This report describes a practical analytical model that can be used for the seismic 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 OOP capacities. 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 small displacement 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 Un iversity of California, Berkeley. Some of the previous work is described, including a previously proposed st rut 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 the OOP 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 relationship is 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. The performance 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 is subjected 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. These functions consider only the effect of record-to-record variability. Finally, the strength variability of the URM infill walls is considered using a first-order, second-moment (FOSM) analys is, and it is shown that the effect of this strength variability is minor compared with that of record-to-record variability. The effect of disregarding the interaction between IP and OOP strength, which is a common design practice, is discussed. The fragility functions produced in this report are found to give failure probabilities lower than those determined by earlier work in the research program, which assumed failure based on IP and OOP elastic forces rather
than inelastic deformations. Conclusions and suggestions for further investigations are given at the end of the report.
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