Structure stability of superstructure in SG-Ⅱ upgrade facility

Based on the fundamental theory of improving structure stability, with the finite-element method (FEM), the research on the structure stability of the superstructure was performed. The results indicates that the third modal shape of the superstructure is the most important factor in the stability of the large aperture mirror, and the the weak link is the frame. With the stiffener with optimized position used between the frame and the concrete-filled steel tubular structure adopted for the frame, etc, the stability of the superstructure could be improved effectively. At last, the stability scheme of the superstructure was proposed which could satisfy the stability requirement of large aperture mirrors.