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Seismic Evaluation and Rehabilitation of Low-Rise Reinforced Masonry Buildings with Flexible Diaphrams

Cohen, Gregory L.

2004

In response to Executive Order 12941 (1994), the United States government initiated a coordinated effort to assess and mitigate the seismic hazards of its existing owned and leased facilities. This study was contracted to enhance that effort for low-rise reinforced masonry buildings with flexible diaphragms. The study involved the development of systematic methodologies for the seismic evaluation and rehabilitation of such buildings.

First, the seismic behavior of these types of buildings was characterized. Two, half-scale low-rise reinforced masonry buildings with flexible diaphragms were tested on the US Army Tri-axial Earthquake and Shock Simulator at the US Army Engineer Research and Development Center, Construction Engineering Research Laboratory. This testing provided experimental data for analytical modeling, and provided specific technical substantiation for the generally accepted premise that diaphragm flexibility can significantly affect the seismic response of low-rise buildings. Following that testing, diaphragms and attached masonry chords were removed from the shaking-table specimens and subjected to reversed cyclic quasi-static displacements.

Observations and conclusions from physical testing were used to develop and validate a simple tool for the analysis of these buildings. The tool was developed in the general case and then analytically bounded for the particular case of low-rise reinforced masonry buildings with flexible diaphragms. It was validated in the linear elastic and nonlinear ranges using data from shaking-table testing, finite-element modeling, and lumped-parameter modeling. Data from previous flexible-diaphragm tests, performed by others, were reevaluated in the context of performance-based engineering and synthesized with the analysis tool into a coherent evaluation methodology intended to supplement the existing methodologies of Federal Emergency Management Agency (FEMA) Documents 310 and 356.

To assess the efficacy of the proposed methodology, four existing buildings were selected from the Ft. Lewis, Washington inventory and evaluated for seismic deficiencies. The buildings were evaluated using procedures of FEMA 310, US Army TI 809-05, and the proposed methodology. Results conclusively demonstrated that the FEMA 310 methodology does not sufficiently characterize the seismic performance of flexible diaphragm systems, and the proposed methodology is simple, effective, and useful. Recommendations are placed in the context of evolutionary updating of the FEMA methodologies, as applied to specific subsets of the national building inventory.

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