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Seismic retrofit of masonry slabs in existing steel or masonry buildings has found special significance in current codes as failure of unstable jack arch slabs has been reported as a major reason for collapsing structures in Middle East deadly earthquakes. In this paper, three retrofit schemes are investigated and compared. The proposed rehabilitation techniques consist of a single X strapping, SXS, a double X strapping, DXS, and a two-way jack arch slab supported by a steel grid. Using experimental studies, advantages and disadvantages of each scheme are evaluated. In-plane stiffness and capacity of the diaphragm are adopted as the seismic performance index of each rehabilitation scheme. According to the obtained results, the jack arch slab systems designed and constructed based on proposed retrofit methods provide an appropriate alternative to other forms of flooring in seismic zones. DXS can greatly improve diaphragm performance in terms of in-plane stiffness, capacity and even energy dissipation of the diaphragm compared with the other two techniques. The second place belongs to SXS while the steel grid scheme has a minor effect on the in-plane stiffness of the diaphragm.

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To study seismic performance of concrete-encased composite columns with T-shaped steel cross-section, twelve half-scale columns were tested under quasi-stastic cyclic loading. The result indicates that concrete-encased composite columns with T-shaped steel section possess good seismic performance. The failure modes include bending failure, shear-bond failure, shear compression failure and shear-composition failure. Unsymmetrical phenomenon of positive and negative hysteresis loop was shown evidently. Span ratio has a great influence on failure mode. The ductility performance decreases with increasing of axial compression level. As stirrup ratio increases, ductility and bearing capacity of columns are improved greatly, and energy dissipation capacity after yielding is enhanced. Cross tie can enhance ultimate bearing capacity, and lower strength attenuation and stiffness degradation on the later loading stage

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This paper presents a comparative numerical research on the overall seismic behavior of RC frames with different types of rebars (normal versus high strength rebar). A nonlinear numerical model is developed and is validated using experimental results. Comparing the numerical and experimental behaviors shows that the developed model is capable of describing the hysteretic behavior and plastic hinges development of the experimental RC frames with various strength longitudinal steel bars. The validated model is then used, considering the influences of axial load ratios and volumetric ratios of longitudinal rebars of column, to investigate the effects of reinforcement strength on the overall seismic behavior of RC frames. The simulation results indicate that utilizing high strength reinforcement can improve the structural resilience, reduce residual deformation and achieve favorable distribution pattern of plastic hinges on beams and columns. The frames reinforced with normal and high strength steel bars have comparable overall deformation capacity. The effect of axial load ratio on the energy dissipation, hysteretic curves and ultimate lateral load of frames with different strength rebars is similar. In addition, increasing the volumetric ratios of longitudinal rebars can increase the ultimate lateral load of frame and improve the plastic hinge distribution of frame.