAIPEI 101 skyscraper is located near the eastern margin of the Taipei basin.
The TAIPEI 101 building’s TMD with a gross weight of 660 metric tons swayed during the 2005 Typhoon Long-wang and the 12 May 2008 Wenchuan, China earthquake from observations and the 2010 Typhoon Fanapi from seismic data (Chen et al., 2012), but it did not sway during the great 2011 Tohoku-Oki earthquake. This is an interesting problem. Long-period strong ground motions caused by large earthquakes are the main factors responsible for seismic damages of large-scale structures, such as high-rise buildings, long bridges and big tank farms (e.g., Koketsu et al., 2005). Chen et al. (2008) analyzed strong ground motions generated by the 26 December 2006 Pingtung offshore twin earthquakes (ML = 6.96 and 6.99, respectively) and found that the long-period ground motions were amplified by a factor from 2 to 10 in the Taipei basin and Lanyang basin at the northern end of Taiwan which are about 350 km away from the source region. The long-period ground motions generated from the 12 May 2008 Wenchuan, China earthquake, which was about 1910 km from Taipei, caused a significant resonance in tall buildings in the Taipei basin, and thus some people in a few high-rise buildings felt the vibrations.
This study examined vibrations of the super tall TAIPEI 101 skyscraper located at the Hsinyi district of Taipei, Taiwan caused by the 11 March 2011, Tohoku-Oki, Japan earthquake (Mw = 9.0) from the data recorded at stations on the 74th and 90th floors above ground and the fifth floor underground of a newly installed seismic array inside the building and a nearby down hole station. Fundamental and higher mode vibrations, associated with local peak amplitudes, could be clearly delineated in the spectra. The fundamental-mode frequency is approximately 0.15 Hz, which is the natural frequency for the skyscraper. Spectral ratios on the fifth floor below ground to the down-hole station are smaller than 2 when f < 0.3 Hz and are up to about 10 at f = ~1 Hz. This could be due to amplifications of seismic waves in the soft surface layers. Ratios of spectra at the building to those at the down hole station increase from unity on the fifth floor underground to factors of 110 and 146, respectively, on the 74th and 90th floors above ground for the fundamental mode.
The skyscraper is equipped with a 660-metric-ton tuned mass damper (TMD) in the main building between the 87th and 91st floors. The TMD is the largest of its type in the world. The TMD swayed during the 2005 Typhoon Long-wang, the 12 May 2008 Wenchuan, China earthquake and the 2010 Typhoon Fanapi, but it did not sway during the great 2011 Tohoku-Oki earthquake. The frequency content of accelerograms, recorded on the fifth floor below ground from the great 2011 Tohoku-Oki earthquake, appears mainly in a frequency band of 0.015–0.1 Hz, thus being unable to cause high excitation in the TAIPEI 101 skyscraper.
We appreciate two anonymous reviewers for giving us significant comments to improve the article. We thank TAIPEI 101 for providing space and power supply for installation of building array. We appreciate Mr. Jeff Sheu, engineering operation manager of TAIPEI 101, for the assistance in fieldwork. We also thank Mr. Chin-Shang Ku of the Institute of Earth Sciences, Academia Sinica for the assistance in installation and maintenance of TAIPEI 101 building array. This work was supported by the Institute of Earth Sciences, Academia Sinica and the National Science Council, R.O.C. under grant NSC 100-2119-M-001-023.
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