Seismic analysis of multi-story unreinforced masonry buildings with flexible diaphragms
Aleman Hernandez, Juan
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Studies regarding the regional seismicity of New York City (NYC) indicate that earthquakes of magnitude greater than or equal to 5 have a 20-40% probability of occurring in a 50 year period. Considering that more than 50% of the large population of the region (19.1 million) is living in an area where 80% of the buildings are of old Unreinforced Masonry (URM) construction, it is likely that even a moderate earthquake could have critical consequences on public safety and the economy of this area. The main goal of this study was to develop simulation capabilities supported by experimental testing to evaluate the seismic performance of typical unreinforced masonry buildings with flexible diaphragms located in NYC. Simplified nonlinear macro-models of wood diaphragms, masonry walls and floor-to-wall connections were developed and validated individually using data from past experiments. The models are implemented in both commercial and research software to demonstrate their practical use. In particular, a new approach for modeling flexible diaphragms is proposed that is shown to provide similar accuracy to detailed finite element models at a fraction of the computation cost. The models developed were also validated as at the system level, through shake table testing of two full-scale specimens conducted as part of this study. The specimens were designed and constructed to represent the expected loading conditions of a central portion of a one-story URM building and constructed of materials representative of older masonry construction. The tests provide a unique data set that captures the interaction between flexible floors, out-of-plane walls and their connections at, full scale. The validated macro-models can be used for performance-based seismic assessment of unreinforced masonry building in NYC and elsewhere. To demonstrate their use, preliminary studies were conducted to develop out-of-plane URM fragility curves, a building-specific collapse fragility function, and estimate the seismic response of the building when subjected to a ground shaking intensity similar to the 2011 Virginia Earthquake. The probabilistic framework and new performance definitions provided by the FEMA P58 project were used to conduct a performance-based seismic assessment for an archetype unreinforced masonry building in New York City with an emphasis on out-of-plane behavior. This preliminary study served to demonstrate the applicability of the proposed models for applications in performance-based design.