A NEW VIBRATION DAMPING FACILITY FOR STEEL CHIMNEYS

Transcription

1 Gerb_Chimney.doc Prof. Dr. Klein A NEW VIBRATION DAMPING FACILITY FOR STEEL CHIMNEYS H.W. Klein University of Paderborn, now FH Südwestfalen -University of Applied Sciences, Meschede Germany W. Kaldenbach Ing.-Büro Kaldenbach Lohmar, Germany ABSTRACT When designing slim structures such as columns and chimneys, their dynamic behaviour has to be taken into account in calculating their dimensions. When exposed to wind, steel structures are particularly susceptible to vibrations because of their low damping. Various kinds of stimuli can initiate vibrations in a slim structure. Vortex excitation occurs when the frequency of the vortex coincides with the natural frequency of the structure. In addition to high amplitudes ovalling of cylindrical shells can also occur at low pressures. The vortex-induced amplitudes may be reduced by means of aerodynamic devices (vortex breakers) or damping devices applied to the structure. Aeroelastic instabilities and interference effects may also cause damage to such structures. One form of this instability is so-called galloping. Galloping is a self-induced vibration of a flexible structure in transverse bending mode. If such structures are arranged in groups, they can affect one another mutually. This phenomenon is known as interference galloping. These effects depend principally on the ration between the distance and the diameter of the structures in question. In the case of high amplitudes, special steps have to be taken to reduce the resulting stresses. As a rule poor structural damping is the reason for strong movements caused by wind. By increasing the damping, it is possible to construct very tall slim buildings with a high degree of safety. The article outlines the various means of damping and their technical implementation. A concept for passive vibration damping by means of fluid dampers is proposed using a 125-metre high chimney as an example.

2 Gerb_Chimney.doc Prof. Dr. Klein INTRODUCTION High or slim structures such as bridges, high-rise buildings, aerials, or chimneys are extremely sensitive to stresses caused by wind. In the past, many accidents and much damage have been caused by wind-induced vibrations in such structures. In the past 20 years however, considerable efforts have been made to design structures, which are resistant to the effects of wind forces. Various kinds of stimuli can initiate vibrations in a slim structure. Vortex excitation occurs when the frequency of the vortex coincides with the natural frequency of the structure. In addition to high amplitudes ovalling of cylindrical shells can also occur at low pressures. The vortex-induced amplitudes may be reduced by means of aerodynamic devices (vortex breakers) or damping devices applied to the structure [2], [3]. Aeroelastic instabilities and interference effects may also cause damage to such structures. One form of this instability is so-called galloping. Galloping is a self-induced vibration of a flexible structure in transverse bending mode. If such structures are arranged in groups, they can affect one another mutually. This phenomenon is known as interference galloping. These effects depend principally on the ration between the distance and the diameter of the structures in question. In the case of high amplitudes, special steps have to be taken to reduce the resulting stresses. As a rule poor structural damping is the reason for strong movements caused by wind. By increasing the damping, it is possible to construct very tall slim buildings with a high degree of safety. STATUS OF EUROPEAN STANDARDISATION As part of the process of European standardisation by the CEN, the Structural Eurocode 1 "Basis of design and actions on structures" has been compiled. In part 2-4, "Actions on structures - wind actions", this code regulates the procedure for dimensioning structures exposed to wind forces over a wide range of applications. In the case of chimneys for example, the code allows a simplified certification procedure if the ratio of diameter to height does not exceed the limits shown in figure 1. Detailed certification is however required above the line cd = 1.2 in this diagram. This more detailed certification requires the exact measurement of the natural frequencies and, in the case of additional damping measures, determination of the damping parameters.

3 Gerb_Chimney.doc Prof. Dr. Klein Height [m] Diameter [m] Figure 1: Design criteria for lined steel chimneys, ENV 1991 DAMPING POSSIBILITIES If the fundamental structural damping is not sufficient for economical design, special dissipative devices become necessary. For chimneys and high-rise buildings, the so-called pendulum damper has proved to be an extremely effective way of reducing vibrations. Figure 2 shows the basic principles of a system developed by the company KABE Control of Structural Vibration GmbH in collaboration with the University of Technology in Aachen. A pendulum, which is very precisely co-ordinated with the lowest fundamental flexural frequency, reduces the displacements of the structure. The same function is fulfilled by containers of liquid at the top of the chimney. The difficulty with these special dissipative devices is that they have to be extremely accurately adjusted to the lowest fundamental flexural frequency. This means that the frequencies have to be determined beforehand with a high degree of accuracy. A deviation of 10% from the actual frequency considerably diminishes the effectiveness of these devices. Besides this, these dampers cannot be installed until the chimney has been built, which means that it is unprotected during the construction phase itself - unless additional precautions are taken.

4 Gerb_Chimney.doc Prof. Dr. Klein Figure 2: Pendulum Damper (Source: KABE-Engineering GmbH) THE NEW DESIGN The chimney of the power station at Hannover (fig. 3) consists of a steel cylinder with a diameter of D = 4.60 m. Its wall thickness varies from 6 to 15 mm. Its overall length is 77.2 metres. Figure 4 shows how it is installed in the power-station building. The base of the chimney is located at a height of 47.8 metres above ground level within the building, which means that its head is at a height of m. The height of the building roof is m. A damping unit consisting of 8 individual fluid dampers manufactured by Gerb Schwingungsisolierungen GmbH, Berlin (figs. 5 and 6), is installed at roof level. These are standard components in pipe engineering. On one side, the dampers are connected by a steel ring to the chimney, and on the other to the roof of the building.

5 Gerb_Chimney.doc Prof. Dr. Klein Figure 3: The Chimney

6 Gerb_Chimney.doc Prof. Dr. Klein Figure 4. Cross Section Power Station

7 Gerb_Chimney.doc Prof. Dr. Klein Figure 5: Damping device To calculate the damping parameters as required by the Eurocode, the finite element program PAFEC, produced by PAFEC Ltd. Nottingham [4], was used. Figure 7 shows the FE model with beam, damping and spring elements. Figure 8 shows the maximum bending moment in the steel stack as a function of the lowest fundamental flexural frequency for various damping parameters. The actual design has 8 dampers with a characteristic value of k = 1.1*10 5 Ns/m, thus achieving a total damping value of δ = This value is equivalent to about ten times the structural damping prescribed by the code. For a fundamental flexural frequency of 0.62 Hz, the maximum bending moment is only 5000 knm. Without damping, it would be knm. These damping devices therefore reduce the excursion of the chimney head from 2 metres to 110 mm.

8 Gerb_Chimney.doc Prof. Dr. Klein Figure 6: Cross section and top view

9 Gerb_Chimney.doc Prof. Dr. Klein Figure 7: PAFEC finite element model

10 Gerb_Chimney.doc Prof. Dr. Klein Figure 8: Max Bending Moment SUMMARY Using standard pipe-engineering components, a damper unit has been constructed which has none of the drawbacks of the classical chimney-head damper. The unit functions during the construction of the chimney, is effective over the entire frequency range, is easily maintained and can be extended at any time. REFERENCES [1] DINV ENV 1991, Structural Eurocode 1 "Basis of design and actions on structures", 1995 [2] H.W. Klein, Dynamic Behavior of Elastic Supported Stirrers, Proc. Tech. Conf. Pressure Vessel and Piping, Vol.1, pp , Hyderabad, India,1997 [3] H.W. Klein, Wind Induced Vibrations in Steel Columns and Their Damping, Proc. Tech. Conf. Pressure Vessel and Piping, Vol.1, p1, Hyderabad, India,1997 [4] H.W. Klein, Dynamics and Pafec-FE, Proceedings of the PAFEC User Meeting, Frankfurt, 1995 [5] H.W. Klein, W. Kaldenbach, A new Vibration Damping Facility for Steel Chimneys, Proc. Conf. Mechanics in Design, pp , Trent University of Nottingham, UK, 1998