耐震架構全体分散型と外周集約型の鋼構造事務所建物の保有水平耐力と地震応答

Abstract

There are two major types of lateral frame systems in steel buildings, which are space frame system (SFS) and perimeter frame system (PFS). Moment connections are used in most of beam-to-column connections in SFS, while they are limitedly used in the perimeter frames in PFS. In this study, structural characteristics of 7-story standard steel office buildings designed with SFS and PFS are investigated. The 7-story standard office buildings consist of moment frames in the longitudinal direction (X direction) and braced frames in the transverse direction (Y-direction). The column spacing is mostly 6.4m and is 12.8m for the long beams in frames in Y-direction. Superior design solutions are computed by using Multiple Start Local Search (MSLS) for the standard steel office buildings with these two different lateral frame systems, PFS and SFS. The superior design solutions are locally optimized solutions of a problem under constraints on allowable stress design and seismic ultimate lateral strength requirements. The allowable stress design requirements include constraints on stresses under long and short term loads, width-to-thickness ratios, column-to-beam strength ratios, inter-story drift under earthquake loads, and beam sagging under gravity load. The objective function is the total volume of steel and the discrete variables are the steel member sizes. Inelastic pushover analyses are conducted for the evaluation of the seismic ultimate lateral strength requirements. Time-history seismic response analyses are conducted for the obtained superior design solutions. The dynamic performance of the superior design solutions is evaluated. Structural characteristics of the design solutions with the two lateral systems, PFS and SFS, are observed as follows: (1) In both design solutions, it is observed that the constraints on inter-story drifts are critical for moment frames in the longitudinal direction. The constraints of column-to-beam strength ratios are more critical on the design with SFS. (2) The steel volume of the superior solutions with PFS is 13% smaller than that with SFS. A possible reason of this fact is that the column section sizes are controlled by the beam-to-column strength ratio and the long span beams mainly designed for the gravity load increases the column volume. (3) The steel volume of the superior solutions with the both lateral systems is smaller than statistical average of existing buildings. The MSLS design algorithm certainly reduces the steel volume. (4) Time-history seismic response analyses show that the superior solutions do not meet standard dynamic response criteria for new buildings. Therefore, required seismic performance by evaluations with dynamic time-history analyses is higher than that with static analyses, for the studied standard office building. (5) Time-history seismic responses are greater with PFS in both moment frames in X direction and braced frames in Y direction. However, the difference of the steel volume in the two systems can sufficiently compensate this disadvantage. In this research, general structural characteristics of the standard steel buildings designed with two different lateral systems, SFS and PFS, are compared, because the superior design solutions are obtained by the same optimization algorism but not based on engineers' personal experience.