Abstract
An analytical procedure for dynamic stability of CFST column accounting for the creep of concrete core is proposed. The long-term effect of creep of concrete core is formulated based on the creep model by the ACI 209 committee and the age-adjusted effective modulus method (AEMM). The equations of boundary frequencies accounting for the effects of concrete creep are derived by the Bolotin's theory and solved as a quadratic eigenvalue problem. The effectiveness of the proposed method and the characteristics of time-varying distribution of instability regions are numerically surveyed. It is shown that the CFST column becomes dynamically unstable even when the sum of the sustained static load and the amplitude of the dynamic excitation is much lower than the static instability load. It is also found that due to the time effects of concrete creep under the sustained static load, the same excitation, that cannot induce dynamic instability in the early stage of sustained loading, can induce the dynamic instability in a few days later. The critical amplitude and frequency of the dynamic excitation can decrease by 6% and 3% in 5 days, and 11% and 6% in 100 days.
1. Introduction
Steel hollow sections are very efficient in resisting compression forces, and filling these sections with concrete greatly enhances the load-carrying capacity [1,2]. The concrete-filled steel tubular (CFST) structure possesses many mechanic benefits, such as high strength and fire resistances, favorable ductility and large energy absorption capacities, so the CFST members are widely used in modern structures [3]. Moreover, with the advancement in the strength resistance and construction techniques of CFST column, slender CFST columns are frequently adopted to support the roofs of industrial plants, the decks of railways and the floors of multistory buildings [4].
5. 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.
