A unified design method for fatigue loading caused by natural wind for highway overhead sign support structures is needed. Whereas the fatigue design provisions given in the 2001 AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals provide fatigue design criteria for natural wind, these provisions are applicable only within certain limitations. A comprehensive approach was developed for fatigue design for the treatment of sign support structures, considering the dynamic properties of the structures such as the natural frequency and damping characteristics. The approach is applicable for structures in which the dynamic response can be approximated by a single degree-of-freedom system. Typical examples include cantilever-and bridge-type overhead sign support structures. The proposed fatigue design approach incorporates these parameters in a simplified, systematic procedure that can be readily used by the engineer. The analytical work performed involved principles relating to random vibration in use of the vibration response spectrum (VRS). The fatigue load was selected from the VRS in terms of the natural frequency of the structure, the critical damping percentage (a.k.a., damping ratio), and the annual mean wind velocity of the intended site.

This research presents a universal approach for determining the design fatigue load due to truck-induced wind gust for highway overhead sign support structures. The fatigue load is determined on the basis of the dynamic properties of the structure. It is applicable to structures in which the dynamic response can be approximated by a single-degree-of-freedom system. Typical examples include cantilever- and bridge-type overhead sign support structures. The developed approach incorporates these parameters in a simplified systematic procedure that can be readily used by engineers. Principles relating to mechanical vibration and utilization of the shock response spectrum were used to determine the load. The fatigue load is selected from the shock response spectrum in terms of the natural frequency of the structure and the speed of the truck. Comparisons were made with the current truck-induced gust fatigue design provisions given in the 2009 AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals. Additional comparisons were made with dynamic finite element analysis of a modeled support structure.

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