Abstract Summary
Forced vibration tests have always been a reliable method for dynamic characterization of concrete dams. In this field, LNEC has an extensive experience, having conducted a large number of tests on concrete dams. Test methodologies have continuously evolved with substantial improvements in the control of the dynamic actions applied to the dam, the reliability of the structural behavior records, and the processing techniques to identify the structural dynamic parameters. In this context, LNEC uses an eccentric masses vibration generator, which was designed and produced in-house and has been used for several decades in the concrete dam’s field observation. This equipment generates a controlled vibration using a set of masses eccentrically assembled in a rod connected to a vertical rotation shaft. This mechanical component is supported in two bearings inserted in a surrounding steel frame, which is fixed to the dam. The applied excitation force is controlled by the rotation frequency of the electrical engine and by the number and position the masses mounted in the rod. The main objective of this work was the determination of the measurement uncertainty of the excitation force applied by the mentioned dynamic testing equipment. Being a key issue for metrological quality evaluation, the obtained information is essential to assure confidence and rigorous knowledge about the applied excitation force, namely, in extreme situations near dynamical structural safety limits of the observed concrete dam and of the used equipment. In this case, since the excitation force is indirectly measured using a mathematical model, the measurement uncertainty evaluation consisted, in a first stage, in the probabilistic formulation of the input quantities (rotation frequency, masses, radial positions, dimension, diameter and density of the generator’s rod). Experimental work was performed to provide traceability to the International System of Units (SI) in the case of the generator’s masses and rotation frequencies, while information related to the remaining input quantities was obtained from design and production requirements. In a second stage, the dispersion of values related to the input quantities was propagated through the mathematical model, using the Monte Carlo method, allowing the quantification of the excitation force measurement uncertainty and the identification of the main uncertainty contributions by performing a sensitivity analysis. Two experimental cases were studied: (i) the use of five masses in the generator in the frequency range of 1 Hz up to 6 Hz; and (ii) the use of a single mass in the generator in the frequency interval comprised between 5 Hz and 15 Hz. In the first case, the excitation force estimates and expanded measurement uncertainties (considering a 95 % confidence interval) varied between 3,55 kN ± 0,14 kN and 127,68 kN ± 0,91 kN, being rotation frequency the major contribution for the obtained dispersion of force values. In the second case, the excitation force estimates and expanded measurement uncertainties varied between 16,71 kN ± 0,25 kN and 150,4 kN ± 1,8 kN, being the generator’s rod diameter the main contribution for the output measurement uncertainty.
