Because of their inherent fl exibility and low damping ratios, cantilevered mast - arm tra ffi c signal structures are suscepti b le to wind - induced vibrations. These vibrations cause stru ctural stresses and strains to develop in a cyclical fashion which can lead to reduced service life due to fatigue and, in extreme cases, full collapse. In 2001, after the collapse of several of these structures throughout the United States, American Assoc iation of State Highway and Transportation O ffi cials (AASHTO) code standards were updated to include fatigue provisions for tra ffi c signal supporting structures. In New York State, there is particular concern regarding structures spanning longer than 14 me ters which currently do not meet these updated fatigue provisions. To address this concern, a full - scale experiment was conducted using an existing 25 meter mast - arm tra ffi c signal structure, located in Malta, NY, in which the response of the structure was observed in relation to in - situ wind conditions. In previous studies, high - amplitude vertical vibrations of mast - arm tra ffi c signal structures have been shown to be due to vortex shedding, a phenomenon in which alternatingly shed, low - pressure vortices in duce oscillating forces onto the mast - arm causing a cross - wind response. When the frequency of vortices being shed from the mast - arm corresponds to the natural frequency of the structure, a resonant condition is created. The resonant condition causes the l ong - lasting, high - amplitude vibrations , which may lead to the fatigue failure of these structures. Turbulence in the approach fl ow is known to a ff ect the cohesiveness of vortex shedding. Results from this full - scale experiment indicate that the surrounding terrain conditions, which a ff ect the turbulence intensity of the wind, greatly in fl uence the likelihood of occurrence of long - lasting, high - amplitude vibrations and also impact whether reduced service life due to fatigue is likely to be a concern

US Transportation Collection