Dynamic Performance of Cantilevered Sign Trusses for Fatigue

US Transportation Collection2020PDFTech ReportSenior, HunterBennett, CarolineCollins, William E.Li, JianEwing, MarkFadden, MattUniversity of Kansas. Dept. of Civil, Environmental and Architectural EngineeringKansas. Dept. of Transportation. Bureau of ResearchUniversity of Kansas. Department of Civil, Environmental, and Architectural EngineeringKansasUnited StatesAerodynamicsCantileversDesignFatigue (Mechanics)Fluid dynamicsOverhead traffic signsSimulationTrussesVortex sheddingWind pressureStructural analysisK-TRAN: KU-18-3C2106Appendices A1-B (98p.) available as Supporting File.Final Report, July 2017\u2013June 2019Cantilevered overhead sign structures (COSS) experience cyclic loading due to stochastic loads such as natural wind gusts (NWG). Wind loading can produce large deflections in the cantilever and large-magnitude stresses can develop at the box-type connection between the cantilevered arm and mast where fatigue performance is a concern. Modifications to the sign structures pose further concerns as changes to the aerodynamic properties could have unintended consequences. A design consideration of COSS is serviceability of the sign through use of a steel grate walkway that workers can use for maintenance on the sign. The steel grate walkways on COSS are rarely used for maintenance, and thus are being removed by multiple state departments of transportation (DOTs) to prevent vandalism. However, the impact on COSS with the removal of the walkways is unknown. This report describes an investigation in which the dynamic and aerodynamic properties of COSS were studied, and the effect of the walkway presence on structural response was explored. Computer simulations of 32 different COSS configurations were carried out to see the effect of the grate removal on the natural frequency and the mass of the overall structural system. Aerodynamic performance was considered through the use of computational fluid dynamics (CFD) and experimental testing, where wind pressure on the sign in both axial directions and effects of vortex shedding were examined. Findings include: (1) the walkway had a minimal effect on the system\u2019s mass and natural frequencies, (2) the walkway had a minimal influence on the wind-induced force on the sign, (3) vortex shedding was not exacerbated with removal of a walkway attachment, and 4) walkway removal did not significantly change stress magnitudes at the connection between the cantilever and mast. Based on these findings, the removal of walkways from Kansas Department of Transportation's (KDOT\u2019s) COSS was not found to be problematic.120