Plastic Stability Analysis And Bracing Research Of Multi-story Frame-Bracing Structures

In the practice of current steel structures, frame structure is a widely used structural system. In multi-story buildings, bracing systems are often mounted to improve the lateral stiffness of the whole structures. Frame-bracing structure is a kind of dual-system, among which the bracing system may be shear wall, reinforced concrete core, vertical truss, etc. In current design codes for steel structures, multi-story steel frames are divided into two types, sway and non-sway frames. The effective column lengths in the two types of structures are determined using different tables or formulae. When the lateral stiffness of a bracing system is equal to or greater than five times that of the frame itself, the frame is regarded as a non-sway type, otherwise the frame is classified as a sway type, even if a bracing system is present in the whole structure. By this rule, in many braced frame structures, where the lateral stiffness of the bracing system is less than five times that of the frame (weakly braced frame), the effective length column length is determined as if no bracing exists at all. This is unreasonable because the effect of the bracing stiffness on the lateral stability of the frame is entirely neglected. Therefore, more detailed theoretical analysis on the stability of braced frames is quite needed.The lateral deflections of bracing systems are composed of flexural and shear deformation in practical structures. If the shear deformation of a bracing is considered to be small (e.g., concrete core ) the bracing system was regarded as of flexural-type and If the flexural effect is considered to be small (e.g. cross bracing in low rise frames), the bracing system is regarded as of shear-type. Based upon the elastic stability analysis of flexural-and-shear-typed bracings (includes vertical beam and vertical truss), criterions for classifying the types of bracings were presented in this thesis, where the bracings were divided into flexural, shear and flexural-and-shear types.The stability of frames braced by vertical beams was investigated. Theoretical analysis of systems was carried out where beams and columns were hinged rigid bodies while the vertical beams were elastic, then the analysis was extended to systems in which both the frames and the bracing beams were elastic. The FEM analysis was carried out for many practical frames where both the axial forces and the stiffnesses of members were varied story-by-story. Expression to determine the critical load of frames braced byflexural-typed bracings was presented. How the bracing beams influence the buckling modes of the frames when the bracing stiffness is increased was also investigated. Furthermore, based upon the explicit relationship between critical loads of frames and bracing stiffness, the expression of critical bracing stiffness at which the frames will buckle in a non-sway mode is also proposed.The relationship between the critical load of frames and the stiffness of flexural-typed bracings was extended to more general cases where bracing systems are of flexural-and-shear-type. The expression of the critical stiffness for this type of bracings was also suggested.Based upon virtual displacement method and the updated-Lagrangian approach, stiffness matrices for two-dimensional elastic-plastic beam model in large displacement were proposed, in the derivation of which transverse stress was included because it was believed that part of the 2nd order effects of various stresses, which are in equilibrium with each other and with the external loads, will counteract each other and neglecting either part would possibly lead to incorrect results. The proposed beam model satisfied the rigid-body test requirement. By using general displacement control method(GSP method) to determine the incremental load parameter and using incremental tangent method to deal with material non-linearity, the proposed beam model were proved to be able to track the entire procedure of deformation, numerically effective during buckling or snap-through of structure...