Full-scale measurements and damping ratio properties of cooling towers with typical heights and configurations

The standard damping ratio for cooling towers is equivalent to that of high-rise reinforced concrete structures, which is 5%. But considering the unique configuration and material properties of the cooling towers, the actual damping ratio of cooling towers is far smaller than the standard value. Damping ratio is an important input parameter for wind and seismic analyses, its value directly determines the anti-wind and anti-seismic safety of the cooling towers. However, few field tests and damping ratio experiments have been conducted for cooling towers at home and abroad. In this paper, we performed field tests of 8 cooling towers with typical height and configuration in the inland areas of China. Acceleration response signals (vibration signals) at typical points of the tower body were obtained under ambient excitation. The measured acceleration signals were preprocessed using the random decrement method (RDM) and natural excitation technique. The natural frequencies and damping ratios of the first ten modes were obtained by combining three modal recognition methods, namely, auto-regressive and moving average (ARMA) model, Ibrahim time domain (ITD), and spare time domain (STD). The measured values were compared against the results of finite element analysis (FEA) and the errors were assessed. Then the equivalent synthetic damping ratios of the 8 cooling towers were estimated based on modal combination. Finally we provided the calculation formulae for the damping ratios and equivalent synthetic damping ratios of the first ten modes by using the fundamental frequency as the objective function and the measured damping ratios of the 8 cooling towers. The measured frequencies were consistent with the results of FEA for the 8 cooling towers, and the maximum difference in fundamental frequency was 4.4%. The damping ratios recognized of each mode showed a dispersed distribution. The maximum damping ratio of the first ten modes was 2.86%. The equivalent synthetic damping ratios of the 8 cooling towers ranged from 1.13% to 2.16%. Error analysis indicated that the fitting formula for the damping ratio had high precision and high stability. The present research provides valuable clues for the determination of damping ratio for large cooling towers and enhances the understanding on the damping mechanism.