Stability Analysis And Bracing Requirements Of Longitudinal Braced Frames

Bracings are widely used to increase the stability of steel structures. The bracing systems of the industrial longitudinal frames usually consist of the cross bracings and the longitudinal connecting bars whose primary roles are to maintain the frame stability and reduce the out-of-plane effective length of columns and to transfer horizontal loads. Many investigations on the design requirements of bracings of single component and the bracings in the frame to reduce the effective length of columns have been carried out, but stability bracings simultaneously carrying horizontal loads are rarely studied. The specification GB50017 specifies that the bracing force and the horizontal load need not be added has no theoretical basis, so further investigations are needed to provide a reasonable guideline for design.Based on the relationship between the buckling load of a rigid column and the bracing stiffness, the critical stiffness requirement of the bracing for a single column was obtained. The critical stiffness requirements of the ideal frame bracings were derived when the cross bracings or the connecting bars were rigid. The relationship between the buckling load of the longitudinal frame and the stiffness of bracings and the formula for the critical stiffness requirement were put forward using the analytical method. In the actual frame, the bracings should not only satisfy the stiffness requirement but also the capacity requirement. With initial imperfections being considered, the ultimate load and the bracing capacity requirement were obtained, the requirement was amplified to ensure the stiffness not be zero when frame buckling occurs. If the cross bracings are located at the two ends of the frame, bracing requirements can be calculated with half number of columns.The longitudinal bracings not only maintain the frame stability but also carry the horizontal loads. In the hinged frame, horizontal loads are fully resisted by the cross bracing and the vertical loads of the columns connected with cross bracing are changed due to axial force in the diagonal...