Elastic Stability Analysis Of The Top-outrigger-braced Frame-Tube Structures

In practice of current multi-story buildings,the outrigger-braced frame- tube structure is a widely used effective structural system. It belongs in dual system according to mechanic characteristic. But the outrigger-braced frame-tube system exhibits improved capability in resisting side-sway and mainstructure overturning effect than classical dual system.From 1960s, when the structure with outrigger was used in practice for the first time, a lot of multi-story or super high-rise buildings at home and abroad, including steel structure, reinforced concrete structures, steel concrete composite structure, applied outriggers as strengthened story in structures. Meanwhiles, the theoretical research and analysis of structure with outrigger is rapidly developing. In general, the subject on the optimal place and the amount of outriggers is more investigated than structure stability. The stability analysis of the outrigger-braced frame-tube structure , that based on a dual system, can further show the mechanic characteristic and be helpful for designers.In this thesis, the elastic stability of the top-outrigger-braced Frame-Tube structures in a sway mode is investigated. The main jobs are as follows:The structure of a single story frame-tube mode with outrigger on top was studied. The concept of equivalent rotationally rigidity is offered and the formula of rotationally rigidity is obtained. By using slope deflection method, some kinds of models with different constraint conditions were studied, and the critical load in sway buckling mode is established. Because the bending stiffness of outriggers outclass the stiffness of common beams and columns, the overall lateral stiffness of the structure is simplified as summation of the lateral stiffness of tube- outriggers structures plus that of frames. Applying the concept of equivalent negative stiffness of axial forces, the approximate formula for the critical load of the system is suggested, and compared with theoretical results. By analysing adouble-story frame-tube mode with outrigger on top, the theoretical result of critical load in sway buckling mode is obtained. Making use of equivalent principle of second order effect, the load on middle stories is converted into that at top and the approximate formula for the critical load of the system is suggested. Then these formulae are extended to multi-story buildings. Comparison with FEM shows a good correlation between them. The models with outriggers in middle and on top respectively is analysed. By using force method the lateral stiffness of structure is obtained. According to different reduced models, the approximate formula for the critical load of columns under constant vertical force is suggested, and campared with FEM.Analysing the frame- tube structure with one and two outriggers as a whole, the equivalent bending and shear stiffness is reasoned. The approximate formula for the critical load of the structure is suggested. Comparison with FEM shows the formula is of great referenced value while menbers satisfying relative stiffness requirement.