18.01: Effective Vibration Dampers: Mast, towers and chimneys

Abstract

ABSTRACTSlender steel structures such as mast and towers consisting of or supporting cylindrical elements or chimneys can be susceptible vortex shedding. Vortex Introduced Vibrations (VIV) can be induced for slender structures because a fluctuating across wind force is generated from vortices shed alternately from opposite side of the structure. Significant VIV occurs when the frequency of the vortex shedding is the same as the natural frequency of the structure. If the critical wind speed for VIV is often occurring for the structure and the vibration amplitudes caused by the VIV are critical with respect to fatigue it can result in failure of the structure. Here an effective method of damping the vibrations can be the installation of a Tuned Liquid Sloshing Damper (TLSD). This paper focuses on design of a TLSD and compares this to the tuned mass damper (TMD) solution.Compared to 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 providing the most efficient damping of the critical vibration mode. Also with respect to maintenance the TLSD is preferred as it only requires that the liquid level is controlled whereas a TMD needs maintenance and possibly repair or even replacement of damper parts.The 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 wish to be dampened. As the liquid moves in the container the sloshing energy of the water reduces the dynamic response of the structure subjected to VIV. Experiments have shown that the TLSD is rather broad‐banded in its efficiency and also that it is effective with sloshing liquid mass down to values below 1% of the effective mass of the mode that it is designed to dampen.Ramboll has experience with design of both types of dampers TMD and TLSD for broadcast masts and towers and for chimneys. Especially for a broadcast tower in Norway where a TMD has been replaced by a TLSD Ramboll has tested the efficiency of the TLSD for comparison with the replaced TMD. This paper will present these results and give an introduction to TSLD design.