Effective vibration dampers for masts, towers and chimneys

Abstract

Slender steel structures such as masts and towers consisting of or supporting cylindrical elements or chimneys can be susceptible to vortex shedding. Vortex-induced vibrations (VIV) can occur in slender structures because a fluctuating cross-wind force is generated from vortices shed alternately from opposite sides of the structure. Significant VIV occur when the frequency of the vortex shedding is the same as the natural frequency of the structure. If the critical wind speed for VIV often occurs for a structure and the vibration amplitudes caused by the VIV are critical with respect to fatigue, the result can be failure of the structure. An effective method of damping the vibrations can be to install a tuned liquid sloshing damper (TLSD). This paper focuses on the design of a TLSD and compares this with the tuned mass damper (TMD) solution. Compared with the fabrication and installation of a TMD, it is much simpler to make a container and fill it with liquid. The container is often made as a square box or a cylinder that is easily mounted at the top of a mast, tower or chimney, thus providing the most efficient damping of the critical vibration mode. A TLSD is also preferred when it comes to maintenance as only the liquid level has to be checked, whereas a TMD needs maintenance and possibly repair or even replacement of damper parts. A TLSD is designed by calculating the dimensions that will result in a mode shape of the sloshing liquid with a frequency close to the natural frequency of the structure that is to be damped. As the liquid moves in the container, so the sloshing energy of the water reduces the dynamic response of the structure subjected to VIV. Experiments have shown that the TLSD has a rather broadband efficiency and also that it is effective with sloshing liquid mass values down to < 1 % of the effective mass of the mode it is designed to damp. Ramboll has experience with the design of both types of dampers, TMD and TLSD, for broadcast masts and towers and for chimneys. In particular, at a broadcast tower in Norway where a TMD has been replaced by a TLSD, Ramboll has tested the efficiency of the TLSD for comparison with the TMD replaced. This paper will present these results and give an introduction to TSLD design.