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Several mast arm failures have been observed in Florida and other states from fatigue caused by wind-induced vibrations.1 Mast arm structures have been increasingly observed to be susceptible to structural vibrations resulting from both normal and extreme wind conditions. Vibrations are also caused by truck-induced wind gusts along mid- to high-speed roadways. Mast arm cantilevered signal supports are susceptible to fatigue cracking from the numerous oscillations caused by these winds.2 In the past, some practitioners have taken remedial measures to minimize galloping of the mast arms with various levels of success, such as:
* Horizontal signal head mounting or removal of the back plates.
* Relocation of signal heads and signs so to change the mast arm oscillation frequency.
* Use of polycarbonate signal heads to change the mast arm oscillation frequency.
* Use of overlong arm lengths, then cutting the excess in the field to provide a stiffer member.
* Relocating the mast pole and foundation closer to the road to provide a shorter, stiffer arm.
* Installing a flat panel (sign blank) horizontally directly over the signal head acting as a drag against the up and down motion.
* Installing a strand cable to the mast arm upright.
The latest revisions by the American Association of State Highway and Transportation Officials (AASHTO) to address severe wind loads for traffic signal structures are designed to make our streets safer. The high costs involved in producing structures to meet these revisions impact implementation and budgets. The cost of traffic signal installations in many cases have doubled in the past decade, with key contributing factors being the added steel cost of bulkier poles and mast arms, larger foundations, higher costs of labor, and steel costs. Many cities are weary of adopting newer fatigue design recommendations based upon their successful history with the past standards, knowing adoption will add substantial cost for a signal installation.
For the design of traffic, sign, and lighting structures, the Federal Highway Administration (FHWA) requires AASHTO 20133 or later as the minimum design standard for all projects receiving federal funding. Compared to prior additions, this specification update was substantial. This standard changed the risk assessment related to wind loading for most urban intersections. This results in larger poles and foundations that have...