EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS Earthquake Engng Struct. Dyn. 2010; 39:397–417 Published online 10 August 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/eqe.946 Initial stiffness of reinforced concrete structural walls with irregular openings Bing Li ∗, † and Qin Chen School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore SUMMARY Reinforced concrete (RC) structural walls with openings are the primary lateral-load-carrying elements utilized in many structures designed to resist earthquakes. A review of the technical literature shows that there is a considerable amount of uncertainty with regards to the elastic stiffness of these structures when subjected to seismic excitations. Current design practices often employ a stiffness reduction factor to deal with this uncertainty. In an attempt to develop additional information regarding the stiffness of these structures, this paper discusses an approach to determine the initial stiffness of RC structural walls with irregular openings and low aspect ratios. This approach would consider the effect of both flexural and shear deformations. As a part of this study, an analytical approach to determine stiffness was also developed and validated by comparing theoretical and experimental results obtained from six RC shear walls with irregular openings. Simple equations for assessing initial stiffness of RC structural walls with irregular openings are then proposed, based on these parametric case studies. Copyright 2009 John Wiley & Sons, Ltd. Received 31 January 2009; Revised 16 June 2009; Accepted 23 June 2009 KEY WORDS: wall with irregular openings; initial stiffness; flexural stiffness; shear stiffness; stiffness ratio 1. INTRODUCTION The past 15 years have seen major developments in seismic design provisions, with a paradigm shift from a force-based approach to displacement-based one and an increasing in focus on the deformation characteristics of structures. A pre-requisite to this approach would be to determine the actual displacement ductility, displacement and period of vibration of individual members and the structural system as a whole. Accurate assessment of the initial stiffness thus becomes an inevitable requirement. Various researchers have investigated the initial stiffness of reinforced ∗ Correspondence to: Bing Li, School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore. † E-mail: cbli@ntu.edu.sg Copyright 2009 John Wiley & Sons, Ltd.