Experimental assessment of the long-term variation of modal properties and aerodynamic damping of a self-supporting lattice tower

The dynamic properties of a 50 m tall self-supporting lattice tower are investigated using long-term full-scale monitoring. Two years of acceleration, strain, temperature, and wind speed data are analyzed to obtain the natural frequencies, mode shapes, and damping ratios. The environmental variation of the modal characteristics and aerodynamic damping was studied based on the simultaneously recorded temperature and wind data. Natural frequencies decreased with an increase in outside temperature. The damping ratios of the first four modes ranged from about 0.2% at low wind speed to 0.6% at high wind speeds. A clear increase in the damping ratio with the mean wind speed, by more than double, was found for the first four modes. The increasing trend of the damping ratio in the alongwind direction with the wind speed was closely similar to that obtained using the quasi-steady-based formulation. The results of the study are an important addition to the few available studies investigating the dynamic properties of self-supporting lattice towers using full-scale data, which in turn will be an input to design standards. • The dynamic properties are investigated using full-scale vibration monitoring data. • The long-term variability of the dynamic properties with the environment was studied. • Aerodynamic damping was investigated using vibration and wind speed measurement data.