Stiffness degradation of prestressed cable-strut structures observed from variations of lower frequencies

A great attraction of prestressed cable-strut structures is that the initial prestresses can guarantee the structural stability. However, increasing the prestress level of these structures does not necessarily improve the structural stiffness. Here, we investigate the influence of prestress on the stiffness of prestressed cable-strut structures, whereas potential buckling of compression struts is taken into consideration. Symmetry representation of internal mechanism modes and group theory is introduced for parallel and efficient computations on the lower frequencies, which are associated with the internal mechanisms. Nonlinear static equilibrium and improved group-theoretic analysis on these cable-strut structures are iteratively performed. Numerical results show that there is a nonlinear correlation between the prestress level and stiffness. Importantly, continuous increase in prestress for a structure with compression struts can decrease the lower frequencies, and thus weaken the structural stiffness. The first natural frequency tends to be zero when the prestress is close to the critical buckling force of certain compression struts. The presented method can be used as a reasonable choice for force-finding and health-monitoring of prestressed cable-strut structures.