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Confirming Effectiveness of Base Isolation at Takenaka-Owned Base Isolated Buildings During the Great East Japan Earthquake

—Reduced shaking by a maximum of 50 percent in low- and medium-rise buildings—

July 14, 2011Takenaka Corporation

Takenaka Corporation (President: Toichi Takenaka) verified the effectiveness of base isolation and confirmed the high level of safety of Takenaka-owned base-isolated buildings during the Great East Japan Earthquake (M9.0) of March 11. The two buildings are a combined bachelor employees dormitory and residence (FLATS TOYO: completed in 2008) in Koto-ku, Tokyo and a bachelor employees dormitory (Funabashi-Chikuyuryo: completed in 1987) in Funabashi City, Chiba Prefecture. At these two buildings, seismographs installed at the bottom (ground side) and the top (building side) of the base isolation layer measured earthquake shaking. Comparing the measurement data verified the degree that the shaking was reduced by the base isolation structures.
At FLATS TOYO, a computer was used to analyze the shaking of the building and to confirm whether or not the analytical values and measured values conformed, verifying that it is possible to simulate earthquake shaking at an upper structure (upper part of a building) with high precision. At the same time, the scale of shaking of a building hypothesized in a case where a base isolation structure has not been introduced was analyzed, verifying the degree that a base isolation structure reduces shaking.
Takenaka Corporation is proud to be the leader in the construction of base-isolated buildings in Japan. In response to this verification of the effectiveness of base-isolated buildings, Takenaka will aggressively advocate the merits of base-isolated buildings for both new construction projects and renovation projects in order to ensure the safety and security of its customers during large earthquakes and guarantee them that they can continue their businesses stably after an earthquake.

【Results of the verification at FLATS TOYO】

Verification based on observation records

In Toyo in Koto-ku in Tokyo, where FLATS TOYO is located, shaking at seismic intensity 5 upper was observed during the earthquake. In this building, which is a 12-story building, seismographs were installed at [1] the foundation and on [2] the first floor, and on [3] the twelfth floor (Fig. 1). The base isolation layer is between [1] the foundation and [2] the first floor. The results of the verification confirmed that in contrast to the shaking of the foundation (acceleration of about 100 gal※1), the shaking of the first floor of the building was about 50 percent lower (acceleration of about 50 gal), and even on the twelfth floor, which was the top floor, the shaking could be reduced by about 40 percent (acceleration of about 60 gal). (Fig. 2)

※1 gal (gal = cm/s2): A unit of acceleration. It represents the degree which velocity changes per second. Like seismic intensity, its value varies between observation locations during a single earthquake. The larger the gal value, the more violent the shaking, but it is necessary to consider velocity and displacement as well as acceleration to assess damage.

Verification based on analysis

The broken yellow line represents a simulation of the way the building shook based on analysis.
The values measured by the seismographs at the two points, [2] the first floor and [3] the twelfth floor and the simulation results conform very closely, confirming that the system performed as its designers intended.
Comparing the analytical values (broken blue line) and hypothesizing a case without a base isolation structure with the measured values reveals that the shaking on the twelfth floor was between approximately 1/4 and 1/5 lower than the shaking which would have occurred without the system. (Figs. 3 and 4)

【Results of the verification at Funabashi-Chikuyuryo】

Verification by seismographs

In Funabashi City in Chiba Prefecture, where Funabashi-Chikuyuryo is located, shaking at seismic intensity 5 lower was observed during the earthquake. Measured values obtained by seismographs installed at [1] the foundation, at [2] the first floor, and at [3] the roof (Fig. 5), have confirmed that in contrast to ground shaking (acceleration of about 150 gal), shaking at the [2] first floor and [3] roof was lowered by about 1/2 (acceleration of about 70 gal). (Fig. 6) This has confirmed that even in this building with base isolation devices installed for more than 20 years, the devices have maintained their high base isolation effects.

The base isolation technology adopted by the Funabashi-Chikuyuryo, which is promoted as Takenaka’s proprietary technology, the “Viscous Base Isolation® System,” has been applied to semiconductor factories.

【Outline of the building】

Building name FLATS TOYO
Location 3-chome, Toyo, Koto-ku, Tokyo
Design/Construction Takenaka Corporation
Year of completion 2008
Use Employee dormitory
Total floor space approx. 8,037 m2
Number of floors 12 fl. aboveground (no basement)
Structure RC, partly S (foundation isolation)
Materials of base isolation Rubber bearings with lead plugs, natural rubber bearings, ball screw type friction dampers, oil dampers

<FLATS TOYO>

【Outline of the building】

Building name Funabashi-Chikuyuryou
Location Funabashi City, Chiba Prefecture
Design/Construction Takenaka Corporation
Year of completion 1987
Use Employee dormitory and residence
Total floor space 1,530 m2
Number of floors 3 fl. aboveground (no basement)
Structure RC (foundation isolation)
Materials of base isolation Natural rubber bearings, viscous dampers

<Funabashi-Chikuyuryou>