Verification of Design Forces Used in Seismic Design of Buildings By The Equivalent Lateral Force Method

The equivalent lateral force (ELF) method is widely accepted and deemed safe for seismic design of regular buildings; however, dynamic analysis methods such as the Response Spectrum Analysis (RSA) are expected to yield more accurate results as it can explicitly consider the effects of higher modes of vibration. The Thai seismic design standard (DPT 1301/1302-61) and ASCE 7-22 permit the use of the ELF method for analysis and design of all buildings in seismic design category (SDC) C regardless of building heights and irregularities. There is doubt whether dynamic analysis should be required when the building is tall or irregular. This research therefore aims to evaluate adequacy of the ELF method when it is applied to regular tall buildings, and torsionally irregular buildings in SDC C located in Bangkok and tall regular buildings in SDC D located in Chiang Mai. A total of 21 buildings were studied: 7 regular and 7 torsionally irregular buildings in Bangkok, and 7 regular buildings in Chiang Mai, ranging from 6 to 42 stories. All buildings employ special reinforced concrete shear wall systems. The study further investigates whether shear force demands for design of structural walls amplified by factors in ACI 318-19 are adequate when compared to the nonlinear response history analysis (NLRHA) results. The results show that ELF tends to slightly overestimate floor displacements and story drifts. ELF becomes inadequate in estimating story shear forces in tall buildings in Chaing Mai near the base and top of buildings, and in torsionally irregular buildings in Bangkok, particularly at core wall farthest from center of mass. RSA can provide a more reliable estimation of structural responses when compared to those from NLRHA. Furthermore, the ELF method can not well capture the variation of force demands along the building height.