Reliability consideration for fatigue design of sign, luminaire, and traffic signal support structures under wind load

• Evaluated statistics of fatigue stress range and cycle of support structures under natural wind.
• Estimated fatigue reliability of the sign support structure under wind load.
• Fatigue reliability of the support structures designed with current AASHTO varies widely.
• Fatigue design wind pressure for support structures is calibrated for Canadian practice.

Fatigue failure of sign, luminaire, and traffic signal support structures has been observed. One of the causes is attributed to the along-wind loading (i.e., the natural wind gusts). The Canadian Highway Bridge Design Code (CHBDC) is not specific on the fatigue design wind pressure for these structures. The fatigue design wind load for the support structures recommended in American Association of State Highway and Transportation Officials (AASHTO) is developed based on the “infinite-life” approach, and by assuming that the stress range can be estimated considering that the response due to natural wind gusts can be represented as a constant amplitude sinusoid. The adequacy of this assumption and the implied structural reliability by using the recommended fatigue design wind load are unknown. This study assesses the statistics of the stress range caused by natural wind gusts for the support structures that are amenable for simplified structural representation. The statistics are used together with a probabilistic model of hourly-mean wind speed and fatigue capacity to estimate the fatigue reliability. The results indicate that the recommended fatigue design requirement for the support structures in AASHTO results in a widely varying reliability index. This is because the recommendation does not take into account the damping ratio or stress cycles or spatially varying statistics of wind climate. The results are also used as the basis to suggest an alternative requirement for fatigue design. The parameters involved in the requirement are calibrated based on selected target reliability, and a ready to use Canadian wind map for the fatigue design is provided.