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Inelastic Seismic Response of Structures with Mass or Stiffness Eccentricities in Plan.
Bruneau, N.; Mahin, S. A.
National Science Foundation, Washington, DC., September 1987, 316 p.
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A tentative classification scheme for various types of inelastic torsionally coupled systems is proposed. The concept of equivalent nonlinear systems is obtained from the expansion of the incremental equations of motion and the requirements for geometry-independence are enunciated. For mass and stiffness symmetric systems having elements with different yield capacities, torsional coupling is created by the desynchronizing inelastic element responses, despite the existence of symmetry in the elastic domain. An extensive parametric study demonstrates that the element ductility levels remain within reasonable bounds provided the ratio of uncoupled frequencies is not excessively large and the yield level of the weaker element is accurately estimated. A similar extensive parametric study is conducted for initially eccentric systems. A procedure to insure a fair comparison between the coupled system and an equivalent single degree of freedom is formulated; it provides a capability for predicting element ductilities in the coupled system from readily available design tools. Using this new procedure, the resulting ductility demand of the weaker element in the two-element torsionally coupled systems becomes almost equal to that in the equivalent single degree of freedom, independently of the traditional parameters.
Earthquake damage; Dynamic response; Seismic waves; Elastic properties; Earthquake resistant structures; Structural vibration; Vibration; Earthquakes; Structural engineering; Stiffness; Displacement; Nonlinear systems; Ductility