Fatigue loading and design methodology for high-mast lighting towers

Description

Recent failures of high-mast lighting towers (HMLTs) have raised questions about the robustness of the existing inventory of these structures. Safety of the motoring public is an obvious concern as many of the collapsed structures have fallen on, or near, the adjacent roadway. Fatigue failures have occurred at the baseplate-to-column weld, handhole detail, and anchor rods, with several of the failed structures being in service for less than 5 years. The goal of NCHRP Project 10-74 is to increase the reliability of HMLTs through investigation of wind-induced loads, resulting in the development of rational design criteria for fatigue design of HMLTs. Long-term field monitoring was performed to evaluate the in-service response of HMLTs; specifically, the magnitude and frequency of wind-induced loads experienced by these structures. Data were collected from 11 HMLTs over the course of 2 years. Wind speed data and stress-range histogram data were compiled for each HMLT, forming the basis for the proposed fatigue design loads. The concept of a fatigue-limit-state load for infinite life design was investigated, and found to be appropriate. Extrapolating the histogram data for a typical 50-year lifetime shows the total number of load cycles accumulated will exceed the limiting number of cycles at the constant-amplitude fatigue limit for the most common HMLT fatigue details. In addition, the compiled stress-range histogram data do not differentiate between loads from wind gusts and vortex shedding; therefore, the concept of the combined wind effect is introduced. Combined wind effect considers both gusting and vortex shedding effects together, eliminating the need for separate computations for fatigue design loads. The proposed fatigue design load incorporates the fatigue-limit-state load with new importance categories that consider consequence of failure. The fatigue-limit-state load corresponds to a static pressure-range load cycle with a 1:10,000 probability, the established endurance limit for steel. This concept is similar to the fatigue truck used for the Fatigue I limit state in the AASHTO LRFD Bridge Design Specifications, which is used for infinite fatigue life of bridges. A reasonable relationship between mean wind speed and fatigue load also is developed to increase the reliability of structures exposed to sustained wind speeds above the national average. The proposed importance categories intend to increase the reliability of structures whose failure poses a direct risk to the motoring public. In short, the recommended static-pressure range values for the fatigue design of HMLTs, in order of increasing reliability, are 5.8, 6.5, and 7.2 psf. Other relevant findings presented in this report include: effects of vibration mitigation, which can significantly increase life or decrease effective load; dynamic properties of HMLTs, which may be useful for in-depth analysis; aerodynamic properties of the flow around multi-sided sections; and a proposed method for fatigue life evaluation of HMLTs--

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