RESEARCH ON THE CREEP OF CONCRETE FILLED STEEL TUBULAR COLUMNS BASED ON THE GENERALIZED KELVIN CHAIN
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摘要: 混凝土徐变是混凝土材料本身固有的一个时变特性，是结构响应中一个重要组成部分，其计算方法通常是建立在单轴试验和理论基础上。为探讨钢管混凝土徐变特性，该文采用自制的压力自平衡混凝土徐变试验装置对混凝土圆柱和圆钢管混凝土柱进行了徐变试验，结果表明：钢管混凝土柱徐变变形要比普通混凝土柱的徐变变形小，在该文试验中两者相差接近50%，这可能是密闭钢管内核心混凝土无法与外界发生水分交换而不发生干燥收缩和干燥徐变以及钢管围压所致。根据粘弹性理论，引入多参数Kelvin链粘弹性元件模型，建立了求解单轴应力状态下混凝土徐变的Volterra型积分方程，模型参数近似表示为连续粘滞谱。通过离散时间变量t和分步积分，进一步得到了单轴应力状态下混凝土徐变应力-应变增量本构模型。依据徐变叠加原理，考虑Poisson效应，进一步将单轴应力状态下混凝土徐变应力-应变增量本构模型拓展到三轴应力状态，用于钢管混凝土徐变分析。对有限元商用软件Ansys进行二次开发，将反映三轴应力状态下混凝土徐变性能的本构方程引入Ansys提供的用户子程序Usermat中，并采用Fortran语言编程，从而实现了钢管混凝土徐变长期性能的有限元分析计算。将有限元数值解与试验结果进行对比分析，发现该文提出的模型是科学的和有效的。该文提出的方法将为混凝土徐变计算提供了另一条有效途径。Abstract: Concrete creep is an inherent time-dependent behavior of concrete and an important composition of structural responses. Its calculating method is usually established on the basis of uniaxial test and theory. To explore the creep behavior of concrete-filled steel tubular columns, the creep of circular concrete columns and concrete filled circular steel tubular (CFST) columns were measured by a self-made and self-balanced loading device. The results show that the creep deformation of CFST columns is smaller than that of concrete columns. In the present concrete creep tests, the maximum difference between the two aforementioned creep deformations was close to 50%. It is attributed to the fact that the core concrete cannot exchange moisture with its external atmosphere environment so that no dry shrinkage and creep of the core concrete occurred. It is also attributed to the confining pressure of the steel tube. According to the theory of viscoelasticity, a Volterra integral equation for solving the concrete creep under uniaxial stress condition was established by introducing a model consisting of a multi-parameter Kelvin element chain. The model parameters were approximately expressed as a successive retardation spectrum. Therefore, the constitutive relationship of concrete creep stress and strain increments under uniaxial stress condition was derived by discretizing the time variable t and integration by parts. Based on the principle of creep superposition and the Poisson effects, the constitutive model of the concrete creep stress and strain increments under uniaxial stress condition state was extended to a multiaxial stress state to analyze the CFST creep. The commercial finite element analysis software Ansys was secondarily-developed. The constitutive equation under multiaxial stress state for concrete creep performance was compiled with the Fortran language and introduced into the user subroutine Usermat of Ansys. Finally, the finite element numerical analysis for the long-term creep performance of CFST columns was carried out. Comparing the numerical solutions with the test results, it is found that the proposed method provides another effective approach to the analysis of concrete creep.
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Keywords:
- concrete filled steel tubular /
- creep /
- Kelvin model /
- Ansys /
- experiment
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图 5 长期荷载试验过程中温度与湿度变化
Figure 5. Variation of temperature and humidity measured during long-term test
表 1 徐变试验试件一览表
Table 1 List of creep test specimens
序号 试件编号 t/d ts/mm (实测值) NL/kN ε0 素混凝土 C-P-135a 135 − 80.44 −227.5 C-P-135b 135 − 80.44 −126.75 钢管混凝土 C-S-135a 135 3.648 282.31 −360.5 C-S-135b 135 3.688 282.31 −419.67 注：“C-P-135a”中C表示为“creep test”，P表示为“plain concrete”，S表示为“concrete filled steel tube”；a与b为两个平行试件；t为持荷时间；ts为钢管厚度实测值；NL为试件实际施加荷载值；ε0为加载时刻弹性应变。 
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