Nonlinear analysis of elliptical CFST columns at ambient and elevated temperatures

Abstract

The paper studies the application of the concentrated plasticity 3D beam/column finite element on the nonlinear analysis of elliptical concrete-filled steel tube (CFST) composite columns at both reference temperature and during fire conditions. The element based on the generalized plasticity material model is extended and adjusted to analyze elliptical composite CFST columns. It has plastic hinges at the ends and an elastic component in between and accounts for the gradual cross-section plastification and nonlinear geometry under large displacements. The plastic hinge capacity is described by the yield surface. This study presents a novel, highly accurate polynomial expression for the yield surface of elliptical CFST sections. This expression applies to ambient and elevated temperatures and is validated on a wide range of section parameters. Additionally, the paper discusses the definition and selection of other element parameters, such as elastic component properties and average thermal dilatation definition. Through an extensive validation study that includes 117 beam or column tests and one frame example, the response of the presented model is compared to other experimental or numerical studies. The study confirms the element’s capability to capture the overall behavior of elliptical CFST columns with high accuracy (within the 15% margins) and being on the safe side. Besides, the comparison with the fiber-based beam/column element, referring to the required calculation time, proves its high computational efficiency.