Stochastic Subspace System Identification of a Steel Transmitter Mast

The design of transmitter masts is determined by the wind load. The mast should sustain the load without excessive stresses but moreover in order to prevent malfunctioning of the antennae, the rotation at the top has to be limited. Because the time dependent character of the wind, the response is inherent dynamic and susceptible to resonance effects. The dynamic magnification at resonance is largely dependent on the damping ratios of the lower vibration modes. Therefore a vibration experiment was performed on a steel transmitter mast in order to determine these damping ratios. A by-purpose was to derive the stiffness characteristics of the pile foundation from the experiment. Since it is very difficult, if not impossible, to measure the dynamic wind load, only response measurements were recorded. The stochastic subspace system identification method is suited to extract the modal parameters from such output-only data. Due to symmetry of the structure, closely spaced modes are likely to occur. This is confirmed by a Finite Element (FE) Analysis of the mast. Finally, the experimental results are compared with the FE-results.