| The main structure is the primary lateral resistance system that is composed of mega beam and mega column in steel mega-frame structures. Horizontal strengthened storey, which is composed of upper and down floor and the mega beam, plays a great role in controlling structural horizontal displacement. So, studying the numbers, stiffness and location optimization design of mega beam can provide a foundation for further researching the seismic response behavior and aseismic design method of steel mega-frame structures. At the beginning, the concept and development status of steel mega-frame structures, as well as the status of the research on the horizontal strengthened storey is introduced roughly, which can be used for reference. Based on that, the influence of numbers, stiffness and location of mega beam on steel mega-frame structures is further studied and two representative mega steel-frame structures are selected as examples. Changing the numbers, stiffness and location of mega beams for different calculative modes are hypothesized. Five typical earthquake waves and four response spectrums are selected, which represent different site conditions respectively. Using finite element program SAP2000, three-dimensional history time analysis of two steel mega-frame structures with different calculative modes subjected to different earthquake ground motions are conducted. The results of horizontal structural displacement and internal force are obtained. In order to study the influence of mega beams on steel mega-frame structures, results through different calculative modes are compared here. Some conclusions are obtained as follows: 1. It's not reasonable for a steel mega structure with overmuch mega beams, because of the phenomenon of stress concentration, in which results increases of top displacements instead of decreases. 2. The variety of mega beam positions has great influence on responses of structures under earthquake, but little influence under the response spectrum seismic loading. 3. The variety of mega beam stiffness is achieved by varying the cross-sections of the mega beams, while keeping the heights of the mega beams invariable. Comparing the results of internal force and displacement of different modes, it can be known that the variety of cross-sections of mega beams has less influence on the responses of the structures. In addition, by analyzing the results of the different modes, it can be found that the modes of the two real steel mega-frame structures are reasonable. On the basis of these, the relationship between top displacements and linear stiffness ratios of mega beam and mega column subjected to horizontal loads, and between top displacement and positions of mega beams are studied by numerical analysis method. With different numbers of mega beams, the optimum position of mega beams and the optimum linear stiffness ratio of mega beam and mega column are ascertained. And then, the simplified method of the optimum position of mega beams is gained. Using the simplified method with mode 1, 2 and 3 of steel mega-frame structures, the suggested values of the optimum position and the optimum linear stiffness ratio of mega beam and mega column are achieved. In order to supply references, the suggested values can be applied during designing of a steel mega-frame structure.
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