Study On Torsional Response Of Eccentric Structures Under Seismic Action

Real structures will in general undergo torsional vibration in addition to lateral oscillations, when they are subjected to earthquake ground motions. Lots of experiences verified that asymmetric structures show poor seismic performance due to lateral torsional coupled response. Studies on seismic torsional response are in progress while disputes existence in some key issues. The study reported herein focus on effect of torsion on seismic response of structures, issues as follow are studied:1. Parameters pertinent to the problems of accidental torsion were introduced, which caused by stiffness and weight uncertainly distribution and rotational component of earthquake ground motions. And the accidental eccentricity was estimated. According to different results of static and dynamic response caused by accidental torsion, method to calculate accidental torsion effect were suggested;2. The inelastic torsional response of stiffness asymmetric system with mass eccentric system were compared, under the condition that element stiffness and strength were affected each other. Inelastic torsional response due to strength distribution was studied according to the results of three-dimensional nonlinear time-history analysis. Results showed that when strength eccentricity is close to zero, inelastic torsional response of stiffness asymmetric system would be controlled best; while when strength eccentricity is close to weight eccentricity, inelastic torsional response of mass eccentric system would be controlled best. Cases with different design eccentricity were illustrated and compared also;3. Response characteristics of asymmetric structures under excitation of bi-directional earthquake ground motions were concluded. Torsional angle, displacements in orthogonal direction, ductility of flexible and stiff side were quite different with results under excitation of single directional earthquake ground motion. Effects of parameters on torsional response under excitation of bi-directional earthquake ground motions were studied. The critical orientation of seismic incidence and maximum structural response to multi-component earthquake excitation were studied;4. The problem of pushover analysis on asymmetric system was studied. According to the basic assume, the applicability of Pushover analysis to asymmetric systems was studied. The processes of equivalent SDOF(single degree of freedom) system of single story with one-way eccentric system, single story system under bi-directional excitation and multi-story shear structures were inferred. Pushover analyses were performed to obtain the inelastic response of asymmetric frame and frame-shear wall systems, and response characteristics were concluded. A method to combined the orthogonal direction response were verified;5. The problems related to seismic design were studied. Several torsional irregular indexes were compared, discussion about such indexes was presented, and design process based on irregular index was suggested. Equivalent eccentricity of frame-wall system was suggested based on the dynamic characteristics of element bending, shearing, St.Venant torsion and buckling torsion. Quantity analysis of ductility and load support capability of asymmetric system was studied. And dynamic effect of eccentricity was studied.