The Performance of Seismically Isolated Buildings in Japan based on Earthquake Observation Records

Subsequent to the Kobe earthquake, a number of large earthquakes occurred in Japan: 2004 Niigata earthquake (M6.8), 2005 Fukuoka earthquake (M7.0), 2011 Tohoku earthquake (M9.0), 2016 Kumamoto earthquakes (M6.5, M7.3) and 2024 Noto Peninsula earthquake (M7.6). Many response records from seismically isolated buildings were obtained for these earthquakes. The positive benefits of seismically isolated buildings have been clearly demonstrated by analyses of these earthquake records. Nonetheless, several isolated buildings still experienced some damage of their seismic expansion joints (movement gaps). In the M9.0 Tohoku earthquake in 2011, the total duration of shaking was more than four minutes, and many buildings experienced two to three minutes of significant shaking.   For such long durations, the energy absorption capability of the isolation devices must be verified. This important issue has been addressed over a number of years by dynamic cyclic tests of rubber bearings, hysteretic dampers and other types of isolation devices.

This short article gives an overview of earthquake response records obtained in seismically isolated buildings, with particular focus on those from the Kumamoto earthquakes of 2016 and the Noto Peninsula earthquake of 2024.

On April 14 and 16, 2016, two major earthquakes occurred within 28 hours in the Kumamoto region of Japan. In the two earthquakes, which were M6.5 and M7.3, and both with Japan Meterological Agency (JMA) Seismic Intensities of 7. While serious damage to buildings was confirmed in many places in Kumamoto and Oita prefectures, seismically isolated buildings kept residents and users safe, and continued to function without any problems after the earthquakes. Most seismic-resistant buildings avoided collapse, but many sustained damage such as cracking in columns, beams, and walls along with extensive non-structural damage such as toppling of furniture, falling of light fixtures, and rupture of utility service pipes.

At the time of the Kumamoto earthquakes, there were 24 seismically isolated buildings, including 4 under construction, in Kumamoto Prefecture, and 17 of these were surveyed after the earthquakes. Most of the isolated buildings are condominiums, and also medical facilities, offices, and warehouses. There are also some seismically isolated, single-family houses in Kumamoto Prefecture but these were not able to be investigated as part of the survey. The post-earthquake survey included exterior and interior visual inspections of the buildings and also interviews of the occupants and building managers.

Interviews with residents of seismically isolated buildings yielded the following comments: “I was really grateful for the seismically isolated resistance because no dishes fell or broke. My relatives were evacuated and got a good night’s rest.” “When I was lying down, I could feel an earthquake of JMA intensity 2 or 3, but when I was standing or sitting doing something, I often didn’t notice an earthquake of JMA intensity 2 or 3. Because things inside the room did not fall over and dishes and hanging lights did not fall, the children were not startled by the sound and slept soundly during the foreshock and mainshock. Thanks to this, we didn’t panic and we were all able to act calmly together.”

Eight of the buildings included scratch plates, simple devices which record building movement by means of scratch markings on a metal plate, which made it possible to confirm the isolation displacement movements during the earthquakes. Seismographs were not installed in any of the seismically isolated buildings surveyed. In Japan, the installation of seismographs and scrach plate is at the building owner’s option. Structural designers are expected to actively encourage the installation of such equipment.

The peak-to-peak maximum displacements (double amplitudes) measured by the scratch plates were between 60 cm and 70 cm for most of the seismically isolated buildings in Kumamoto City. A maximum double amplitude of 90 cm and a maximum single amplitude of 46 cm were recorded at the medical facility, a hospital in Aso City (Figure 1).

The maximum acceleration of the ground motion observed at a point 3.5 km east of this seismically isolated hospital was 261 gal, with a maximum velocity of 73 cm/s. The period of excitability of the observed ground motion was 3 seconds, and it contained a long-period component.

The building’s seismic isolation system comprises 27 natural rubber bearings and 45 lead-rubber bearings. The diameter of all rubber bearings is 70 cm. The seismic isolation period is approximately 3 seconds. The maximum displacement of 46 cm corresponds to a shear strain of 330% in the rubber bearings. Figure 2 and 3 show the displacement orbit recorded in the M7.3 main shock. To date, this is the largest seismic isolation displacement of a building ever recorded worldwide. In all the isolated buildings there were almost no residual displacements of the seismic isolation systems, and no damage was observed in any of the seismic isolation devices.

Fig. 1. Seismically isolated medical facility in Aso city

Fig. 2. Scratch plate and displacement orbit, medical facility in Aso

Fig. 3. Displacement orbit recorded at medical facility in Aso city

However, at the perimeter of this building, deformation of expansion joints in the area connecting the seismically isolated building and the earthquake-resistant building was confirmed (Figure 4). In addition, an elongated locker installed in a hospital ward was reported to have toppled over, and an LCD TV in the hot water supply room toppled over and a small refrigerator moved approximately 10 cm.

Fig. 4. Damage of expansion joints

On January 1, 2024, a M7.6 earthquake occurred on the Noto Peninsula in Ishikawa Prefecture, Japan. Many old houses and other structures collapsed, and there were 376 fatalities.

There are four seismically isolated buildings on the Noto Peninsula (three are indicated in Figure 5), and several dozen elsewhere in Ishikawa prefecture and the surrounding prefectures of Toyama and Niigata. The largest isolation displacement response was experienced by the Keiju Medical Center in Nanao City where a maximum displacement of 19 cm was observed (Figures 6 and 7).

The Keiju Medical Center hospital building is a seven-story RC structure with a total floor area of 16,000 square meters and was completed in 2013. The seismic isolation system uses high-damping rubber bearings. The isolated building did not suffer any damage, whereas significant non-structural and contents damage occurred in the adjacent non-isolated hospital building.

Fig. 5. Noto Peninsula seismic intensity map (by JMA) and seismically-isolated buildings (by JSSI)

Fig. 6. Keiju Medical Center, Nanao City  (photo: Prof. Akira Wada))

Fig. 7. Noto Peninsula seismic intensity map (by JMA) and seismically-isolated buildings (by JSSI)

Figure 8 shows the acceleration waveforms observed at an isolated building (3-story RC structure) in Shika Town. A maximum acceleration of 656 gal was recorded below the seismic isolation layer, and the maximum acceleration on the first floor and rooftop was 144-152 gals, a decrease of about three quarters.

Fig. 8. Observed acceleration waveforms for an isolated building in Shika Town

Figure 9 shows the maximum response accelerations observed in over 50 seismically isolated buildings for four major Japanese earthquakes. The horizontal axis of this figure is the maximum acceleration at the basement below the seismic isolation level, and the vertical axis is the ratio of the maximum accelerations (amplification factors) of the 1st floor (1FL) and the highest floor (Top floor) against that of the basement below the seismic isolation level. From all the observed results it can be seen that the isolated superstructure accelerations were significantly reduced from those of the basement (input acceleration) and that the larger the input acceleration (at the basement level), the greater is the seismic isolation effect. In the case of small shaking and low input accelerations it can be seen that the amplification factor may be greater than 1.0.

Fig. 9. Amplification Factor of Observed Acceleration of Seismic Isolated Buildings

These investigations and many other detailed studies have verified the performance of seismic isolation in many earthquakes in Japan. Some of the observed seismic responses were at levels comparable to the design seismic ground motions for large earthquakes. No structural damage occurred in any of the buildings, confirming that the seismic isolation systems performed as intended. Some buildings showed slight damage to seismic isolation expansion joint details, and careful attention should be paid to the design of expansion joints and other non-structural details.

The investigation of the response of isolated buildings, and the collection of such seismic observation records is very important because it contributes to the development of better seismically isolated structures and improvements in the reliability of seismic isolation technology.

Recording the response of seismically isolated buildings to earthquakes is very useful for evaluating the effectiveness of seismic isolation and for monitoring the structural health of the seismic isolation systems (devices). Furthermore, the displacement of the seismic isolation level can be tracked by inexpensive orbit devices (scratch plates) without the need for more expensive electronic monitoring systems.

The use of seismically isolated structures that exhibit high safety and maintain their functionality during earthquakes is an effective way of ensuring life safety and reducing earthquake damage. Over time, seismic recordings have indicated that the largest observed earthquakes are in some cases more severe than what is assumed in design and it is therefore important that sufficient allowances and safety margins are considered in the design of seismically isolated structures.

References:
1. Mineo Takayama, The Performance of Seismically Isolated Buildings based on Earthquake Observation Records in Japan, 18WCSI, Turkey, 2023
2. Mineo Takayama, Keiko Morita, Observed Response of Seismically Isolated Buildings during the 2016 Kumamoto Eearthquake,17th U.S.-Japan-New Zealand Workshop on the Improvement of Structural Engineering and Resilience, New Zealand, 2018
3.Mineo Takayama, Keiko Morita, Building Damage and Lessons Learned from the 2016 Kumamoto Earthquakes, 17WCSI, New Zealand, 2017
4. Keiko Morita, Mineo Takayama, Behavior of Seismically Isolated Buildings during the 2016 Kumamoto Earthquakes, 17WCSI, New Zealand, 2017