During a day with a sustained wind speed of 12 mph at the ground level, I captured vibrations of the spire atop the Freedom Tower from approximately 1,800 feet (550 meters) away. After processing these vibration videos with CHI Consulting Engineers, LLC’s in-house computer vision program, StructuralVision, it was determined that the tip of the spire vibrated within a range of ±1.0 inch (25mm). Additionally, three distinct modal frequencies were observed: 0.140 Hz, 0.341 Hz, and 0.906 Hz. The first modal frequency closely matches the estimated fundamental frequency of the Freedom Tower (0.148 Hz) as calculated using ASCE 7 equations. The remaining two frequencies are believed to correspond to the spire's own vibration modes, which were corroborated by a finite element (FE) model of the spire. This FE model utilized exterior dimensions estimated by StructuralVision, along with assumed wall thicknesses and weights based on engineering experience.
This case study showcases the robustness of the StructuralVision program, demonstrating its readiness for displacement and vibration monitoring applications in various bridge and building projects.
Published by Qi Ye, P.E. Founder and Managing Principal of CHI Consulting Engineers, LLC.
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