New Horizons in Structural Design April, 2008

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構造デザインの最前線　　　　FACT　NIKKEN SEKKEI09sized and low-rise buildings are more likely to be heavily damaged.Project-Based Sway AssessmentIn studying earthquake safety, an important indicator for a building is its "natural period." This term refers to the time taken by the building to return to its original position after swaying. In general, higher buildings have longer natural periods. In the case of a 200-m-high superskyscraper, the natural period is around 5 seconds. Various cycles of waves are contained in earthquake motion. In this way, a seismic event resembles a musical tune, which is com-posed of various sounds ranging from low pitch to high pitch. An earthquake is composed of different types of motion, some involv-ing powerful short-period waves and others entailing strong long-period waves. When the powerful periodic band of an earthquake conforms to the natural period of a building, "resonance phenom-ena" are caused and the building sways considerably. The site ground also influences the properties of seismic waves. Therefore, it is necessary to understand how a building sways, taking due account of both earthquake and ground characteristics during the design process.A high-rise building with a long natural period and a seismically isolated building require careful examination of sway. In such cases, it is necessary to conduct analyses using actual seismic waves. Such buildings are vulnerable to the long-period ground motion caused by plate-boundary earthquakes, so we apply the type of ground motion that would be likely to occur in the event of an earthquake. For example, we implement computer simulations to check how a given building would have swayed in the Kanto Earthquake for the Tokyo metropolitan area and in the Nankai and Tonankai Earth-quakes for the Kansai region. We also apply inland epicentral earth-quake solutions to buildings when appropriate. We make such deci-sions based on the nature of each project, taking the importance of the building in question into account.Introduction of Input Earthquake Motion SystemThe first thing the designer should do in studying aseismic capacity is to evaluate earthquake motion after selecting parameters for a projected earthquake and properly reflecting the ground condi-tions, which may differ according to area. Then, the designer con-ducts a simulation using obtained earthquake motion data to verify the earthquake safety of the particular building. As a tool for realizing the above process, we have developed over several years a design input earthquake motion system. This system allows us to easily calculate sway under different conditions. Based on the results, it becomes possible to clarify aseismic capac-ity for different cases in numerous design projects, as well as to promote aseismic capacity design. This system is not only applicable to new buildings—it is also useful in evaluating aseismic capacity and earthquake retrofitting for existing buildings.Of course, we still have much to learn about complex natural phenomena. Accordingly, we installed an earthquake recorder in our own building in order to verify the characteristics of the build-ing that had been projected via the design process and simulation results. By analyzing the observation data, we could check the nat-ural period of the building and the adequacy of the analysis model used for simulation. We continue to use our accumulated data and knowledge to further clarify the relationships between earthquakes and buildings in order to provide more reliable, safer structures in all of our projects.Toru Kobori, Takashi Yamane資料：日本の地震活動（地震調査研究推進本部）"Earthquake Activities in Japan"Based on Headquarters for Earthquake Research Promotion website

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