The study of the dynamic response of slender structures due to downburst outflows through full-scale monitoring

The wind-excited response of structures has been extensively researched in the past many years and as a result, proper approaches and testing procedures have been proposed to avoid undesirable vibration and failure of structures due to wind. However, these researches and approaches are addressed to synoptic winds, which are large-scale phenomena covering thousands of kilometers and lasting up to a few days. The wind speed time history during these events is characterized by a constant mean wind speed averaged over 10 minutes to 1 hour and a zero mean stationary fluctuating component. Recently failure of structures due to transient events that are temporally and spatially small, and characterized by non-stationary wind speed fluctuations are being documented. One of these transient events that are documented to be responsible for the failure of light slender structures is downburst outflows. In the past 20 years, Wind excited response of structures due to downburst outflows has been studied using wind speed data, wind tunnel simulations, and computational fluid dynamics applications. These studies resulted in analytical approaches for the wind field model and dynamic excitation calculation. However, unlike synoptic winds, there is no design approach that is collectively agreed upon by researchers and codified in guidelines for design against downburst winds. Full-scale wind and structural response monitoring is practiced in wind engineering since the earliest construction of tall structures. It has been used to validate theoretical design approaches and wind tunnel testing procedures against atmospheric boundary layer winds. Its application for small-scale events such as downburst outflows is difficult because of the unpredictability of the occurrence of these events. In addition, their small spatial and temporal scale also make the selection of the monitoring site difficult because the probability of registering a downburst event at the monitored structure can not be predicted with high certainty. On the other hand, the validation of the analytical models of downburst wind can not rely on wind tunnel and computational fluid dynamics simulations because of the unavailability of 3-D wind field data during downbursts. This makes full-scale wind and structural response monitoring of structures the most reliable method of validating theoretical models. In light of this, continuous long-term monitoring of the wind-and-structural response of three slender structures was implemented by the GS-WinDyn research team at the University of Genova. The study presented in this dissertation focuses on one of the three monitored structures which is a slender lighting pole located in La Spezia, Italy. First, the ambient vibration data of the structure was used to investigate the dynamic properties of the structure through operational modal analysis. Then, case studies of downburst events were extracted from the registered monitoring data to analyze the wind and structural response during these events and to investigate the correlation between wind speed and structural response parameters. The response of the structure was calculated in the time domain considering simplified models and assumptions for the wind field and aerodynamic loading. The result was compared with the registered response of the structure to highlight the level of uncertainties in the considered models and assumptions. Furthermore, a detailed review and validation of selected analytical dynamic response calculation methods, that have a complete framework for engineering applications, was done. The simplicity of the monitored structure, and the possibility of obtaining both the quasi-steady and resonant components of the structural response using strain registrations, will make this study a benchmark for the validation of methods and simulations of downburst wind load and response.