Fatigue Design of Sign Support Structures for Loading Caused by Natural Wind Loads

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.