考虑相关稳定的非线性金属轴压柱承载力直接强度法

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

非线性金属结构材料主要包括不锈钢和铝合金两类,其应力-应变关系曲线没有明显的屈服点和屈服平台。材料的比例极限强度往往低于其名义屈服强度,随着构件截面应力水平增至超过材料比例极限强度后,材料的切线模量逐渐降低,但其应变硬化能力较为显著,直接影响截面组成板件的局部稳定和构件的整体稳定承载力。目前考虑局部-整体相关稳定的轴压柱承载力计算一般采用有效截面法,但该类方法针对复杂截面的计算过程较为繁琐,且未能充分考虑截面板件间的约束作用。汇总国内外现有关于考虑局部-整体相关稳定的不锈钢和铝合金轴压柱的试验数据,包括不锈钢焊接工字形和箱形截面以及铝合金挤压成型工字形和箱形截面共4类,采用通用有限元软件ABAQUS创建精细数值模型,准确考虑不锈钢和铝合金材料的非线性特性、构件的局部和整体几何初始缺陷以及截面残余应力分布等,通过模拟试验过程得出轴压柱的承载力与失稳破坏形态,根据与试验结果的比较验证所建立有限元模型的准确性。基于验证可靠的有限元模型,对影响轴压柱局部-整体相关稳定性能的关键因素开展参数分析,建立了大量的数值分析算例,着重考察材料归一化屈服强度、应变硬化指数、局部和整体几何初始缺陷以及焊接残余应力对轴压柱稳定承载力的影响规律。结合汇总的试验数据和有限元数值算例结果,对现行AISI S100-16《北美冷成型钢结构设计规范》中的直接强度法计算公式进行了评估,表明其计算公式不能准确预测非线性金属轴压柱的相关稳定承载力,而且公式计算结果偏于不安全。采用数值拟合方法,针对不同材料、不同截面形式轴压柱的相关稳定承载力分别提出直接强度法修正计算公式。根据现行中国标准GB 50068—2018《建筑结构可靠性设计统一标准》中给出的可靠指标以及GB50429—2007《铝合金结构设计规范》和CECS 410∶2015《不锈钢结构技术规程》规定的抗力分项系数取值,且参照GB 50068—2018选定4种不同的荷载组合形式,在12种荷载工况组合下对建议的修正计算公式进行可靠度分析,计算得出的构件可靠指标均大于目标可靠指标3.2,表明所提出的直接强度法计算公式满足中国标准的可靠度要求,可以用于计算考虑相关稳定的非线性金属轴压柱承载力。

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.