Joint Slippage Investigation of double angle on Wind Turbine Lattice Towers Using Finite Element Modelling

Abstract: Although fossil fuel resources are declining, their use also pollutes the environment. Also, due to the increase in the average wind speed in the world, the use of wind turbines, which are classified in structures with new and renewable energy, will be very cost-effective. Wind turbine towers can be made of concrete, steel, conical, lattice, wood, or multi material. Given that the investment cost to build a wind farm and connect it to the transmission network is 75 to 85 percent, and the cost of building the structure is 15 to 25 percent of the total cost. Steel lattice towers can reduce the cost of building a wind turbine structure by 30 percent and therefore, a complete and correct model for analyzing these types of structures will be very important and economically noteworthy. Wind turbine lattice towers are usually made and executed with bolt connections. In this case, the number of bolts is very high, which increases the need for cyclical and reciprocal loads. Joint slip in these structures refers to the relative displacement of bolt connection under the influence of force. Therefore, creating a joint slip will be inevitable due to the ease and speed of execution in which the bolt hole is made of a larger bolt diameter. Joint slip increase the displacement of lattice towers So much so that the maximum displacement of the tower is twice as high as that of static methods. And not considering it will destroy the tower and assuming the reliability factor will make it uneconomical. In this type of structure, the tower is often made of angle and single angles are used in cross members and bracing and double angles are mostly used in the bases of lattice towers. With this explanation, in this study, the force curve of the displacement of the of three samples with single angle section in the laboratory and four samples with double angle section in Abaqus software was modeled and were affected by reciprocating loads and then the results of numerical modeling were validated with laboratory samples. In the models modeled in the software, after sensitivity analysis, the type and size of the mesh is precisely minimized the resulting error. In this investigation available data in joint slip develops bolt connections which include angles with equal leg. It offers force-displacement curve of different connections for double angles and their connections damping ratio are calculated likewise and the effectiveness of each variable on the slip of the node is expressed in double angles. The results show that joint slip occurs during service loads and this effect depends on the number of bolt, the diameter of the bolt, the bolt cross-sectional area, the thickness of the angle and the effective cross-sectional area among these, screw diameter is the most important variable for predicting joint behavior. Also, the viscosity damping ratio for single and double angle connections is almost equal and can be assumed to be 42.5. This ratio increases with increasing number and decreasing bolt diameter. This investigation is beneficial for designing wind turbine lattice towers and in it, provides structure behavior to the designer more accurately.