Seismic design of asymmetric ductile systems.
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The research promotes a better understanding of the response of torsionally unrestrained and restrained ductile systems by examining the mechanism developed during the torsional response of systems as they are affected by the dynamic actions of the translational and rotational mass. A simple but effective design strategy for the seismic design of torsionally asymmetric systems is suggested based on the estimate of the system displacement ductility capacity and the distribution of the estimated system strength to its elements. The strength eccentricity is considered the main parameter to influence the ductile response of asymmetric systems. The possible success of the design strategy to limit displacement demands of the elements to less than their displacement ductility capacity, for zero and increasing strength eccentricities, was examined against the effects of key parameters expected to influence response. These parameters are: strength eccentricity and the associated increase of system strength, mass eccentricity, ratio of radii of gyration of strength and mass, reduced system displacement ductility capacity, transverse elements and their degree of torsional restraint, the ratio of element nominal yield displacement, i.e., α=Δye2/Δye1. and associated stiffness eccentricity, uncoupled translational period, consideration of different earthquake records and their direction of application. Elements are modelled with a realistic relationship between element strength, stiffness and nominal yield displacement. The stiffness is strength dependant and the nominal yield displacement is a geometric and material property independent of strength. The centre of strength and stiffness are, therefore, not independent parameters. This research focuses on analytical studies of torsionally unrestrained and restrained single-mass asymmetric systems. Single, two and multi-element systems were examined. An experimental programme was also undertaken on single-mass models to verify some of the analytical findings. The findings suggest that the suggested design strategy is successful in limiting the displacement demands on elements to less than their displacement capacity for zero and increasing strength eccentricities. No differentiation is required between systems having or not having mass eccentricity. The proposed design strategy is slightly different for torsionally unrestrained and restrained systems. It promises to be straightforward, rational and in terms of design efforts most user friendly.