Conclusions
An analysis procedure for assessing the dynamic stability of slender CFST columns due to the time effects of the creep of the concrete core is proposed in this work based on the creep model of the ACI committee 209 and the effective modulus evaluation from the AEMM. Through the detailed numerical investigations on a typical CFST column, the validity and effectiveness of the proposed method is verified and the time-dependent characteristics of dynamic stability of the CFST column are explored. For the typical CFST column under a sustained central axial load, its natural frequency and Euler buckling load decrease by about 5.5% and 11.2% in 65 days and 110 days, respectively. Since the natural frequency and Euler load are two key parameters directly influencing the regions of dynamic instability, and their absolute day rates of change become less than 0.01% in about 100 days, the time of 100 days of the creep of the concrete core can be used to evaluate the time effect of the creep of the concrete core on dynamic stability of the CFST column. The dynamic stability of CFST column is significantly affected by the creep of the concrete core. Dynamic instability of a CFST column under a sustained central axial load much lower than its static lateral buckling load such as P0/Pcr = 0.3 would occur in a few days when the creep of the concrete core shifts the amplitude and frequency of the excitation into the instability region of the column. With the development of creep, the CFST column would become dynamically unstable under a dynamic excitation with a very small amplitude Pt such as Pt/ P0 = 2%. The critical amplitude Pt and frequency θ of the excitation can decrease by about 6% and 3% in 5 days of the first loading and respective 11% and 6% in 100 days. Under the same excitation, the dynamics of the CFST column could convert from stability to instability or changes in the opposite direction owing to the creep of the concrete core.
