Seismic Behavior of Wood Diaphragms in Pre-1950s Unreinforced Masonry Buildings

This paper documents an experimental testing program on the lateral in-plane behavior of pre-1950s existing and rehabilitated wood floor and roof diaphragms in unreinforced masonry buildings found in the Central and Eastern regions of the United States. Three diaphragm specimens were constructed with elements and connection details typical of pre-1950s construction. The specimens were tested, retrofitted, and retested again using different rehabilitation methods, including enhanced shear connectors and perimeter strapping, a steel truss attached to the bottom of the joists and connected to the vertical lateral force resisting system, and unblocked and blocked plywood overlays connected to the sheathing and joists. Specimens were tested under quasi-static reversed cyclic loading to evaluate their in-plane lateral deformation performance at selected locations of the diaphragm. The measured in-plane lateral response was used to develop backbone curves defining the relationship between the applied lateral force and the diaphragm midspan displacement. These backbone curves provide the basis for bilinear curves that define yield strength and displacement, effective stiffness, and post-yield stiffness. These parameters, based on experimental testing, were compared with the provisions for wood diaphragms in the FEMA guidelines for seismic rehabilitation of buildings (FEMA 273 and its update FEMA 356). For the diaphragms tested, FEMA 273 tended to overpredict the stiffness and significantly underpredict yield displacement and ultimate deformation levels, while FEMA 356 tended to underpredict stiffness and overpredict yield displacement. However, the updated FEMA 356 guidelines tend to conservatively estimate the diaphragm response in terms of strength, stiffness, and deformability.