Thermal Analysis of Highway Overhead Support Structures

Abstract

Dynamic message signs are increasingly used on freeways, expressways, and major arterials to manage traffic flow better by providing accurate and timely information to drivers. Overhead truss structures that allow for wide displays across multiple travel lanes are typically used to support those signs. Recently, cracks at welded toes in these structures have been identified in multiple states. The AASHTO standard specification defines the loads to be considered during design as wind load, dead load, live load, and ice load, and fatigue design for wind galloping, vortex shedding, natural wind gust, and truck-induced gust. Notably missing from this list is any consideration for thermal loads. Although the number of thermal cycles is relatively low, the temperature variations are relatively large. Should these large temperature ranges result in large stresses, the potential for fatigue failures to initiate, principally from thermal variations, exists. The objectives of this study were to investigate the influence of thermal- and wind-induced loads on bridge-type overhead truss support structures and to provide a resource aimed at improving current design specifications for thermal design. To provide a solid baseline for this work, field monitoring of several overhead trusses was conducted. The evaluation of the collected data was important for understanding the structural behavior and for the advancement of any future code provisions. In addition, finite element modeling was conducted to further study strain and stress magnitudes, which were compared with the field monitoring data. The fatigue life of the truss structures was also estimated on the basis of AASHTO specifications and the numerical modeling results.