Two traffic signal structures recently collapsed in Wyoming. The collapse was the result of a fracture at the connection between the cantilever signal light support arm and the pole connected to the foundation. The Wyoming Department of Transportation (WYDOT) has confirmed that the traffic pole failed at the toe of the welds as a result of fatigue cracking. The structures did not fail under an extreme-event wind, and the fatigue crack growth was most likely caused by vibrations at lower wind speeds.
As a result of the failures the University of Wyoming began researching the signal structure problems. A test pole was supplied by WYDOT, that was originally located in Cheyenne, WY. The pole was removed from service and reassembled by WYDOT personnel at the maintenance yard located in Laramie, WY.
Cantilevered signal structures have very low damping (0.15 to 0.45% of critical damping). In addition, the in-plane (tip displacement perpendicular to the ground) and out-of-plane (tip displacement parallel to ground) motion occur at approximately the same frequency (~0.8 Hz for the Wyoming test pole). The nearly constant winds in Wyoming combined with the low damping result in almost constant motion in the first mode. To reduce the number and magnitude of the cycles the pole sees, UW has been investigating damping devices that can mitigate this vibration problem and lead to longer signal pole service life.
Free and forced-vibration tests are currently being conducted on these devices. Below are photos, videos, and short description of each of these devices and the test results. Further information can be obtained by contacting Trey Hamilton (firstname.lastname@example.org) or Jay Puckett (email@example.com).