A free-vibration experiment was conducted in Tsukuba to assess changes over 40 years in seismic-isolation systems. The test focused on a building completed in 1986 as Japan's first practical seismic-isolated building and confirmed whether the changes remained within the range predicted in the design.
A free-vibration experiment was conducted at the company's Technical Research Institute on the four-story reinforced-concrete Administration Building, which was built 40 years ago. The building was supported by 25 natural-rubber laminated bearings, each 50 centimetres in diameter. The 12 steel-rod dampers were removed, leaving the building supported only by the laminated rubber bearings, and 2 hydraulic jacks applying 200 tonnes of pressure were used to push it horizontally by 10 centimetres before measuring its free vibration.
The history of practical seismic-isolated buildings spans only approximately 40 years, and data from long-term assessments of actual buildings are limited. The measurements showed that the stiffness of the laminated rubber bearings had increased by approximately 10% after 20 years, approximately 11% after 30 years and approximately 12% at the 40-year mark, remaining within the range predicted at the design stage. The design assumes that safety can be maintained even if the bearings become up to 30% stiffer after 120 years.
The company plans to continue the experiments at the 50- and 60-year marks and share the resulting data across the industry through academic presentations and other channels. Okumura-gumi President Okumura Takaten (奥村太加典) said the 40-year assessment may have increased public confidence in seismic-isolation technology and expressed the intention of promoting the spread of seismic-isolated buildings in earthquake-prone Japan.
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