Modal analysis of an arch dam combining ambient vibration measurements, advanced fluid‐element method and modified engineering approach

Modal analysis, aiming at estimating modal characteristics such as natural frequencies and mode shapes, is fundamental for studying the dynamic behaviours of a structure. This paper presents a modal analysis of an arch dam using three different techniques. The reference one is based on the statistical analysis of the ambient vibration data collected from the dam crest. The two other approaches are both numerical but with different methods (fluid-element and Westergaard) for the modelling of the reservoir and its interactions with the dam. By applying the three techniques to the studied dam and comparing their results, it is demonstrated that: (1) analysing the ambient vibration data through an operational modal analysis method is able to extract the dam modal characteristics; (2) the fluid-element method is effective for arch dams since the first 10 natural frequencies can be accurately predicted once the material parameters are calibrated on the first three modes; and (3) the Westergaard method, a technique with only additional masses, produces significantly under-estimated frequencies for the first few modes if same parameters are used as the fluid-element method; the underestimation can be corrected for several modes by using a higher stiffness parameter but the required value is unrealistic for the case study. Furthermore, a modified Westergaard method is introduced in this paper by using a reduced added-mass coefficient. This method, once the coefficient is calibrated on the 1(st) mode, is able to well predict the partially coupled modes as illustrated with the case study of the Saint-Guerin dam.