Identification of building rotational modes using an ambient vibration technique Rocco Ditommaso, Marco Vona, Marco Mucciarelli, Angelo Masi Department of Structures, Geotechnics, Engineering Geology University of Basilicata, Potenza, Italy. ABSTRACT: Heavy structural damage on buildings subjected to seismic motion is frequently due to torsional effects. These effects have been extensively studied in the last years and incorporated in seismic codes. The standard approach for the experimental evaluation of such effects involves the installation of a multi-channel accelerometric system on buildings, either for continuous monitoring of earthquakes or to record forced vibrations. This time- and resource-consuming approach is usually applied to single buildings, but cannot be used in large scale studies in order to validate building code provisions or in quick estimations of building dynamic properties during vulnerability evaluations. In this paper we propose a simpler experimental set-up based on ambient vibration recordings, which is widely used for the identification of translational modes. To validate the methodology we compare our results with a full empirical and numerical experiment performed on a two-story steel frame. A single high-resolution seismometer proves to be able to identify at least the first two torsional frequencies, while with more instruments it is possible to identify also the mode shapes. Keywords: Structural Monitoring, Dynamic Identification, Rotational Modes, Ambient Vibration 1. INTRODUCTION Heavy structural damage on buildings subjected to seismic motion is frequently due to torsional effects. In fact, due to torsional response, an uneven distribution of lateral loads can result, which can increase damage at key points in a structure, particularly when subjected to strong earthquakes. Torsional effects can be caused by: - irregular configuration in plan and/or in elevation; - irregular distribution of masses and stiffnesses in plan and/or in elevation. The configuration of a structure can significantly affect its behaviour under seismic actions. Past earthquakes have frequently shown that buildings having an irregular configuration suffer higher damage than the regular ones. The evaluation of torsional effects is an important topic in modern seismic engineering, both for new (e.g. CEN, 2003; UBC, 2000) and existing buildings having poor seismic design (e.g. NZEES, 2006; CEN, 2004; OPCM, 2003). Among the basic principles governing the conceptual design of new buildings, some are relevant to building configuration and regularity, such as uniformity, symmetry, torsional resistance and stiffness. Specifically, uniformity in plan is achieved through an even distribution of the structural members, so that a short and direct transmission of the inertia forces created in the distributed masses of the building is allowed. Further, besides lateral resistance and stiffness, building structures should possess adequate torsional resistance and stiffness in order to limit the development of torsional motions which tend to stress the different structural members in a non- uniform way. Criteria for regularity in plan and in elevation are provided in Eurocode 8 (CEN, 2003), so that building structures can be categorized into being regular or non-regular. As regards plan regularity, an important condition is minimizing the distance between the center of mass, where horizontal seismic floor forces may be assumed to be concentrated, and the center of stiffness. This result can be achieved if lateral stiffness and mass distribution of the building structure are approximately symmetrical in plan with respect to two orthogonal axes. As regards elevation regularity, both the lateral stiffness and the mass of the individual story shall remain constant or reduce gradually, without abrupt changes, from the base to the top. The distinction between regular and non-