The purpose of this paper is to develop a simple and approximate method with sufficient accuracy for predicting the effect of shearing strains iii the middle surface of the walls, which reflect the shear lag phenomenon, on the lateral buckling of thin-walled open member. An energy equation for the lateral buckling has been derived in which the effects of torsion, warping of the member and, especially, the shearing strains in the middle surface of the wall are taken into account. The energy equation can be applied for a prismatic thin-walled member with any kind of open cross-section, for any loading system, and for any end boundary conditions. Some numerical examples by using optimization techniques are given to show the accuracy and applicability of the proposed method here. The results are compared with known results from experiment and finite element method, and good agreement is obtained. Finally, the effect of the shearing strains in the middle surface of the walls on lateral buckling of the thin-walled open member and the effect of the number of Simpson's nodal points taken on optimal values are discussed. (C) 1997 Elsevier Science Ltd. [References: 12]
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