Behaviour Factor for seismic design of moment-resisting steel frames M. Ferraioli, A. Lavino & A. Mandara Department of Civil Engineering, Second University of Naples SUMMARY Existing seismic codes are based on force-controlled design or capacity design, using the base shear concept. The most important parameter in this approach is the behaviour q-factor, which is based on the maximum capacity of structure to dissipate energy during the plastic deformations corresponding to ultimate limit state criterion. In this paper, the existing methods for determining the behaviour factor of moment-resisting steel frames are reviewed for both regular and irregular in elevation multi-storey moment-resisting steel frames. The effects of storeys, spans and regularity in elevation of frames on the behaviour factor were considered. Keywords: Moment resisting frames, behaviour factor, nonlinear analysis. 1. INTRODUCTION The main limit of traditional force-based design approach is that the performance cannot be predicted because the seismic behaviour of the structure is governed by phenomena which are not adequately captured in the simple design process. In fact, during the occurrence of an earthquake ground motion it is possible for steel moment-resisting frames to enter a region of non-linear behaviour. However, the nonlinear dynamic analysis requires step-by-step integration, which is very time consuming. As a consequence, the current design codes calculate the design force to be used for elastic analysis of structures from spectra based on linear behaviour together with the use of a behaviour factor that accounts approximately for the non-linear effects. In other words, an approximate inelastic spectrum is defined for specifying design actions of structures which are expected to respond inelastically to the design earthquake. Even if the distribution of forces resulting from such analyses may have little similarity to that expected during the actual earthquake, the concept of a factor used in design to reduce forces is adopted by most seismic codes in order to account for the nonlinear response of the structure associated with the material, the structural system and the design procedures. This factor is called behaviour factor (q-factor) in the European Code (2004) and response modification factor (R) in the American Codes (Uniform Building Code (1997), NEHRP Provisions (2003)). In SEOAC Guidelines (1999) R is termed “the structural quality factor” or “the system performance factor”. Some considerable differences in the numerical values of the behaviour factors specified in various codes for the same type of structure may be found. These discrepancies also derive from different partial safety factors used in each code for material resistances and applied loads. 2. FORCE REDUCTION FACTORS FOR SEISMIC DESIGN Although the inelastic spectra are rigorously applicable to the inelastic behaviour of single-degree-of- freedom (SDOF) systems, these spectra are usually applied with satisfactory accuracy to multi-degree- of-freedom (MDOF) structures. It is also possible to use the design spectra - that represents a scaled down form of elastic pseudo-acceleration spectrum - to approximately estimate the response of inelastic MDOF systems. In other words, most seismic codes use the concept of design response spectrum defined by dividing the elastic response spectrum through a reduction factor. The ductility-