Structural Behavior And Design Method Of Hollow And Concrete-Filled Elliptical Steel Tubular Columns

In recent years, coupled with their aesthetical appeal and sound structural efficiency in bending, elliptical tubular structures are emerging gradually as new members of tubular structure families upon the field of modern construction practice. In order to investigate the distinct mechincal properties of members with elliptical sections from other tubular sections, the thesis presents the experimental, analytical and numerical studies on these members around the following two main contents:one is the buckling performances of elliptical cylinderial shells and hollow elliptical tubular columns; the other is the compressive behavior of concrete-filled elliptical tubular columns. The research work of this thesis is aimed to provide advices for design of elliptical members, so that application of these members in practice can be further developed. The layout of this thesis is organized as follows:Chapter 2 derives an analytical solution for the elastic buckling of elliptical cylindrical shells under axial compression on the basis of energy method. During the theoretical derivation, a damped exponential is introduced into the displacement function to reflect the deformation behavior of elliptical shells under axial compression. The Ritz’s method is employed to solve the established energy equation, leading to the analytical solution for the critical buckling load of axially-compressed elliptical shells. The elastic buckling loads of circular cylindrical shells under axial compression, which are obtained by degeneration of current analytical solution for elliptical shells, are found to be in good agreement with the classical solutions in the existing literature. The accuracy of the analytical solution is further verified by comparing its predictions with results obtained from finite element analysis. In addition, a parametric study based on the proposed analytical solution has also been conducted.Chapter 3 presents an analytical solution for the elastic buckling of elliptical cylindrical shells under external pressure on the basis of energy method. During the theoretical derivation, a damped exponential is introduced into the displacement function to reflect the deformation behavior of elliptical shells under external pressure. The Ritz’s method is employed to solve the established energy equation, leading to the analytical solution for the critical buckling load of externally-pressurized elliptical shells. The elastic buckling loads of circular cylindrical shells under external pressure, which are obtained by degeneration of current analytical solution for elliptical shells, are found to be in good agreement with the classical solutions in the existing literature. The accuracy of the analytical solution is further verified by comparing its predictions with results obtained from finite element analysis.In addition, a parametric study based on the proposed analytical solution has also been conducted.Chapter 4 theoretically deduces analytical solutions for the warping displacement, normal stress and shear stress in elliptical cylindrical shells under restrained torsion based on Umansky’s theory for closed thin-walled members. The analytical solutions include the numerical integrated expressions and their simplified formulas, with the former and latter defined as exact and approximate solutions, respectively. The accuracy of approximate solutions is evaluated by comparing the approximate solutions among exact solutions and the corresponding numerically predicted responses via ABAQUS for elliptical cylindrical shells with different aspect ratios and shell thickness. Furthermore, the application of the proposed approximate solutions in the simplified analysis for elliptical framed tube structures under torsion has been investigated.Chapter 5 provides test results of 8 cold-formed hollow elliptical stub columns with diffrenct aspect ratios, as well as their corresponding numerical predictions calculated from ABAQUS software. In the finite element models, residual stresses and geometric imperfections are both considered. Validation of this numerical method is also conducted by comparing simulation results with the corresponding test data.Charpter 6 describes the test procedure and test results of an experimental study on the compressive behavior of carbon fiber reinforced polymer (i.e. CFRP)-confined elliptical concrete columns. The designed experimental program considers the effects of key parameters, including the CFRP thicknesses and aspect ratios of elliptical section. The accuracy of a finite element model for CFRP-confined elliptical concrete columns is then demonstrated through comparisons with the test results. On the basis of the test data as well as the numerical results obtained with the verified finite element model, a simple and accurate stress-strain model for FRP-confined concrete in elliptical columns is formulated for use in design.Charpter 7 carries out the uniaxial compression tests of 14 concrete-filled elliptical stub columns with different aspect ratios and concrete strengths, with aim to getting a better understanding of the mechanical properties and failure modes of the concrete-filled elliptical columns. Followed by organsing and analysing the test results, the effects of aspect ratios and concrete strengths on the axial compressive behavior of concrete-filled elliptical columns have been obtained.Charpter 8 develops a finite element model to predict the axial compressive resistances of concrtete-filled elliptical stub columns via the ABAQUS software. The constitutive model of confined concrete under non-uniform confinement is established in a modified plastic-damage model within the theoretical framework of the Concrete Damaged Plasticity Model (CDPM) in ABAQUS. The dilation angle of concrete is taken to be independent with confining pressure, while the dependences of the strain-hardening/softening rules and damage parameters on the confining pressure are considered. In addition, the distinct characteristics of non-uniformly confined concrete are mainly reflected in the definition of an effective confining pressure. The proposed finite element model is verified with test results in Charpter 7.Charpter 9 proposes a design-oriented model for concrtete-filled elliptical stub columns based on the assumption that the confining pressure provided from the outer steel tube is a constant. The flexural and axial capacity formulas for concrtete-filled elliptical stub columns are also derived through conducting section analysis using the proposed design-oriented model. In addition, design equations for concrtete-filled elliptical stub columns are finally put forward followed by introducing an equivalent stress block according to the Chinese code for design of concrete structures.Charpter 10 obtained the theoretical model for concrtete-filled elliptical slender columns by means of numerical integration method. The slenderness limit expression for short concrtete-filled elliptical columns is also proposed based on the numerical results of a parametric study using the developed theoretical column model. Employing the nominal curvature method, design equations for concrtete-filled elliptical slender columns are formulated, of which the validity is demonstrated through comparsion with existing expertimental results.