日本の構造設計体系からみた米国の鋼構造耐震設計に関する一考察

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  • A STUDY OF SEISMIC DESIGN OF STEEL BUILDINGS IN THE US FROM THE STANDPOINT OF JAPANESE STRUCTURAL DESIGN
  • ニホン ノ コウゾウ セッケイ タイケイ カラ ミタ ベイコク ノ コウ コウゾウ タイシン セッケイ ニ カンスル イチ コウサツ

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<p> The design of steel buildings in Japan and the US differ in a number of aspects, including the material utilized, structural design methods, and approaches. I-shaped columns with limited moment connections between the columns and beams (called the “perimeter frame system (PFS)”) are used in the US and other countries, while most beam-to-column connections are moment connections with square hollow structural section (HSS) columns (called the “space frame system (SFS)”) in Japan. In this paper, a popular elastic building design procedure in the US, the equivalent lateral force (ELF) method, is assessed from a Japanese structural design perspective. A design example of a 12-story steel moment-frame office building shown in FEMA P-1051 is studied. Furthermore, the required lateral strength by ELF and the Japanese allowable design procedure is compared for moment-frame and braced-frame buildings with respect to the natural period. The findings are as follows:</p><p> </p><p> (1) The lateral stiffness of the FEMA P-1051 design example required by ELF is significantly lower than that required in Japanese buildings. The first natural periods are 3.48 and 3.13 sec., while the approximate periods estimated for the same building height are 1.56 and 1.40 sec. in the US and Japanese practice, respectively. Furthermore, the first natural periods of the building designed in Japanese system, SFS, in accordance with Japanese design standards are 1.70 and 1.75 sec. It is confirmed in this example that moment resisting steel buildings with significantly lower lateral stiffness are designed and constructed even in California (CA) State, a seismic zone in the US.</p><p> </p><p> (2) The steel volume in the example in FEMA P-1051, designed with PFS, can be reduced to 85%, if it is designed in Japanese style (SFS) using square HSS columns and moment connections in almost all beam-to-column connections. This is because the cross-sectional area of the columns and beams needed for the relatively low lateral stiffness is not significantly larger than that needed for the strength against the gravity load. All frames are lateral frames in SFS and they contribute to the lateral stiffness. When the building is designed in accordance with Japanese design standards with a higher lateral stiffness requirement, the steel volume in PFS is close to that in PFS. It may be true that concentrating additional steel in limited perimeter frames in PFS is more effective for the higher requirement of lateral stiffness.</p><p> </p><p> (3) The required lateral strength by ELF and the Japanese allowable stress design procedure is compared. Two building sites in California are selected for the comparison. One is Stockton, which is an FEMA P-1051 example location, and the other is Stanford, where the seismic load is almost the maximum in the US. The ratios of required lateral strength in the US, QUS, with respect to that in Japan, QJP, is defined as αQ (=QUS/QJP), and are calculated for moment-frame and braced-frame buildings with various natural periods. Assuming that the natural periods of braced-frame buildings are shorter than 1 second, then αQ can be as large as 1.4 in highly seismically active regions in the US. Contrarily, assuming that the natural periods of moment-frame buildings are higher than 1 second, then αQ is mostly less than 1, i.e., the required lateral strength of moment-frame buildings in the US is lower than that in Japan.</p>

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