Field Monitoring and Numerical Investigation of Constrained Layer Damper for Mitigation of Wind-induced Vibration of High Mast Illumination Poles

Abstract

High-mast illumination poles (HMIP) are tall and slender structures with low inherent damping. The Kansas Department of Transportation (KDOT) discovered premature cracking near the handhole of several HMIPs in Western Kansas. In addition, multiple video recordings captured significant HMIP vibrations under high wind. This thesis investigates the mechanism of wind-induced vibrations of HMIPs and the potential of using a constrained layer damper (CLD) to mitigate such vibrations. To examine the underlying mechanism of the wind-induced vibrations of the HMIP structures, the fundamental natural frequencies were extracted from the recorded videos using computer vision algorithms and signal processing. Finite element modeling was then used to confirm that the recorded vibration is dominated by the first bending mode. Subsequently, a 100-ft-tall HMIP in west Kansas was selected for long-term vibration monitoring using wireless smart sensors. Data analysis with the long-term monitoring data indicates that buffeting-induced vibration was the leading cause of the excessive vibrations of the monitored HMIP. This finding leads to the decision to use structural dampers for vibration mitigation. To this end, this thesis proposes a new design of a constrained layer damper for reducing the buffeting-induced vibration of HMIPs. The conventional CLD uses a single continuous piece of constraining layer, making it ineffective when applied to circular sections due to the overlapping neutral axes between the constraining layer and the base structure. To overcome this challenge, the proposed CLD has several longitudinal slits added to the constraining layer such that the neutral axes of the slitted constraining layer are separated from the base structure. As a result, the viscoelastic layer is able to develop shear strain under the bending deformation of the base structure, developing viscoelastic damping for vibration mitigation. A series of comprehensive numerical simulations were performed to: 1) compare the damping capability of different viscoelastic materials; 2) investigate the impact of different CLD parameters on the damping improvement for tubular structures, including the thickness of the viscoelastic layer, the thickness of the constraining layer, and the percentage coverage in the longitudinal direction. The study found that the proposed CLD can increase the damping level of the HMIP to 235% of the inherent damping of the HMIP and reduce its steady-state response at resonance by 57%.