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.

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]. It is known when a slender column is subject to an axial compression, it could fail owing to lateral instability [5]. The instability of slender CFST columns under axial static compression has been experimentally and numerically studied by many researchers [4,6–۸]. These studies have shown that slender CFST columns are prone to global buckling under static loading. In addition to the static loading, the service loads of slender CFST members also involve the dynamic loading. For example, the slender CFST piers in modern bridges are subject to the dynamic vehicle loading, and the high CFST pillars supporting large span roofs are loaded by dynamic wind loading. The behavior of CFST columns subjected to cycles of compressive loading has also been reported by many researchers [9–۱۱]. Under a sustained centric axial static load, the concrete core of a CFST column creeps with the time and the creep of the concrete core may change the lateral stiffness and the lateral natural frequency of the CFST column significantly. If the column under the sustained load is further excited by an axial dynamic excitation at some stage, the column may suddenly lose its stability laterally due to dynamic resonance when certain relationships between the frequency of excitation and the natural frequency of the column are satisfied and the amplitudes of the excitation are sufficiently high. Because the creep of the concrete core develops with the time and changes the lateral natural frequency, the relationships between the frequency of the excitation and the natural frequency of the column and the required amplitude of the excitation inducing the dynamic instability of the CFST column may change greatly with the time. Such dynamic instability may occur even when the sum of the amplitude of the excitation and the sustained static load is much smaller than the static instability load of the column. Meanwhile, the engineering structures are commonly subject to sustained static loads and sudden dynamic excitations [12,13].