Study On The Critical Bracing Stiffness Of Brace-Steel Frame Based On The Overall Buckling Concept

Based on the formula to calculating the flexural buckling critical force of an elastic bar under axially compressing force, and the first-order buckling modal of the brace-frame when the bracing stiffness is relatively small was considered, after decomposing the total deformation into the entire flexural deformation component which is dominating and the storey shearing deformation component, the formula to calculating the equivalent flexural stiffness which is corresponding to the entire flexural deformation component and the formula to calculating the storey shearing stiffness which is corresponding to the storey shearing deformation component are introduced. When calculating the equivalent flexural stiffness of the brace-frame,allowing for the influence of shear lag caused by the finite flexural stiffness of beam, a coefficient accounting for the departure from beeline of the axial deformation of all the columns in one storey was introduced in the formula used to calculate the equivalent flexural stiffness. The storey shearing stiffness of both the brace and frame were added together to form the total storey shearing stiffness, and then the inter storey horizontal rotation under unit horizontal force can be calculated. After the determination of the equivalent flexural stiffness and the inter storey horizontal rotation under unit horizontal force, the equivalent slenderness ratio of the brace-frame was defined and calculated, and the formula to calculating the critical force of brace-frame was deduced in succession.From the formula to calculating the critical force of brace-frame, we can find that the reciprocal of the critical force of brace-frame is equal to the sum of the reciprocal of the flexural buckling critical force component and the reciprocal of the shearing buckling critical force component. So we can get a better known about the influence of the bracing stiffness to the critical force of brace-frame from two points bellow:â‘ The influence of the flexural stiffness of the bracing system to structural flexural stiffness and the flexural buckling critical force component;â‘¡The influence of the storey shearing stiffness of the bracing system to storey shearing stiffness and the shearing buckling critical force component;â‘¢Combining the two influences above, a complete understanding of how the bracing stiffness influence the critical force of brace-frame was get.Based on the understanding above, some concepts about the critical stiffness and corresponding formulas were brought forward, which were defined and deduced for the...