尺寸效应对大直径钢管混凝土柱轴压承载力的影响

Concrete-filled steel tube (CFST) is an ideal structural solution for modern large-span or high-rise applications subjected to heavy loadings, which has been widely used in skyscrapers and large-span arch bridges. The requirement for larger CFST members is inevitable as the height of buildings or the span length of bridges keeps increasing. Size effect should affect the mechanical behavior of CFST columns because the core concrete is a quasi-brittle material. This is an even more complicated issue for CFST columns due to the confinement effect on concrete core. Current designing codes are made based on the experimental results from small dimension CFST specimens, which can not consider the size effects. This may lead to an overestimation of the ultimate capacity of CFST members with large cross sectional dimension. In order to ensure the safety of CFST applications, it is necessary to provide an exquisite designing method to predict the static response of CFST members with the consideration of size effect. Unfortunately, available research works are not adequate to provide such designing equations. In this context, this project will experimentally and numerically investigate size effect on the ultimate capacity of axially loaded circular CFST specimens with different cross-sectional dimension, concrete strength and ratio of cross-sectional area of steel tube over concrete core. Shrinkage is also measured during the test to figure out if it is different for specimens with different cross-sectional dimension, and how this difference will influence their ultimate capacity. The mechanism of how size effect influences the confinement of steel tube to concrete core will be explained based on the experimental results. Based on the experimental and numerical results, the designing equations and methods will be proposed for large diameter CFST members with the consideration of size effects.

钢管混凝土适应现代工程向大跨、重载、高耸方向发展的需要，在高层建筑与大跨拱桥中得到了广泛应用。随着建筑高度和拱桥跨度的不断增加，钢管混凝土构件的巨型化是未来发展趋势与必然。但由于核心混凝土的存在，大尺度钢管混凝土构件的尺寸效应显著，且由于钢管与核心混凝土的相互作用，使其尺寸效应问题更为复杂。现有研究和相关规程规范均是基于小尺度钢管混凝土试件的研究成果，无法准确反映大尺度钢管混凝土存在的尺寸效应，可能高估了其承载力。因此，亟待提出适用于大尺度钢管混凝土构件的设计方法，以确保大尺度钢管混凝土结构的可靠性。项目以圆形钢管混凝土轴压短柱为研究对象，以截面尺寸为主要研究参数，并考虑混凝土强度、含钢率、核心混凝土收缩等因素，采用试验研究与理论分析相结合的研究方法，分析核心混凝土尺寸效应对钢管混凝土约束效应与受力机理的影响，建立考虑尺寸效应的大直径圆钢管混凝土轴压承载力的设计方法与计算公式。

钢管混凝土具有抗压性能好、承载力高、塑性和韧性好、施工方便等优点，适应现代结构向大跨、高耸、重载方向发展的趋势及承受恶劣环境的需求，因此在高层、超高层建筑及大跨桥梁中得到了较广泛的应用。随着建筑高度和桥梁跨度的增加以及结构形式的多样化，钢管混凝土构件的截面尺寸不断增大。然而，现有规范公式均是基于小尺寸钢管混凝土试件的试验提出的，考虑到核心混凝土尺寸效应的影响，现有是否能够准确评估大尺寸钢管混凝土构件的轴压承载力值得商讨。目前关于钢管混凝土尺寸效应研究相对较少，本项目主要完成工作如下：.1）进行混凝土轴压短柱尺寸效应试验（24个试件），考察其弹性模量、峰值应力、峰值应变的尺寸效应规律，进而建立考虑尺寸效应影响的混凝土轴压短柱应力-应变关系模型。.2）对混凝土轴压短柱进行细观数值模拟，从有限元的角度分析混凝土峰值应力的尺寸效应，验证前文建立模型的有效性。.3）进行钢管混凝土轴压短柱尺寸效应试验（36个试件），获得了试件截面峰值应力、峰值应变及弹性模量的尺寸效应规律，研究参数范围内尺寸效应使钢管混凝土试件截面峰值应力减小9.7%、峰值应变减小约20%，而弹性模量基本不存在尺寸效应。.4）基于文献数据及约束混凝土理论建立了考虑尺寸效应影响的主动约束混凝土模型，分析了主动约束与被动约束的关系，进而建立了钢管混凝土尺寸效应分析模型，模型能够较好地预测钢管混凝土峰值应力及峰值应变的尺寸效应规律。.5）通过系统地参数分析发现，钢管混凝土的尺寸效应随含钢率及屈服应力增大而减弱，随混凝土强度增大而增强，对于工程中的大尺寸钢管混凝土构件，截面峰值应力折减幅度接近15%。基于参数分析，本文提出了考虑尺寸效应影响的钢管混凝土峰值应力及峰值应变计算公式。.本课题组研究对钢管混凝土的尺寸效应进行定量描述，为工程中的大尺寸钢管混凝土设计奠定了理论基础。