Study On Form-finding Methods And Free Vibration Characteristics Of Cable-membrane Structure

Cable-membrane structure is a kind of novel space structure which developed from the middle of 20 century. It is made of soft fabric and clenched by fexible cables or rigid supports so as to obtain prestress producing stiffness and bearing loads. In this paper, initial form finding and free vibration characteristics of cable-membrane structure are studied on the basis of summarizing research status both at home and abroad.The formulation of geometrical non-linear finite element is deduced for analyzing cable-membrane structure. Then the incremental theory of geometrical nonlinearity of linear cable element and plane triangular membrane element on the basis of updated lagrange coordinate are obtained.Different from traditional structures, cable-membrane structure is all-tension flexible system. It is necessary to find initial equilibrium shape meeting boundary condition and prestress distribution. Form-finding is the first and extremely important process in analysis and design of cable-membrane structure. Traditional dynamic relaxation method is improved and form-finding analysis program is compiled. The form-finding method using ANSYS is given. Example results show that the methods presented in this paper is accurate and effective. The improved dynamic relaxation method is better than that using ANSYS at accuracy and stability. The effects of deadweight and prestress ratio on form-finding results are analyzed. Study finds that deadweight brings large effect on the cable-membrane structure having large changes in surface curvature and deadweight in pre-unit projected area. And form-finding results depend on prestress in cable when prestress in cable is far greater than that in membrane, but, the two factors should be considered when prestress in cable is close to that in membrane.Cable-membrane structure is wind sensitive structure. Response under wind load correlates with free vibration characteristics. Free vibration characteristics of cable-membrane structure are analyzed. Results show that the free vibration frequency is small and distributes in succession, and the vibration modes is complex. The prestress in membrane and the structure's span are two main structural parameters influencing free vibration characteristics of cable-membrane structure. The free vibration frequency increases greatly along with increase of the prestress in membrane, and it reduces greatlyalong with increase of the structure's span. In addition, tensile stiffness of cable hardly affects the free vibration frequency, especially the first free vibration frequency.The research results of this paper establishs a solid base for the cutting and dynamic analysis of cable-membrane structure.