Determination of buckling mode for braced elastic-plastic frames

The instability of braced frames was studied by geometric and material nonlinear analysis accounting for residual stresses, initial sway imperfection and member initial bow. The change of buckling mode with increasing of bracing stiffness was analyzed and the relationship between the ultimate load capacity and the bracing rigidity was found. The threshold stiffness for the bracing to be just enough to make frames buckle in a non-sway mode was obtained. Horizontal loads produce stresses in the bracings, and thus make the bracings yield prematurely and result in decrease of bracing stiffness. This will change the buckling mode of the braced frame. Nonlinear analysis of braced frames loaded both vertically and horizontally was also made. A simple and easy-to-use criterion was established for judging the buckling mode of braced frames including the effect of the horizontal loads. The proposed equation was compared with FEM analysis. (c) 2007 Published by Elsevier Ltd.