Effect of Wind Induced Unsteady Vortex Shedding, Diurnal Temperature Changes, and Transit Conditions on Truss Structures Supporting Large Highway Signs TR-687, March 2018

Highway sign structures are called upon to support a variety of signs including large dynamic message signs (DMS) to better manage traffic flow by providing accurate and timely information to drivers. Overhead truss structures are employed to support DMS cabinets. DMS cabinets are much heavier, and have different depths and aspect ratios than typical highway signs. The current American Association of State Highway and Transportation Officials (AASHTO) LRFD Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals (AASHTO 2015), which is the main document used for design of sign support structures by state DOTs in the US, do not give clear guidance for estimating wind loads in these situations. This increases the uncertainty in estimating stresses induced in the members of the truss structure supporting the DMS cabinet. Having detailed understanding of stresses caused during the service life of the trusses supporting DMS cabinets is crucial for their safe and economic design. In recent years, there is increasing evidence that the truss structures supporting a variety of large and heavy signs are subjected to much more complex loading than those typically accounted for in the codified design procedures. Consequently, some of these structures have required frequent inspections, retrofitting, and even premature replacement. In order to reliably predict the behavior of these structures, and to design them properly, detailed knowledge of the wind forces is obviously necessary. The first objective of this study is to accurately estimate unsteady wind loads acting on the DMS cabinets and other traffic signs and on the members of the truss structures supporting these signs. The cyclic oscillations of the total wind load associated with vortex shedding behind the signs may be a main contributor to premature fatigue failure. This is because these cyclic oscillations that occur even under steady incoming wind conditions can create a resonance condition. Besides wind loading, the highway sign structures may be subjected to fatigue induced by stresses caused during the transport of trusses to the site and those caused by large diurnal temperature variations. Thus the second objective is to investigate possible fatigue failure due to vibrations during transportation from fabricator to the site where the truss and DMS cabinet will be deployed. The third objective is to investigate diurnal temperature effects on the fatigue life of structures