Wind turbines are kinds of electricity generators, proliferating nowadays due to their consistency with the environment. The rotation of the wind turbine blades duo to wind, burdens some frequencies on the wind turbine tower. Therefore, in addition to common design due to service loads, the wind turbine tower should be checked in the frequency analysis so that its natural frequencies do not coincide with the frequencies caused by blades rotation, resulting in the resonance of the tower and its failure. This makes the design of these structures to be complex. In this research, the frequency analysis of the lattice tower of a three-blade horizontal-axis wind turbine including different sources of nonlinearities, i.e., geometric, material and joint slip effect is carried out and the natural frequencies are obtained using the developed Nonlinear Analysis Software for Towers -NASTower-. It is observed that the joint slip effect can substantially reduce the natural frequencies of the lattice tower, which are significant in the design of these structures. In addition, the wind turbine displacements due to different design load cases are investigated, indicating that incorporating the joint slip effect into the analysis substantially increases the wind turbine displacements, which could affect on its performance.

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