EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS Earthquake Engng Struct. Dyn. 2007; 36:1065–1087 Published online 13 February 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/eqe.672 Pseudodynamic response of torsionally unbalanced two-storey test structure S. N. Bousias ∗, † , M. N. Fardis, A.-L. Spathis and A. J. Kosmopoulos Department of Civil Engineering, University of Patras, Patras, Greece SUMMARY Two one-way eccentric, two-storey, one-by-one-bay reinforced concrete (RC) structures are pseudodynam- ically tested under unidirectional ground motions. Theoretical considerations about the effect of torsional coupling on modal periods and shapes agree with modal results of the test structure, considering member stiffness is equal to the secant stiffness to yielding in skew-symmetric bending. Modal periods of such an elastic structure are in fair agreement with effective periods inferred from the measured response at the beginning of a test of a thoroughly cracked structure and at the end of the test. A time-varying stiffness matrix and a non-proportional damping matrix fitted to the test results may be used to reproduce the measured response approximately by modal superposition and identify the role of the four time-varying modes. Flexible side columns sustained very large drift demands simultaneously in the two transverse directions and suffered significant but not heavy, damage at lap-splices. RC-jacketing of the flexible side columns practically eliminated the static eccentricity between the floor centres of twist and mass as well as the torsional response. Inelastic time-history analysis with point-hinge member models, using as elastic stiffness the secant stiffness to yielding and neglecting post-ultimate-strength cyclic degradation of resistance in members with plain bars and poor detailing, predicted fairly well the response until the peak displacements and member deformations occurred. After that, it underestimated displacement peaks and the lengthening of the apparent period and missed the gradual drifting of the response towards a permanent offset. Copyright 2007 John Wiley & Sons, Ltd. Received 9 May 2006; Revised 16 December 2006; Accepted 18 December 2006 KEY WORDS: asymmetric buildings; inelastic seismic response; irregular buildings; pseudodynamic tests; torsional seismic response ∗ Correspondence to: S. N. Bousias, Department of Civil Engineering, University of Patras, Patras, Greece. † E-mail: sbousias@upatras.gr Contract/grant sponsor: NATO Contract/grant sponsor: European Commission Contract/grant sponsor: Greek General Secretariat for Research and Technology Copyright 2007 John Wiley & Sons, Ltd.