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Practical advanced analysis for design of laterally unrestrained steel planar frames under in -plane loads

Purdue University
Publication Date
  • Engineering
  • Civil
  • Design
  • Mathematics


Currently, advanced analysis procedures that are suitable for practical design of steel planar frames subjected to in-plane loads cannot detect out-of-plane instability. As such, structural design using these procedures requires separate checks to ensure that out-of-plane instability does not govern the frame strength. It is thus the objective of this research to extend in-plane advanced analysis procedures to account for inelastic out-of-plane buckling. Through the use of small-displacement, small-strain, and linear-stability assumptions, it is demonstrated that in-plane and out-of-plane behaviors are uncoupled within a linearized step. This permits independent development of out-of-plane analysis procedure and does not require a major modification to the in-plane analysis procedures. Derivation of the out-of-plane second-order elastic stiffness matrix is based on the finite element analysis approach. The effects of inelasticity, residual stresses, and geometric imperfections on out-of-plane buckling are accounted for by replacing the elastic stiffnesses EIy, ECw, and GJ with their effective values which were determined from calibration against the AISC LRFD column and beam strength equations. It is found that the current AISC LRFD specification, which is based on member-by-member design approach with the use of effective length factors, does not allow a complete assessment of the interactions between framing members and may lead to a conservative design for out-of-plane buckling. Because advanced analysis treats the frame as a system and simultaneously considers the key factors influencing its behavior, advanced analysis provides a more accurate assessment of frame strength and gives insight on the potential failure characteristic of the frame. Based on a set of attributes found in typical building frames, it is demonstrated that out-of-plane buckling is likely to govern the design of nonsway frames and may control the design of sway-frames. In light of this, it is recommended that the developed advanced analysis procedure be used in the design of both types of planar frames. ^

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