Seismic performance of rehabilitated wood diaphragms

The objective of this research was to experimentally and analytically investigate the behavior of existing rehabilitated floor and roof wood diaphragms under in-plane lateral loads. The study focused on wood diaphragms characteristic of pre-1950's unreinforced masonry (URM) buildings found in the central and eastern regions of the United States.; Three diaphragm experimental specimens were built, tested, retrofitted and re-tested again under quasi-static reversed cyclic loading. The specimens differed in the type of sheathing, nailing pattern and the framing orientation with respect to the loading. One specimen represented a floor diaphragm, while the other two were more typical roof diaphragms, with one having a corner opening. A total of four retrofit methods were used on the specimens: (1) steel perimeter strapping and enhanced bolted connections; (2) a steel truss with enhanced bolted connections; (3) an unblocked plywood overlay and (4) a blocked plywood overlay. The in-plane lateral responses measured for the diaphragms were used to develop backbone curves, which were compared with those given in the FEMA 273 and FEMA 356 guidelines for seismic rehabilitation and some differences were found.; An analytical study of the diaphragm specimens was undertaken using two-dimensional finite element models. Inelastic quasi-static analyses were used to calibrate nailed connection slip behavior, which involves selecting hysteretic parameters for strength deterioration, stiffness degradation and pinching, to correlate with the experimentally measured diaphragm behavior under lateral loads. The analysis was extended to predict the lateral in-plane response for unretrofitted and retrofitted diaphragms of varying aspect ratios.; The experimental testing showed that all the retrofit methods provided an increase in strength and stiffness in varying degrees. The steel truss provided the most significant increase in strength and stiffness, followed by the blocked plywood overlay. The analytical data indicated that the FEMA 273 guidelines underpredict yield shear strength and overpredict stiffness, while the FEMA 356 guidelines underpredict the yield shear strength and stiffness of the diaphragm response. The results of the experimental and analytical studies were used to develop a proposed change to some of the parameters used to construct backbone curves for similar wood diaphragms according to the FEMA 356 guidelines.