Modal Parameters for the Analysis of Inelastic Asymmetric-Plan Structures Jui-Liang Lin a) and Keh-Chyuan Tsai, b) M.EERI The conventional inelastic response spectra constructed by using the single-degree-of-freedom (SDOF) modal systems representing the relationship of roof translation versus base shear are short of the capability to accurately estimate rotational seismic demands for asymmetric-plan structures. This paper illustrates the inelastic response spectra for one-way asymmetric-plan structures constructed from the two-degrees-of-freedom (2DOF) modal systems. The 2DOF modal system can simultaneously represent the roof translation versus base shear and the roof rotation versus base torque relationships for each vibration mode of any one-way asymmetric- plan structure. Thus, the proposed inelastic response spectra are able to estimate not only the translational but also the rotational seismic demands. This new type of response spectra is named as the T-R response spectra. In order to construct the T-R inelastic response spectra, this research identifies three independent elastic 2DOF modal parameters: vibration period, T n , frequency ratio,  n , and modal eccentricity, e n . The relationships between the inelastic 2DOF modal parameters and the strength ratio are established in this study. The ranges of the 2DOF modal parameter values are investigated for the general one-story, one-way, asymmetric-plan buildings. The translational and rotational constant-strength response spectra constructed from the 2DOF modal systems are illustrated. This paper demonstrates that the proposed inelastic response spectra are more suitable for computing the seismic demands of one-way asymmetric-plan structures than the conventional SDOF inelastic response spectra. DOI: 10.1193/1.3238566 INTRODUCTION The conventional response spectra constructed from the inelastic single-degree-of- freedom (SDOF) systems have been extensively studied (Newmark and Hall 1973, Nas- sar and Krawinkler 1991, Vidic et al. 1992, Miranda and Bertero 1994) and widely ap- plied in engineering practice. The force-deformation relationships of the inelastic SDOF system represent the roof translation versus base shear relationship of a physical multi- degree-of-freedom (MDOF) building (Shibata and Sozen 1976, Priestly et al. 1996). For inelastic structures, the vibration “modes” may be viewed as varying from time to time. Nevertheless, converting the inelastic force-deformation relationship of roof translation a) Associate Research Fellow, National Center for Research on Earthquake Engineering, 200, Section 3, XinHai Rd., Taipei 106, Taiwan, R.O.C. E-mail: jllin@ncree.org. b) Director, National Center for Research on Earthquake Engineering 200, Section 3, Xin Hai Road, Taipei 106, Taiwan, R.O.C. E-mail: kctsai@ncree.org. and Professor, Department of Civil Engineering, National Taiwan University, Taipei, Taiwan, R.O.C. 821 Earthquake Spectra, Volume 25, No. 4, pages 821–849, November 2009; © 2009, Earthquake Engineering Research Institute