New Horizons in Structural Design April, 2008

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28FACT　NIKKEN SEKKEI　　　　構造デザインの最前線Different building categories typically call up associations with dif-ferent types of structure. For instance, a timber structure is usually selected for family homes, a concrete structure for condominiums, and a steel structure for large indoor space like a gymnasium. The multipurpose buildings we introduce here incorporate different structural styles that have been combined vertically. These build-ings are designed to deliver the best performances in the appropri-ate places. Seismic Isolation Structure Attenuates Swaying All buildings in Japan must be designed to withstand earthquake forces. It is possible to address this need by employing thicker pil-lars in the framework of a building or by increasing their number. It is common to achieve strength by adding diagonal members deployed between pillars, such as struts and braces. However, we cannot always meet expectations in terms of function and cost solely through the use of thick pillars, struts, and braces. When a high aseismic capability is required, we first come up with a seismically isolated structural system. This structure pro-tects against earthquake effects by isolating in a way, the building from the ground.In a seismically isolated structure, it is common to deploy the seismic isolator between the foundation and the building. This approach is called "foundation isolation." In recent years, however, there has been a gradual spread of an approach by which the seis-mic isolator is installed between the lower and upper stories of a building. This is because the lower part of the structure, if designed to be very strong, will work just like a foundation. This approach is called "mid-story isolation system."Mid-Story Isolation System Enables Many PossibilitiesA seismic isolator deployed at the mid-level of a building improves its aseismic capability and helps to upgrade the functions of the building. It also adds value by responding to the different purpos-es, structures, and shapes of the stories above and below the seis-mic isolation layer. Good examples can be found in Shiodome Sumitomo Building and Iidabashi First Building/First Hills Iidabashi. Installation of mid-story equipment that attenuates earthquake-generated sway enables us to design multipurpose buildings with columns deployed in different spans or structures that differ between upper and lower stories. In Iidabashi First Building, a rein-forced concrete residential building has been constructed atop the steel structured office building. An important feature of this technology can be noted in the fact that in the event of an earthquake the stories above the seis-mic isolation layer experience minimal sway. This in turn reduces seismic forces affecting the lower stories. The large atrium of Shiodome Sumitomo Building makes good use of this effect. Such structural technology plays a key role in improving aseismic capa-bilities and architectural potential. Nevertheless, it is always vital to carefully check elements that run vertically through the seismic iso-lation story, which experiences greater swaying, and to keep site-specific factors firmly in mind. Effective for Aseismic ReinforcementInstallation of seismic isolators in intermediate stories works very well in large-scale buildings where different structures are com-bined vertically. This technique is also being applied for seismic retrofitting to existing buildings that need to be preserved—an area that is expected to see further development in the future. Yasuhiro Tsuneki, Shingo Torii, Katsuhide Murakami上部構造住居オフィス免震層下部構造基礎構造Structure of Iidabashi First Building/First Hills Iidabashi飯田橋ファーストビル・ファーストヒルズ飯田橋断面構造Shiodome Sumitomo Building and Iidabashi First Building/First Hills Iidabashi異なる機能や構造形式を重ねる中間層免震構造A Combination of Different Structural Styles汐留住友ビル&飯田橋ファーストビル・ファーストヒルズ飯田橋

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