Seismic collapse capacity analysis of degrading SDOF systems with uncoupled negative and positive stiffness mechanism

The negative-positive-uncoupled stiffness device (NPUSD) is a novel variable stiffness system recently developed by the authors, designed to efficiently and cost-effectively achieve multi-level seismic fortification in structures. This study aims to integrate the NPUSD with a viscous damper in parallel, forming an innovative negative-positive-uncoupled stiffness amplifying damper (NPUSAD). Additionally, it establishes the collapse capacity spectra for a degrading single-degree-of-freedom (SDOF) system equipped with the NPUSAD under three distinct seismic record sets. The study investigates the influence of both the SDOF structural parameters and the NPUSAD parameters on the collapse capacity spectra. The results indicate that the NPUSAD-SDOF system significantly enhances the collapse resistance of long-period structures, with improvements of 30%, 40%, and 50% under far-field non-impulsive, near-field non-impulsive, and near-field impulsive seismic record sets, respectively. Among the parameters of structural element, the ductility ratio, soften stiffness coefficient, and stability coefficient have a significant impact on the collapse capacity spectra. For the NPUSAD parameters, the transition displacement ratio has the greater influence, followed by the positive-to-negative stiffness ratio, while the connecting stiffness ratio has the least impact.