Operational modal analysis of a self-supporting antenna mast

Abstract

This paper presents the results of an operational modal analysis on a self-supporting antenna mast. The structure is instrumented with seismic accelerometers, which are moved in different setups for a detailed characterization of the mode shapes. In addition, eight strain gauges are installed, two at the bottom of each leg of the tower. Two particular challenges in the modal analysis are highlighted. First, due to the symmetry of the tower, the bending modes occur in pairs with near to identical natural frequencies and orthogonal mode shapes. These mode shapes change from one setup to another, which complicates the merging of partial mode shapes obtained from different setups. Second, the acceleration signals are polluted by radio signals. It is shown how these two issues are solved, resulting in the identification of five bending modes. The identified natural frequencies and mode shapes are very close to those predicted by a detailed finite element model of the tower, where the vertical columns are modeled using beam elements and the connections between the columns and the braces are assumed pinned. This confirms the validity of such detailed finite element models for the assessment of the dynamic response of these lattice towers under wind loading.