Traffic Signal Pole Research: FHWA-WY-04/03F
Recent traffic signal pole failures have been a catalyst for various research projects in the United States. The two failures that occurred in Wyoming were due to fatigue cracks in the pole that initiated at the toe of the fillet weld between the box connection and the pole (Figure 1).
The fillet weld in question connects the box plate to the vertical pole. Multiple cracks initiated at the toe of the fillet weld along the length of the weld and coalesced prior to failure. Several defects in the column tube plate were found at mid-depth of the crack. In addition, crack growth appeared from both the inside and outside face of the plate, suggesting that crack growth occurred from both the inside and the outside of the tube. It was also determined that failure was caused by applied cyclic loads and not weld defects or procedures .
Visual inspections were performed by the Wyoming Department of Transportation (WYDOT) and indicated that one-third of approximately 840 poles in the state inventory appeared to have fatigue cracks. These crack lengths were estimated to range from 0.25 inches (6 mm) to 20 inches (508 mm).
Visual inspection methods only detect cracks that have propagated to the surface. In addition, cracks in the galvanization can lead to false positive indications. Consequently, the visual inspections may not have given a true indication of the extent of the actual fatigue damage. Due to a growing concern for the structural integrity of existing poles, research was funded by WYDOT at the University of Wyoming (UW).