Effective Mitigation of Wind Induced Vibration of Traffic Signal Structures and the AASHTO Specification
Mast arm structures, including cantilevered signal supports, are often susceptible to fatigue cracking from the numerous oscillations caused by both normal and extreme winds. Vibration mitigation devices provide an alternative for Departments of Transportation (DOTs) and agencies to consider in lieu of larger poles, more steel, larger foundations, or other previous remedial measures. Such devices act to reduce wind induced vibration oscillations, keeping structure size and costs manageable and adhering to the minimum AASHTO fatigue design recommendations. The vertical metal tubes mount directly onto the end of the mast and utilize both eddy-current damping and pneumatic damping to effectively damper even high-amplitude mast arm movement.
A series of laboratory tests of the vibration mitigation device were conducted at the University of Connecticut. Additionally, the device was field tested in Utah in summer 2016 where seven poles at two separate intersections were retrofit with the damper.
A cost comparison of four traffic signal structures at an intersection in Douglas County, Colorado was performed to demonstrate the cost savings achieved when using an effective mitigation device (the device reduces the need for a galloping fatigue loading case, resulting in smaller and cheaper structures).
- Laboratory tests showed that the performance of the damper is in the range of 97 percent to 99 percent reduction of vibration amplitude.
- Field Testing showed a vibration response reduction from 86 percent (75 ft arm) to 92 percent (45 ft arm).
- The Colorado cost comparison found that the structures’ weights were dramatically reduced in the range of 32 percent to 50 percent, when the damper was used, translating to a 27 percent ($27,500) overall cost reduction for all four pole structures at the intersection.
- Additional qualitative benefits include:
- Better visibility of the traffic signals, especially in installations where programmable traffic signal heads are used.
- Improved vehicle detection when non-intrusive devices such as cameras and radar are used.
- Better stable imagery when surveillance cameras are used.
- Less field maintenance of retightening hardware due to vibration.
- Increased public safety when high profile vehicles pass under mast arms during large amplitude galloping vibration.
- Increased public safety by protecting the structure from fatigue failures due to wind induced vibration, which also limits liability risk.