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Modeling of Unreinforced Masonry Infill Walls Considering In-Plane and Out-of-Plane Interaction

Stephen Kadysiewski Khalid M. Mosalam University of California, Berkeley

PEER 2008/102 january 2009

Modeling of Unreinforced Masonry Infill Walls Considering In-Plane and Out-of-Plane Interaction

Stephen Kadysiewski Department of Civil andEnvironmental Engineering University of California, Berkeley Khalid M. Mosalam Department of Civil and Environmental Engineering University of California, Berkeley

PEER Report 2008/102 Pacific Earthquake Engineering Research Center College of Engineering University of California, Berkeley January 2009

This report describes a practical analytical model that can be used for theseismic evaluation of unreinforced masonry (URM) infill walls located within a reinforced concrete (RC) frame. The model, which consists of diagonal beam-column members utilizing fiber element cross sections, is suitable for use in a nonlinear time history analysis. The model considers both the in-plane (IP) and out-of-plane (OOP) response of the infill, as well as the interaction between IP and OOPcapacities. The behavior is elastoplastic, and limit states may be defined by deformations or ductilities in the two directions. These limit states may be chosen to conform to various codes and guidelines, or they may be developed independently by the engineer. The model is composed of elements that are available in commonly used structural analysis software programs, and is based on smalldisplacement theory, so it is rather straightforward to implement. For each infill wall panel modeled, one additional degree of freedom and two beam-column members are added to the overall structural model. This report is part of a larger research program of investigation into RC frames with URM infill, carried out in recent years at the University of California, Berkeley. Some of the previous work isdescribed, including a previously proposed strut and tie (SAT) model. The behavior of that SAT model is investigated, and it is found that under certain circumstances, problematic issues are encountered. The newly proposed infill wall model is idealized as a single diagonal beam-column member, composed of two beam-column elements, with a node at the midspan. The midspan node is assigned a mass in theOOP direction to account for the inertial forces in that direction. The beam-column elements used in this report are force-based elements with inelastic behavior concentrated at the hinge regions. These regions are modeled using inelastic fibers, whose strength and locations are calculated to produce the desired IP-OOP strength interaction relationship for the panel. The interaction relationshipis based on previous work conducted in an earlier phase of the research program. Additionally, the elastic stiffness and area of the fibers are determined such that the IP and OOP elastic dynamic properties of the infill panel and the overall model strength properties are preserved. The performance of a simple one-panel model is demonstrated using static pushover and dynamic analyses. Theperformance of the model is


shown to be generally satisfactory. However, possible limitations and drawbacks of the model are discussed, as well as possible improvements. The proposed infill model is incorporated into a larger five-story model of a RC moment frame building with URM infill walls. The building model is the same as that used for earlier investigations in the research program. It issubjected to 20 sets of ground acceleration time histories, at five different levels of spectral acceleration. Collapse of the infill panel is assumed to occur at critical displacement ductilities in the IP and OOP directions, with interaction between the ductilities considered. Fragility functions, giving the probability of collapse as a function of spectral acceleration level, are calculated....
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