Vibration-based Assessment of Tensegrity Structures

Sammanfattning: Vibration structural health monitoring (VHM) uses the vibration properties to evaluate many civil structures during the design steps, building steps and service life.The whole function, expressed by stiffness and frequencies of tensegrity structures are primarily related to the level of pre-stress. The present work investigates the possibilities to use this relation in designing, constructing and evaluating the tensegrity structures.One of the aims of the thesis was to improve the current models for resonance frequency simulation of tensegrities.~This has been achieved by introducing the bending behaviour of all components, and by a one-way coupling between the axial force and the stiffness.~From this, both local and global vibration modes are possible to evaluate.~The resonance frequencies are seen as non-linearly dependent on the pre-stress level in the structure, thereby giving a basis for diagnosis of structural conditions from measured frequencies. The new aspects of tensegrity simulations are shown for simple, plane structures but the basic methods are easily used also for more complex structures.The environmental temperature effects on vibration properties of tensegrity structures have been investigated, considering primarily seasonal uniform temperature differences. Changes in dynamic characteristics due to temperature variations were compared with the changes due to decreasing pre-tension in one of the cables. In general, it is shown that the change in structural frequencies coming from temperature changes could of several magnitude as those from damage. Different combinations of materials and boundary conditions are also investigated. These are shown to have a significant impact on the pre-stress level and the natural frequencies when the environment temperature is changed.Coinciding natural frequencies and low stiffness are known issues of tensegrity structures. The former can be an obstacle in VHM, while the later normally limits their uses in real engineering applications. It has been shown that the optimum self-stress vector of tensegrity structures can be chosen such that their lowest natural frequency is high, and separated from others. Two approaches were demonstrated, based on how the self-stress vector will be evaluated: scaling from a base module or considering all modules at once. Both approaches gave the same optimum solutions, even the numerical settings were different.The environmental temperature effects on vibration properties of tensegrity structures were revisited to find a solution such that the natural frequencies of the tensegrity structures are not strongly affected by the changes in the environmental temperature. An asymmetric self-stress vector can be chosen so that the criterion is fulfilled as well as possible. The level of pre-stress can also be regulated to achieve the solution, particularly when a symmetric self-stress vector is chosen. The last part of this thesis, services as a summary of the work. Suggested steps are given to design tensegrity structures for better effective VHM procedures.