Dynamic characteristic of tall industrial chimney estimated from GPS measurement and frequency domain decomposition

This paper is concerned with the analysis of the capabilities and limitations of the use of horizontal dynamic displacements measured by the Global Positioning System (GPS) technology along a vertical profile of a tall industrial chimney under wind excitations to investigate its dynamic characteristics, i.e. the natural frequencies, mode shapes and structural damping ratios. The analysis was performed using Frequency Domain Decomposition (FDD) approach. The basic characteristics of the background noise of GPS technology are examined in the time and frequency domain based on the result of a static test. The field tests were conducted on the chimney, located in Belchatow Power Plant in Poland, using three GPS rover units. The GPS units were installed at three levels of the chimney to measure its horizontal deformation along a vertical profile due to the wind. The GPS receivers were able to measure only the first natural frequency of the chimney. The reasons of the harmful multipath and shielding effects and their influence on the reliability of the measurement results were explained. In order to mitigate the spectral leakage effects as well as to improve the accuracy of damping ratio estimation the windowing procedure was adopted with a selected number of samples LN, determining the duration of a window function. The influence of the LN number on the damping ratio estimation was examined and suitable LN number was recommended. The dynamic characteristics of the chimney were investigated based on the dynamic wind response considered in selected directions in the horizontal plane. The obtained experimental results were compared with the predicted values derived from the finite element analysis as well as with the experimental results based on a measured tip displacement of the chimney by GPS technique using Random Decrement Method. It was shown that the structural dynamic characteristics as well as displacement monitoring of tall slender structures under wind excitations can be effectively determined based on GPS measurements at various levels along the vertical profile.