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Earthquakes and Structures
  Volume 14, Number 5, May 2018, pages 411-424

Inelastic behavior of systems with flexible base
Luciano R. Fernandez-Sola and Juan E. Huerta-Ecatl

    This study explores the inelastic behavior of systems with flexible base. The use of a single degree of freedom system (ESDOF) with equivalent ductility to represent the response of flexible base systems is discussed. Two different equations to compute equivalent ductility are proposed, one which includes the contribution of rigid body components, and other based on the overstrength of the structure. In order to asses the accuracy of ESDOF approach with the proposed equations, the behavior of a 10-story regular building with reinforced concrete (RC) moment resisting frames is studied. Local and global ductility capacity and demands are used to study the modifications introduced by base flexibility. Three soil types are considered with shear wave velocities of 70, 100 and 250 m/s. Soil-foundation stiffness is included with a set of springs on the base (impedance functions). Capacity curves of the building are computed with pushover analysis. In addition, non linear time history analysis are used to asses the ductility demands. Results show that ductility capacity of the soil-structure system including rigid body components is reduced. Base flexibility does not modify neither yield and maximum base shear. Equivalent ductility estimated with the proposed equations is fits better the results of the numerical model than the one considering elastoplastic behavior. Modification of beams ductility demand due to base flexibility are not constant within the structure. Some elements experience reduced ductility demands while other elements experience increments when flexible base is considered. Soil structure interaction produces changes in the relation between yield strength reduction factor and structure ductility demand. These changes are dependent on the spectral shape and the period of the system with fixed and flexible base.
Key Words
    dynamic soil structure interaction; inelastic behavior; ductility demands; equivalent single degree of freedom model; RC buildings
Luciano R. Fernandez-Sola and Juan E. Huerta-Ecatl: Departamento de Materiales, Universidad Autónoma Metropolitana-Azcapotzalco, San Pablo #180 Z.C. 02200, Mexico

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