Keywords: wind towers, structural dynamics, steel structures, computational modelling, non-linear analysis.

This paper presents a numerical modelling, performed with the aid of the finite element program ANSYS [1], on a typical wind turbine supporting steel tower. The study is initially centred on the evaluation of the dynamical response of these structures. The model calibration was made by comparisons to experimental data, acquired in a full scale wind tower structure, and proved to be able to fully represent its structural dynamic performance. This paper also presents a geometric and material non-linear finite element analysis to investigate the ultimate and serviceability limit states that control the wind tower structural response [2].

A finite element model for the wind tower studied was developed using four-nodes thick shell elements - SHELL181, that considers bending, shear and membrane deformations using ANSYS [1]. The mesh was refined along the tower using well proportioned elements to avoid numerical problems. The model was composed of 16625 elements with 16667 nodes. The tower main structure was constructed with a S355 steel grade with a bilinear constitutive law considering a 5% strain hardening Young's modulus. The response of the nacelle and the turbine was considered as linear elastic. The adopted Young's Modulus and density was equal to 210kN/mm² and 7850 kg/m³. This strategy enabled an easy evaluation of the tower self weight load. The adopted boundary conditions simulated a cantilever model with a fixed support at the tower base plate.

It can be clearly noticed from results obtained, that there is a very good agreement between the structural model natural frequencies calculated using finite element simulations and the experimental results measured by Rebelo and Silva [3]. Such facts validate the numerical model presented here, as well as the results and conclusions obtained throughout this work. The geometrical and material nonlinear analysis also produced trustworthy results for the model provided,. Future steps of the current research project will envisage a parametric analysis with the developed finite element model to determine the accuracy of the recommendations present in Eurocode 3 [4,5] for these particular structures.

1

ANSYS, Swanson Analysis Systems, Inc., Houston PA, USA, Version 5.5. Basic analysis procedures, second ed., 1998.

2

Sirqueira A. da S., "Comportamento Estrutural de Torres de Aço para Suporte de Turbinas Eólicas", (Structural Behaviour of Wind Turbine Steel Tower Structures), MSc Dissertation (In Portuguese), Civil Engineering Post-Graduate Programme, PGECIV, State University of Rio de Janeiro, UERJ, Rio de Janeiro, Brazil, 2008.

3

Rebelo C., Silva L.A.P.S. da, "Measurement Plan of a Wind Steel Tower in Espinhaço de Cão, Lagos, Algarve, Portugal (Revision 1), HISTWIN - High-Strength Steel Tower for Wind Turbines, Civil Engineering Department, Faculty of Sciences and Technology, University of Coimbra, 2007.

4

ENV 1993-01-01: Design of Steel Structures, CEN, Brussels, 2003.

5

ENV 1993-01-06: Strength and Stability of Shell Structures, CEN, Brussels, 2004.

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