Improved uniaxial Bouc-Wen model for seismic dynamic response analysis of inelastic system

An improved uniaxial Bouc-Wen model was developed by taking the P-Δ and pinching effects, strength and stiffness degradations, as well as strain hardening into account. According to the nonlinear seismic dynamic responses of an inelastic single-degree-of-freedom (SDOF) system under 69 selected earthquake records, the influence of P-Δ effect on both the mean and coefficient of variation (COV) of seismic ductility demands were quantitatively investigated. The probability distribution type and prediction equation of seismic ductility demand for an inelastic SDOF system with P-Δ effect were also developed. The analysis results show that the P-Δ effect induced by the gravity affects significantly the seismic ductility demand, while the effect induced by the vertical seismic excitation is negligible. Linear correlation coefficients between seismic ductility demand and seismic parameters, such as the moment magnitude, epicentral distance and shear wave velocity, are usually unobvious. It also implies that for a short-period system the seismic ductility demand can be modeled as either a Lognormal or Frechet distribution variable, while for a long-period system, the Frechet distribution variable is preferred.