Wind farm towers are the steel fabricated-tube towers, typically 80 m (262 ft.) in height, serving as the support structures for large, utility-scale power-generating wind turbines. The modern multi-megawatt wind turbine machine is a complex piece of high-tech equipment costing 2 to 3 million dollars each. The turbine's design applies state-of-the-art aerospace and mechanical engineering. In contrast, the structural design of its 20-story tall steel support tower structure is an orphan of structural engineering, for no dedicated US design standard exists for wind farm towers.In US practice, in the absence of specific design guidelines, demonstrating basic building code (i.e., "Code") compliance can be a challenge. It is a struggle that has been faced for many years by structural engineers in the fledgling domestic wind industry. Code compliance is determined using a variety of standards—from AISC, ASME, Eurocode, and miscellaneous textbooks—cobbled together using engineering judgment. Further, wind turbine and tower commercial certification by European agencies requires compliance with various European standards and agency-specific guidelines. For this reason, the application of any US standards must then be rationally reconciled with the European standards serving as the basis of the commercial certification.Further complications are unusual design issues that are seldom encountered in routine building design for which the Code is targeted. Modern utility-scale wind farm towers are thin-shell tubular structures sensitive to local buckling. Subject to turbine operational loads, towers undergo very high-cycle fatigue and must meet frequency separation criteria to avoid resonance. Sensitivity to vortex shedding must be considered. Towers are subject to European certification loads that may in some instances exceed typical Code lateral forces.This paper presents an overview of Code compliance issues and describes some practical approaches and solutions developed over the last decade of wind farm tower structural design.
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