M.A.Sc. Thesis / Most conventional seismic design intends for key structural members to yield in order to limit seismic forces, leading to structural damage after a major earthquake. To minimize this structural damage, self-centering systems are being developed. But how to estimate the peak seismic displacement of a self-centering system remains a problem for practical design. This thesis addresses this need by presenting a parametric study on the seismic displacement demands of single-degree-of-freedom (SDOF) systems with flag-shaped hysteresis considering 13,440,000 nonlinear time history analyses. Ground motion records that represent seismic hazards in active seismic regions with stiff soil and rock site conditions are used. The influences of the four independent parameters that define a flag-shaped hysteresis are presented in terms of median displacement ratios, facilitating the design-level estimation of nonlinear displacement demands on self-centering systems from the spectra displacements of elastic systems. The influence of initial period on self-centering systems is similar to its influence on traditional systems with elastoplastic hysteresis, but a much lower linear limit can be adopted for self-centering systems while achieving acceptable peak displacements. Supplemental energy dissipation suppresses the peak displacement but additional energy dissipation becomes less effective as more is added. The effect of nonlinear stiffness is small as long as it is positive and close to zero, but a negative nonlinear stiffness can lead to unstable response. Self-centering systems located on rock sites usually have smaller displacement demands than those on stiff soil sites. When the damping ratio is increased or decreased, the displacement ratios do not necessarily decrease or increase consistently. A tangent stiffness proportional damping model is considered, leading to a significant increase in displacement demands but similar overall trends. Based on the observations, regression analysis is used to develop a simplified equation that approximates the median inelastic displacement ratios of self-centering systems for design. / Thesis / Master of Applied Science (MASc)

Concrete filled steel tubes are regarded as ideal frame members in seismic resisting systems, as they combine large axial and flexural capacity with ductility. The combination of the two materials increases the strength of the confined concrete and avoids premature local buckling of the steel tube. These benefits are more prominent for circular than for rectangular concrete filled steel tubes. However, most common connection configurations for circular concrete filled tubes are not economic in the US market due to (a) the desire of designers to use only fully restrained connections and its associated (b) high cost of fabrication and field welding. Research indicates that well designed partially restrained connections can supply equal or even better cyclic behavior. Partially restrained connections also possess potential capability to develop self-centering system, which has many merits in seismic design. The goal of this research is to develop a new connection configuration between circular concrete filled steel columns and conventional W steel beams. The new connection configuration is intended to provide another option for rapid assembling on site with low erection costs. The proposed connection is based on an extended stiffened end plate that utilizes through rods. The rods are a combination of conventional steel and shape memory alloy that provide both energy dissipation and self-centering capacity. The new connection configuration should be workable for large beam sizes and can be easily expanded to a biaxial bending moment connection. / Ph. D.

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