Wind-induced vibrations of cantilevered traffic signal structures

Under certain conditions of wind speed and wind direction, some cantilevered traffic signal structures undergo large amplitude vibrations in wind speeds of 10 to 20 mph. These vibrations are steady 'up and down' motions perpendicular to the wind direction. The vibration amplitudes sometimes reach as high as 2 ft from peak to peak. These large amplitude vibrations may lead to fatigue failures, in addition to being a distraction to passing motorists.

The three primary objectives of this research are: (1) to identify the conditions required for large amplitude vibrations in cantilevered traffic signal structures. (2) to understand the mechanism that produces the large amplitude vibrations. (3) to develop strategies to mitigate vibrations in cantilevered traffic signal structures.

Preliminary structural analysis was done using finite element models of the typical traffic signal structures to obtain the fundamental frequency and the stress levels reached due to static loading. A better understanding of the across-wind vibration problem, which takes place in steady winds was obtained by conducting tow tank experiments. The vibrations are attributed to a galloping phenomenon, which primarily takes place when the wind is blowing normal to the cantilever arm from the back side of a traffic signal attached with a back plate. This configuration was found to have negative aerodynamic damping from the wind tunnel experiments conducted using a one quarter scale dynamic model.

The most effective mitigation measure was found to be a horizontal wing attached above the signal light. A horizontal wing attached above the signal light increased the aerodynamic damping to make the structure aerodynamically stable.