Blast resistant analysis and design of historic structures can be a challenging design question. Historic construction materials and methods must be considered and analyzed for abnormal loading scenarios which were never part of the original design intent. A very common material in historic structures is unreinforced masonry. When an external explosion imposes an air-blast load on a heavy, external wall system, the air-blast pressure must overcome the inertial forces associated with the mass of the wall itself before putting the system into motion, thus causing damage. In the case of thick, unreinforced masonry walls, the inertia forces of the walls are very large and the resulting damage predictions are therefore less than one would intuitively imagine. This paper presents lessons learned from the analysis and design of mitigation solutions for a historic building with unreinforced masonry infill walls which was retrofitted to meet GSA design criteria for air-blast loads. Retrofit solutions considered and discussed here are evaluated based on a number of considerations, which include structural configuration, safety, cost and ease of construction. A progressive design approach is taken to analyze the wall systems looking at multiple failure modes. Results of testing and actual explosive events which support the analysis will also be discussed.
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