Vortex Shedding Induced Loads on Free Standing Structures

1 Structural Vortex Shedding Response Estimation Methodology and Finite Element Simulation 1 Vortex Shedding Induced Loads on Free Standing Structures Author: I. Giosan, Introduction Estimating the wind- Induced response of Structures is a main topic in the area of the wind engineering research over the last 40 years [1]. Analyses are usually carried out through theoretical formulations, numerical algorithms, wind tunnel tests, full scale experiments and code provisions. A dominant property of modern wind engineering is that accurate analyses of complex problems are seldom possible by using one of these methods and instead they require the joint application of different techniques For instance, wind tunnel and full-scale experiments frequently provide the input for theoretical solutions, numerical simulations and code provisions; theoretical and numerical methods are often the bases for organizing developing and processing measurements.

2 The growing importance of wind effects on Structures , joined with theoretical, computational and instrumental advances, has given great impulse to these techniques; in the meanwhile, the evolution of each technique is linked and even more inspired by the growth of complimentary methods. All these powerful analytical and simulation tools are required when investigating complex wind Induced phenomena like Vortex Shedding . Vortex Shedding (Fig. ) is the instance where alternating low pressure zones (blue colors) are generated on the downwind side of the stack, as shown in this figure.

3 Fig. Vortex Shedding phenomenon Induced by wind flowing over a cylinder. These alternating low pressure zones cause the stack to move towards the low pressure zone, causing movement perpendicular to the direction of the wind. When the critical wind speed of the stack is reached, these forces can cause the stack to resonate where large forces and deflections are experienced. Every stack has a critical wind speed at which Vortex Shedding occurs. Vortex Shedding can be observed also at larger scale as a meteorological phenomenon.

4 In [Fig. ] is shown an image taken from space of the Vortex Shedding phenomenon behind the island Juan Fernandez along the Chilean rim. Structural Vortex Shedding Response Estimation Methodology and Finite Element Simulation 2 Fig. Vortex Shedding as a meteorological phenomenon. Vortex Shedding is a complex physical phenomenon, especially when it degenerates into lock-in condition. As large vibrations may occur at moderate and frequent wind velocities, Structures may undergo a great number of stress cycles that lead to damage accumulation and may determine structural failure without exceeding the ultimate limit stress.

5 Considering the potential Vortex Shedding fatigue Induced damage it is very important the design procedures to account in a realistic manner for the Vortex Shedding dynamic Induced Loads . Although an immense analytical and experimental effort has been made during the last fifty years to improve the analytical Vortex Shedding prediction models, the design standards are still lacking in presenting concise and easy to use analytical methodologies [3-13]. Because of the complexity of Vortex Shedding phenomenon at this moment there is no general analytical method available to calculate the response of the structure to the Vortex Shedding dynamic Induced Loads [2].

6 Avery detailed comparison on wind Induced response of major codes and standards was made and presented by Working Group E Dynamic Response set up at the First International Codification Workshop held in Bochum, Germany on September 15th 2000 [11]. A comprehensive comparison of the along wind Loads and their effects on Structures was conducted on the major international codes and standards: the US Standard ASCE 7-98, 200 [8], the Australian Standard AS1170-2, 1989 [9], the National Building Code of Canada NBCC, 1995 [3], the Arhitectural Institute of Japan AIJ-RLB, 1993 [10] and the European Standard Eurocode ENV1991-2-4 [6].

7 The comparison study concluded that there are considerable scatter in predictions among these standards and is very important to understand the underlying differences in order to develop unified international codes. An historical overview in the research and simulation areas of Vortex Shedding phenomenon is presented by Giovanni Solari [1]. Structural Vortex Shedding Response Estimation Methodology and Finite Element Simulation 3 Some of the most important pioneering research contributions in the area of wind dynamics have come from the University of Western Ontario (UWO) in Canada and the Technical University Aachen (RWTH) in Germany.

8 The Canadian research team was lead by Dr. Alan G. Davenport since 1965 when he founded the Boundary Layer Wind Tunnel Laboratory. This laboratory has carried out innovative design studies for major Structures . For example, of the 40 tallest buildings in the world, roughly two thirds have been studied at Western Ontario Boundary Layer Wind Tunnel Laboratory. Dr. Davenport has pioneered in the application of boundary layer wind tunnels to the design of wind sensitive Structures . The mathematical spectral model proposed by the Canadian research group is used as the basis for the Canadian National Building Code [3].

9 It gives accurate results for relatively stiff Structures , such as concrete chimneys, in which the aeroelastic effects are relatively small or moderate. For more flexible Structures like steel tubular towers, antenna towers, transmission or highmast towers some of the Canadian research team results are incorporated into Canadian Bridge Design Code [4,5]. The German team research is focused on accurate modeling of large aeroelastic effects and it is incorporated into Eurocode [6]. As mentioned the study of dynamic response of vertical Structures to Vortex Shedding begun in the 60 s thanks to the pioneering contribution of Davenport.

10 In his studies [16,17] he defined the main parameters necessary to analyze the structural wind actions: mean static wind pressure, mean alongwind static displacement, gust response factor and equivalent static force. Studies developed in the 70 s by Vickery [18] and Simiu [19] perfected the method introduced by Davenport especially with respect to wind and aerodynamic modeling. On the other hand ESDU [20] and ECCS[21] introduced procedures to determine the maximum values of structural effects by using influence function techniques.