The 14 th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China MODELLING OF THE HORIZONTAL SLAB OF A 3D IRREGULAR BUILDING FOR NONLINEAR STATIC ASSESSMENT R. Pinho 1 , C. Bhatt 2 , S. Antoniou 3 , R. Bento 4 1 Assistant Professor, Structural Mechanics Department, University of Pavia, Italy 2 Doctoral Student, Instituto Superior Técnico, Technical University of Lisbon, Portugal 3 Managing and R&D Director, SeismoSoft, Chalkida, Greece 4 Associate Professor, Instituto Superior Técnico, Technical University of Lisbon, Portugal 2 Email: rui.pinho@unipv.it, cbhatt@civil.ist.utl.pt, s.antoniou@seismosoft.com, rbento@civil.ist.utl.pt ABSTRACT The manner in which the horizontal slab in 3D buildings, or better the rigid diaphragm effect, is modeled may influence the response of the structure when subjected to seismic action. In this work different ways of modelling the rigid diaphragm effect are tested and compared, through the application to the irregular 3D SPEAR building, a full-scale specimen tested under pseudo-dynamic conditions at JRC Ispra, representing typical old three-storey buildings built in the Mediterranean region in the early 1970s. The dynamic properties of the different models (periods, modes of vibration and effective modal mass percentages) are analysed, and the corresponding interaction with seismic action is studied by means of nonlinear static and dynamic analyses; different response measures, such as capacity curves, interstorey drifts and displacements are evaluated for the two orthogonal directions. The results of this study show that the most accurate and reliable way of modelling the floor’s behaviour is the Rigid Diaphragm with Lagrange Multipliers nodal constraints model. KEYWORDS: numerical simulation, diaphragm effect, nodal constrains, 3D irregular RC building, seismic response 1. INTRODUCTION Typically, in reinforced concrete buildings floors usually exhibit a very large in-plane stiffness when compared with the lateral stiffness of vertical elements. Hence, such slabs may be modelled assuming a rigid diaphragm behaviour in plan, i.e. a rigid body motion of the floor without deformation in their own plane. There are many techniques that can be used to model such rigid diaphragm effect. In this study, several solutions are therefore tested, and their results are then compared, in terms of floors displacements, interstory drifts and capacity curves in both directions. The structure under analysis is the SPEAR building, designed and tested under the framework of the European project SPEAR - Seismic Performance Assessment and Rehabilitation (Fardis and Negro, 2006). This structure is intended to represent typical existing three-storey buildings in the Mediterranean region, and as such was designed for gravity loads only, following the concrete design code implemented in Greece between 1954 and 1995, with the construction practice and materials used in Greece in the early 1970s, including smooth