The Direct Strength Method for Interactive Buckling Resistance of Axial Compression Members Made of Non-Linear Metallic Materials

Stainless steel and aluminum alloy were classified as the non-linear metallic materials due to the absence of obvious yielding point and yielding plateau. When the member cross-sectional stress level increases beyond the proportional limit strength that is usually lower than the nominal yield strength, there exists gradually decreased tangent modulus but remarkable strain hardening capacity, which has significant impact on the local plate buckling and overall column buckling resistances. The commonly adopted effective cross-section approach has been introduced into the calculation methods for predicting the column resistances that account for the local-overall interaction effect, while this may result in complicated computation process for irregular cross-sections and neglect the restraint conditions between adjacent constitutive plates.
The available experimental results on local-overall interactive buckling of axial compression members made of stainless steel and aluminum alloy were summarized, including welded stainless steel I-sections and box sections, and extruded aluminum alloy I-sections and box sections. The elaborated numerical models that were developed by means of the general finite element (FE) software package ABAQUS incorporated the representation of material non-linearity of stainless steel and aluminum alloy, the initial local and global geometric imperfections and the welding residual stresses. The numerically simulated buckling resistances and failure modes were compared with the test results, and thus the accuracy of the developed numerical models were verified. By using the validated FE models, a large number of axial compression members subjected to interactive buckling were generated, and the obtained numerical results were used to carry out parametric studies. The influences of the normalized nominal yield strength, the strain hardening exponent, the initial local and global geometric imperfections and the welding residual stresses on the column buckling resistance were explored.
Based on the summarized test results and obtained numerical data points, design expressions of the direct strength method (DSM) in the current North American Specification for the Design of Cold-Formed Steel Structural Members (AISI S100-16) were evaluated, and it was revealed that the existing expressions provided inaccurate and unsafe predictions for the interactive buckling resistance of axial compression members made of non-linear metallic materials. Newly modified calculation formulae of the direct strength method (DSM) for predicting the interactive buckling resistances were therefore proposed by regression analysis for various materials and cross-section types. By referring to the reliability index in the Chinese Unified Standard for Reliability Design of Building Structures (GB 50068- 2018) and the resistance partial factors provided in the Chinese Code for Design of Aluminum Structures (GB 50429-2007) and the Chinese Technical Specification for Stainless Steel Structures (CECS 410:2015), the reliability analysis of the newly proposed expressions were conducted under a total of four different load combinations and twelve load cases, and it was shown that the computed reliability index values were all higher than the target value of 3. 2. It can therefore be concluded that the proposed DSM expressions satisfy the reliability requirements set in the Chinese standards, and their applicability to predict the interactive buckling resistances of axial compression members made of non-linear metallic materials has also been demonstrated.