Fatigue assessment of sign structures for truck-induced gusts

Abstract

In this thesis, a fatigue assessment was performed for structural support such as highway signs, luminaires, and traffic signals. A particular focus was on truck- induced gusts that occur when trucks pass beneath a structure in order to estimate whether fatigue stress due to truck-induced gusts is larger than the threshold stress or not. Because it is difficult to simulate truck-induced gusts in a wind-tunnel test, this research was carried out through field tests. The target of the field tests was involved the use of a variable message sign (VMS) which is subjected to the biggest fatigue stress according to the AASHTO due to its large projected area in the horizontal plane. Three types of the sensors were used in the field tests with a focus on the fatigue-critical part which is the lower mast arm. Three and eight strain gauges were installed on the bottom and the front of the mast arm, respectively, in order to estimate the range of the fatigue stress. Three pressure sensors were installed on the bottom and the front of the VMS, respectively, to understand the characteristics of truck-induced gusts and an anemometer was installed at the same height of the VMS for use in distinguishing the effects between truck-induced gusts and natural wind gusts. Measured nominal stress was calculated in order to obtain the fatigue stress range by the rainflow counting algorithm and compared with the threshold stress suggested by details of the fatigue-critical part according to the AASHTO. In addition, the characteristics of truck-induced gusts, as analyzed from measured wind pressure data, were verified through data from both wind pressure gauges and strain gauges. The measured fatigue stress range was compared with an equivalent static gust pressure. In other words, the equivalent static gust pressure which accounts for the measured fatigue stress range was inversely calculated and compared with the design value for fatigue wind loads suggested the AASHTO. During the field tests, truck-induced gusts were only estimated because the high wind velocity was not measured. However, if the a larger fatigue stress range had occurred when effect of natural wind gusts was added to truck-induced gusts, it became necessary to analyze the effect of natural wind gusts in a subsequent research.