Flexural stiffness factors for stability design of stainless steel planar frames

Abstract

The Direct Analysis Method is the primary method for the stability design of carbon steel frames in AISC 360-16. The method refers to a second-order elastic analysis performed on members whose tangent stiffness moduli are reduced using adequate factors. The aim of this research is to provide a set of stiffness reduction factor formulation for the stability design of the particular case of stainless steel frames. The proposed stiffness reduction factor is aligned to AISC 360-16 and it is aimed at facilitating greater and more efficient use of stainless steel, whose material nonlinearity must be accounted for in design. The focus of this research is the development of flexural stiffness reduction factor formulation for the in-plane stability design of stainless steel elements and frames with cold-formed square hollow section and rectangular hollow sections. A beam-column stiffness reduction factor accounting for the deleterious influence of material non-linearity, residual stresses and member out-of-straightness is proposed. The use of a second-order elastic analysis coupled with the proposed reduction factor eliminates the need for member buckling strength checks and thus, only cross-sectional strength checks are required. For the case of beam-columns, two types of reduction factors are developed: analytical and approximate. The analytical expression presumes knowing the maximum internal second order moment within a member. It is developed by means of extending the formulations for evaluating the elastic second order effects to the inelastic range. The approximate expression is of considerable usefulness since in practical design, the second order moment is not known in advance. As a result, an approximate expression of the reduction factor, which is assumed to be a function of relevant variables, is proposed by fitting variables to the analytically determined expression. For the purpose of developing this approximate expression, column flexural stiffness reduction factors as well as and beam flexural stiffness reduction factors are derived from stainless steel column strength curves and from the moment-curvature relationship, respectively.