INTENSITY MEASURES FOR THE COLLAPSE ASSESSMENT OF INFILLED RC FRAMES Gerard J. O’REILLY 1 , Mohsen KOHRANGI 2 , Paolo BAZZURRO 3 , Ricardo MONTEIRO 4 ABSTRACT A ground motion intensity measure (IM) represents the interface between the ground shaking intensity and structural response. An ideal IM should be efficient, sufficient and practical. The first aspect, which is the focus of this paper, encompasses the choice of an IM that exhibits an adequately low dispersion in structural response prediction. The presence of masonry infills in RC frame buildings is well-recognised as a critical aspect that significantly modifies the RC frames’ behaviour, especially in old buildings with no seismic design provisions. This renders common IMs such as spectral acceleration at the first mode period of structures, Sa(T1), somewhat inefficient because of the abrupt changes in stiffness of these structures, and thus of T1, due to a sudden brittle break of the infills during shaking. This paper addresses the collapse assessment of gravity load designed (GLD) infilled RC frames by exploring different IMs based on single spectral ordinate, Sa(T), or multiple spectral accelerations averaged in a period range, AvgSa. As such, a pool of candidate IMs are selected and evaluated to address efficient collapse assessment of these buildings. Furthermore, a discussion about the choice of building specific and generic IMs for portfolio seismic assessments is provided. Keywords: collapse; assessment; intensity measure; dispersion 1. INTRODUCTION The seismic assessment of existing reinforced concrete (RC) frames with masonry infills has been the focus of much research in recent years. In particular, buildings designed before the introduction of adequate design codes in the Mediterranean area, often referred to as gravity load designed (GLD) RC frames, have received considerable attention (e.g., Kohrangi et al. 2016; O’Reilly and Sullivan 2017b) given their common presence in the existing building stock and high vulnerability. This is owing to the fact that these GLD RC frames were designed prior to the introduction of seismic design force provisions and whose members were sized for vertical loading only. This was often done with no consideration of what are now well-established concepts in seismic design, such as capacity design and adequate core confinement in RC members to ensure ductile response. While the structural performance of GLD RC frames is well known, some issues relating to the numerical modelling are still under development. Recent work (O’Reilly and Sullivan 2017a) has proposed a numerical modelling approach for GLD RC frame members with low levels of reinforcement and smooth reinforcing bars in addition to beam-column joints with no transverse shear reinforcement and end- hook anchorage. These details were quite common practice in Italy prior to the 1970s. The modelling parameters for the beam-column members and joints were calibrated using experimental test data available in the literature. O’Reilly and Sullivan (2017a) have also shown how the proposed approach captured the hysteretic behaviour, deformed shape and damage mechanism, whereas more traditional modelling techniques were shown not to be representative for any of these. The development of improved intensity measures (IMs) for seismic design and assessment of structures has been the recently focus of a large body of research. Some of the typical IMs used in 1 Research Assistant, Scuola Universitaria Superiore IUSS Pavia, Italy. gerard.oreilly@iusspavia.it 2 Research Assistant, Scuola Universitaria Superiore IUSS Pavia, Italy. mohsen.kohrangi@gmail.com 3 Professor, Scuola Universitaria Superiore IUSS Pavia, Italy. paolo.bazzurro@iusspavia.it 4 Assistant Professor, Scuola Universitaria Superiore IUSS Pavia, Italy. ricardo.monteiro@iusspavia.it