Italian Workshop on
Shell and Spatial Structures
Charis Gantes
National Technical University of AthensBiography
Charis Gantes is Professor and Director of the Institute of Steel Structures in the School of Civil Engineering at the National Technical University of Athens (NTUA). He attended the German Highschool of Athens (Dörpfeld Gymnasium), and then obtained a Civil Engineering Diploma from NTUA in 1985, and a Master’s (1988) and Ph.D. (1991) from the Massachusetts Institute of Technology (MIT). Since 1994 he is faculty member in the Institute of Steel Structures at NTUA, where he is teaching the courses “Steel structures I”, “Steel structures II”, “Nonlinear behavior of steel structures”, “Cable and membrane structures” and “Steel structures for marine applications”.
His research activity is in the area of structural behavior, analysis and design under extreme loads, including seismic, wind and blast, leading structures to nonlinear response, with emphasis on steel structures, and in the evaluation of the ultimate strength of steel members, connections and structures. He is particularly interested in steel structures for energy applications, such as wind turbine towers, buried pipelines and power plant facilities. He is author of one book in English, on deployable structures, and three books in Greek, on design of unconventional steel structures, nonlinear behavior of steel structures and tension structures, and editor of two collective volumes as well as author of two more. He is also author of more than 10 book chapters, 110 peer-reviewed journal papers and 175 conference papers. His research work has received more than 2500 citations, excluding self-citations and citations by co-authors.
He is member of the Greek mirror Committee of CEN/TC250, ELOT/TE67 Committee on “Eurocodes”, coordinator of the Greek mirror Group of CEN/TC250/SC3, ELOT TE67/OE3 “Eurocode 3” and representative of Greece in Committee CEN/TC250/SC3 for Eurocode 3 of the European Committee for Standardization, CEN. He was Member of two CEN Project Teams on Eurocode 3, namely SC3/T1 on EN 1993-1-1 (General rules and rules for buildings) and SC3/T11 on EN 1993-3 (Masts, Towers and Chimneys), which were part of the development of the second generation of Structural Eurocodes. He is Editor-in-Chief of the Journal of the International Association for Shell and Spatial Structures (IASS) and correspondent for Greece of Structural Engineering International (SEI), the journal of the International Association for Bridges and Structural Engineering (IABSE). He is member of several Greek and international scientific and professional organizations, reviewer for more than 50 international journals and co-organizer of Greek and international conferences. He has been invited speaker at several Greek and international Universities and at continuing education seminars in Greece and Cyprus.
In addition, he is active in structural design and consulting in Greece and abroad, having participated in design projects including steel and reinforced concrete buildings, condition assessment and strengthening of old steel, reinforced concrete and masonry structures, long-span steel structures for athletic, industrial and commercial facilities, power plant and waste treatment plant structures and other energy related industrial facilities, wind turbine towers and their foundation, buried pipelines transporting oil and natural gas, port and marine facilities, guyed towers, temporary bracing for deep excavations, structures for the 2004 Athens Olympic Games, highway infrastructure projects including structures for “Attiki Odos”, the Athens peripheral highway, underground structures including tunnels and stations for “Attiko Metro”, the Athens subway and he has participated in expert committees for the resolution of technical differences.
Structural Challenges Encountered in the Design of Tubular Steel Wind Turbine Towers
Considering the global trend of gradually replacing fossil fuels by renewable energy sources, wind energy is gaining great attention nowadays as a mature type of cost-effective renewable energy. The power of wind turbines has increased over time and is still growing rapidly. In 1985, standard wind turbines had a nominal power of 0.05 MW and a propeller diameter of 15m, while today they have a power of 4 to 5 MW for onshore and 6 to 10 MW for offshore installations. Horizontal axis wind turbines have prevailed nowadays.
Several structural systems have been proposed and implemented for wind turbine towers, such as lattice structures and tripods, but the dominant type in the last decades is the free-standing tubular steel tower, which can structurally be classified as a cantilever shell. While the shell diameter may remain constant or vary continuously over the tower height, leading to an overall cylindrical or conical shape, or a combination thereof, the wall thickness varies stepwise, in accordance with the fabrication process. Moreover, tower sections are connected by means of ring flanges, which also act as stiffeners.
Such towers are manufactured by roller bending steel plates into the desired cylindrical or conical shape, and then welding the two adjacent edges together, to obtain a closed shell, known as “can”. The length of cans depends on the width of plates that are roller bent, often varying between 2.5m and 3.0m. The wall thickness of each can is constant, equal to the thickness of the corresponding plate from which it is produced. Cans are then welded to each other in the factory, to obtain longer cylindrical or conical shells, known as “sections”. Ring flanges with pre-drilled holes are welded to the sections at both ends, to enable bolted connections of each section to the adjacent ones. The large tower diameter enables bolting from inside, thus ring flanges are usually internal, with the exception of the bottom ring flange used to connect the tower to the foundation, which in most cases extends both internally and externally to accommodate a larger number of anchor bolts. In the last phase of works taking place in the factory, sections are painted and fitted with internal equipment, such as ladders and cabling.
Then, sections are transported to the wind farm location by trucks. The associated transportability constraints, dictated by the truck size but also by the obstacles along the itinerary, such as narrow, winding roads, as well as passing through tunnels and under bridges, determine the diameter and length of sections, which are commonly under 4.5m and 30m, respectively. At the site, sections are lifted by cranes or, sometimes, by helicopters, and are bolted together with fully preloaded bolts, using the ring flanges at their ends. The fabrication, transportation and erection sequence described above, is closely related to the geometric and structural features of the tower, and must, therefore, be considered carefully for the structural verifications.
The structural action effects on such towers are dominated by axial compression due to the weight of the tower itself and the imposed weights at its top, due to nacelle, hub, gear box and blades, combined with bending due to lateral wind pressures on the tower and, mostly, acting on the rotating blades and transmitted by the rotor at the tower top. The most common types of structural failures, and corresponding required verifications, comprise local buckling failure of the tower shell, fatigue failure at its connections, and resonance between the tower and the rotating blades.
In this presentation the research activities carried out at the Institute of Steel Structures of National Technical University of Athens towards improved design solutions for wind turbine towers will be outlined. The following issues will be discussed:
(a) Tower modeling and analysis for buckling verifications.
(b) Experimental and numerical investigation of manhole and ventilation openings in the tower shell.
(c) Experimental and numerical modeling and analysis of connections between adjacent tower sections.
(d) Tower shell thickness optimization with respect to resonance.
(e) Tripod alternatives for onshore wind turbine towers.