About the research
The Load and Resistance Factor Design (LRFD) Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (LRFD-SLTS, First Edition 2015) requires the use of a nominal stress range for fatigue design of connections. These stringent criteria for the sizes of the cut outs in the tubes are based on the results of National Cooperative Highway Research Program (NCHRP) Project 10-70: Cost effective connection details for highway sign, luminaire, and traffic signal structures where full size specimens with reinforced and unreinforced hand holes were fatigue tested. The hand holes and the reinforcement evaluated were designed based on Wyoming Department of Transportation (WYDOT) standard drawings (only one available at the time of the project).
While the NCHRP evaluated hand hole configuration covers many design situations, in certain cases particularly for smaller diameter poles in the respective classes of structures (sign/signal supports and high-level luminaire support), the required hand hole exceeds 40% of the pole diameter. A limited review of the currently fabricated structures reveal that the high level luminaire structures in the range of 55 to 100 ft. height are mostly affected, where openings as large as 87% of the pole diameter (or about 28% of the pole perimeter) have been used. In sign and signal support structures, poles of 10 in. diameter or less appear to be the most impacted. Unfortunately, there are a significant number of these structures in service and planned for future installation. In other situations, multiple openings may be needed on a particular section that cumulatively exceeds 40% of the pole diameter.
Application of the stress concentration factor of 4.0 on these poles results in either:
- Increasing the thickness of the tubes
- Increasing the dimeter at the elevation of the hole which results in requiring larger foundations and also more thickness
In one example of the impact, the Oklahoma DOT (ODOT) had poles with diameter of 7.8 in. that with a 4.69 in. hole resulted in a 60.2% opening. With the new requirements, their poles need to be designed with a diameter of 11.72 in. This additional diameter resulted in a more expensive pole and pole system.
As such, a more comprehensive set of design guidelines that is based on appropriate sizes representative of the roadway lighting poles with smaller diameters need to be developed.