This paper introduces a numerical analysis method to evaluate the residual fire-resistance of steel-concrete composite structures. The proposed analysis method consists of two procedures, a transient heat transfer analysis and a non-linear structural analysis. To precisely simulate the structural response with temperature, the material properties of concrete and steel according to the two representative temperature conditions, “under-fire” and “after-cooling”, have been taken into account. Furthermore, non-mechanical strains of concrete and steel such as thermal strain, transient strain, and creep strain, which change with temperature variation induced by fire, are implemented into the formulation. Upon validation of the introduced numerical method through a correlation study between experimental data and numerical results, the importance of the exact consideration of the non-mechanical strains as well as material properties of concrete and steel corresponding to the changing temperature has been emphasized. Moreover, through a comparison of the numerical results with the design code EN1992-1-2, it has been concluded that the design code should consider the influence of temperature decrease after experiencing high temperature to ensure the safety of fire-damaged structural members.
In addition to fire-resistance evaluation, a numerical model for the analysis of bond-slip occurring in concrete filled steel tube (CFT) columns is introduced. Unlike the classical bond-link or bond-zone element using double nodes, the introduced model considers the bond-slip effect without taking double nodes by incorporation of the equivalent steel stiffness. Moreover, while solving the system equation to evaluate the slip behavior, the mechanical properties for steel and bond-slip have been changed and updated through an iteration procedure and the validity of the introduced numerical model is verified by comparing the experimental data with the analytical results for CFT columns subjected to axial force and bending moment. Finally, a simple design equation to predict the resisting capacity of circular CFT columns is introduced, and the accompanying design procedure entails two phases: construction of the linearized P-M interaction diagram for a circular CFT column section and its adjustment according to the slenderness ratio. To construct the linearized P-M interaction diagram of circular CFT columns without a rigorous nonlinear analysis, simple equations are proposed in this paper. The ultimate resisting capacities calculated by the proposed design equation are compared with those constructed from rigorous nonlinear analyses and from the AISC design guideline and the Eurocode with the objective of establishing the relative efficiencies of the proposed equation. Finally, the numerical model of a CFT column subjected to high temperature is suggested considering with the concept of gap conductance and modified bond-slip relation. Comparisons with experimental results verified that the suggested model well describes the change of composite behavior at the interface between steel and concrete under the elevated temperature.
강재-콘크리트 합성구조는 콘크리트의 낮은 열전도도에도 불구하고, 화재와 같이 고온에 지속적으로 노출되었을 경우에는 강도가 저하되어 전체 구조물의 안전성에 심각한 손상을 초래하게 된다. 특히 CFT 기둥과 같이 강재가 콘크리트의 외부를 둘러싸고 있는 경우에는 고온에 의해 훨씬 치명적인 손상을 입을 수 있으므로 화재상황에서 보다 정확하게 구조물에 대한 분석이 이루어져야 한다. 이러한 필요에 의해 본 논문에서는 고온에 노출된 강재-콘크리트 합성기둥의 거동을 수치해석적인 방법을 이용해 분석하였다. 먼저 내화해석을 통해 고온에 노출된 강재와 콘크리트의 재료적 성질 변화를 역학적 변형률과 비역학적 변형률로 나눠서 고려하였다. 이 때, 고온 상태 (under-fire) 뿐 아니라 냉각 후 상온상태를 “after-cooling”으로 정의하고, 냉각 시 콘크리트에 발생하는 미세균열에 의한 잔류저항강도 감소를 적용한 수치해석방법을 제안하였다. 고온에 의한 물성변화가 적용된 수치해석 결과는 여러 연구자들의 실험결과를 통해 검증하고, 유로코드에 나와 있는 설계기준과 비교하였다. 또한, 강재-콘크리트 합성구조 기둥의 거동분석을 위해 CFT 기둥에서 발생하는 부착-슬립 효과를 이중절점을 사용하지 않고도 강관의 강성을 등가강성으로 치환하여 계면에서의 슬립거동을 모사할 수 있는 모델을 제안하였다. 새로운 부착-슬립 모델이 적용된 해석 결과를 가지고, CFT 기둥의 저항력을 파악할 수 있는 P-M 상관도의 작도법을 제시하고 설계기준에 나와있는 작도법과 비교하였다. 최종적으로 CFT 기둥이 고온에 노출되었을 경우, 계면에서의 Gap conductance 를 고려하고, 온도에 따른 재료적, 구조적 거동의 변화를 모사하는 모델을 구성하여 실험 결과와 검증하였다.
