The objectives of this paper are (1) to identify the basic parameters which control the earthquake response of torsionally coupled systems composed of resisting elements providing force interaction during yielding; (2) to clarify differences in response among an elastic system (E), an elasto-plastic system without force interaction (EP), and an elasto-plastic system with force interaction (EPI), and (3) to evaluate effects of magnitude of eccentricity. A single-story structure with a rectangular deck and four resisting elements was used to examine these objectives. First, dimensionless equations of motion were formulated containing the basic parameters which control earthquake response and, then, parametric studies were carried out to determine the effects of such parameters on elasto-plastic coupled translational-torsional response. The principal conclusions of this study are the following : 1. The response spectra for EPI-systems are generally smooth functions of natural period T_x which is in contrast with the response spectra for E- and EP-systems which are quite irregular. 2. Force interaction effects do not always produce larger maximum displacements. Force interaction effects on inelastic response have a tendency to balance the two components of displacement response for those cases involving large plastic deformations. 3. Translational displacement responses at the center of mass of the EPI-systems, ν_x and ν_y, are insensitive to changes in the values of eccentricities, especially, for those systems having long uncoupled natural periods T_x; however, torsional response about the center of mass, ν_θ, increases almost linearly with increasing values of the eccentricities. 4. Torsional component of coupled translational-torsional response is very sensitive to characteristics of input ground motions. 5. Upper bound values of a torsional displacement to a translational one, ru_θ/u_x, are 0.25 and 0.45 for systems with values of eccentricities, e_y/r=0.2 and 0.4, respectively.