Overhead sign support structures (OHSSs) are a significant infrastructure item with 231 in service in Winnipeg and an additional 105 under the jurisdiction of the Province of Manitoba as of 2016. Proper design of these structures is paramount because of their proximity to roadways and the risk to commuter safety should they fail. This paper endeavours to provide a review of the advancements made in the governing codes (specifically the _AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals_ (AASHTO Specifications)); review practices across various jurisdictions with respect to selecting and implementing fatigue categories from the various editions of the AASHTO Specifications; review a Manitoba case study covering a significant update to the Standard Designs for Traffic Signals; and finally review a Manitoba case study for typical OHSS rehabilitation measures.
The current AASHTO Specifications suggests designing OHSSs for a minimum 50 year design life and the Specifications have evolved through time to provide the tools necessary to design structures that provide this minimum service life. One of the developments that enables the design of OHSSs with the expectation of 50 years of service is the improved consideration of the fatigue limit state. The combination of the large surface area of the attachments and the long, thin structural members of the OHSSs increases fatigue stresses initiated by wind loading in the form of natural wind gusts, truck-induced gusts, and galloping fatigue, making these important considerations when designing the structures. As recently as the 1994 edition, the AASHTO Specifications referenced the _AASHTO Specifications for Highway Bridges_ for fatigue design of OHSSs. The Specifications have evolved markedly since 2001 when fatigue requirements were first implemented as a stand-alone chapter, independent of the fatigue requirements in the Specifications for Highway Bridges.
The AASHTO Specifications provide recommendations for the fatigue category (essentially an importance factor) selection based on considerations such as the amount of traffic and traffic speed on the roadway where the OHSS is to be installed. Ultimately, however, the decision rests with the jurisdiction that governs the location to select the fatigue category. This has led to varying design practices across jurisdictions depending on several factors, including how the structures have performed historically in that area, the perception of how conservative the designs that stem from each of the three fatigue categories are, and the consideration of the initial cost of the structure.