Date of Award


Degree Type


Degree Name

Doctor of Philosophy


This thesis describes the development of a Pivot Mode Activator System (PMA) for the measurement of motion-induced forces on wind tunnel models of tall structures. The impetus for this was the desire to refine the established base balance wind tunnel technique of predicting wind loads. The system developed forces a sinusoidal oscillation of the test model in a pivoting mode about its base at controlled amplitude and frequency. The wind forces acting on the model are measured, and the motion-induced forces are extracted by correlating the total force with the motion. In this way, both the in-phase and out-of-phase components of the motion-induced forces are distinguished. These quantities are denoted by a complex aerodynamic impedance whose real and imaginary parts correspond to aerodynamic stiffness and damping parameters.;As a demonstration of the use of the system, two substantial experimental studies have been carried out. The first is a comprehensive study of a square prism. This study provides extensive data on the aerodynamic impedance for tip oscillation amplitudes less than about 15% of the model width. The effects of varying the turbulence intensity of the wind and the aspect ratio of the model are also investigated. To complement these results, additional experiments have been carried out for the same model shape. These include forced oscillation experiments on a pressure model which provide information about the spatial and spectral distribution of the forces, as well as free oscillation experiments on several aeroelastic model structures. Rms response predictions based on base balance data, augmented with the measured data on motion-induced forces, are found to agree very well with the corresponding results from an aeroelastic model test.;The second experimental study involved base balance experiments and PMA experiments on various cross-sectional shapes. This study provides a large data set, which along with some judicious interpolation, is expected to be of valuable use to designers of tall chimneys.;The Pivot Mode Activator System has surpassed the initial design expectations. The aerodynamic impedance for a model can be measured with a high degree of resolution. It is hoped that the system will provide a means for expediting fundamental research into motion-induced force models as well as contributing to practical wind tunnel studies.



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