SUPERIOR DESIGN SOLUTIONS OF SECTION SIZES IN STEEL BUILDINGS FOR DIFFERENT LATERAL FRAME SYSTEMS AND COLUMN SHAPES

Abstract

 Superior design solutions of section sizes in seven-story steel buildings are obtained for three types of structural systems: (1) a space frame system with rectangular HSS columns (SFS), (2) perimeter frame systems (PFS) with I-shaped columns (PFSH), (3) PFS with rectangular HSS columns (PFSB). Moment connections are used in most beam-to-column connections in SFS, while they are limitedly used in the perimeter frames in PFS. SFS is a commonly used structural system in Japan, whereas PFSH is commonly used in other countries. In this research, structural characteristics of SFS, PFSH and additionally PFSB are evaluated for evenly rationally designed office buildings using an optimization algorithm. The superior solutions are derived by multiple start local search (MSLS), minimizing steel volumes. The solutions satisfy multiple requirements of the allowable stress design and ultimate lateral strength. The discrete design variables are the section sizes of grouped structural members. Approximately 100 constraints and 40 variables are applied. Dealing with these large numbers, the proposed MSLS algorithm works and superior solutions are obtained for various types of buildings, such as moment frame, braced frame and mixed frame buildings, in the three types of structural systems, SFS, PFSH and PFSB. Pipes or buckling restrained braces (BRB) are used in the braced frame buildings. The findings are as follows:

 

 (1) Superior solutions for moment frame buildings are obtained for the base-shear coefficient of the ultimate lateral strength, C_QUN1, as 0.3 and 0.6. Although the value of 0.6 for C_QUN1 is given by referring to responses in the time-history analyses for very rare (L2) earthquake ground motions, the superior design solutions do not satisfy the standard design criteria against L2 earthquakes. The maximum inter-story drift ratios are 1.4-1.5%, which are greater than the standard criteria of 1.0%. PFSH can be advantageous for the moment frame building in terms of steel volume.

 (2) Superior solutions of the braced frame building are obtained for 0.35 and 1.0 of C_QUN1. The sections of the braces are steel pipes. The differences of steel volumes between PFSH, SFS and PFSB are relatively small. The steel volume is slightly lower in SFS and PFSB, because axial forces are the primarily derived member forces under earthquake lateral load in these braced frame buildings and rectangular HSS columns are advantageous. The C_QUN1 value needed for the L2 time-history analysis is nearly 1.0, which is very different from the 0.35 required by the design standard.

 (3) Superior design solutions using BRBs are obtained for the braced frame building with 0.5 of C_QUN1. The steel volume in SFS and PFSB is slightly lower, as observed in the braced frame building with pipes. The steel volume excluding the braces in the superior solutions with BRBs is 70-90% of that with pipes for the braces (C_QUN1 =1.0).

 (4) A comparison of the superior design solutions for mixed structures shows that the steel volume of SFS solution is higher than those of PFSs. Irregularity in the beam spans or different lateral systems in two horizontal directions causes an increase in the steel volume in SFS, controlled by some critical constraints, such as uniform beam height in a single floor and strong column weak beam ratio.

 

 Superior design solutions are obtained by using the optimization algorithm but not based on engineers’ personal experience. Therefore, although the number of cases studied in this research is limited, the discussion and findings comparing these different structural systems are of interest.