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

Project Details







Iowa Department of Transportation
Iowa Highway Research Board

Principal Investigator
M. Asghar Bhatti
Co-Principal Investigator
Brent Phares

Bridge Research Engineer, BEC

About the research

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 typically employed to support DMS cabinets allowing wide display over more lanes. 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 loadings than those typically accounted for in the codified design procedures (Stam et al., 2011). Consequently, some of these structures have required frequent inspections, retrofitting, and even premature replacement. Wind and strain resulting from temperature changes are main loads that affect the structures during their lifetime. In order to reliably predict the behavior of these structures, and to design them properly, detailed knowledge of the wind forces and diurnal temperature variations is obviously necessary. The important effect due to diurnal temperature variation was a recent Iowa Department of Transportation (DOT) sponsored project completed by the present research team TR612: ‘Wind loads on dynamic message cabinets and behavior of supporting trusses’ has shown that the diurnal temperature variations may play an important role in explaining the development of cracks observed at some of the trusses supporting DMS cabinets. The study also identified other possible causes for the development of cracks (e.g., unsteady loads induces by vortex shedding behind the DMS cabinets, fatigue induced during the transport of trusses to the site). The study proposes to further investigate probable causes of stresses leading to fatigue cracking in trusses supporting a variety of highway signs including DMS cabinets. The study will be a joint University of Iowa – Iowa State University project and will involve computational fluid dynamics (CFD) simulations, field monitoring and structural analysis. A detailed field investigation will be carried out to study the range of stresses induced by diurnal temperature variations. Detailed unsteady CFD simulations will also be carried out to study cyclic oscillations of the total wind load associated with vortex shedding behind large traffic signs. A complementary field investigation will be carried out to verify CFD simulations. A truss will be monitored during transport from fabricator to the site and the number of fatigue cycles will be estimated to get an idea about whether or not strict guidelines should be put in place on how the trusses should be transported to the site. The ultimate goal would be to prepare a set of detailed recommendations for inclusion in the specifications for design of highway sign support structures. The objectives of this study are to investigate wind and thermal effects in the bridge type overhead DMS truss structures and improve the current design specifications. In order to accomplish this objective, it is necessary to study structural behavior and detailed strain-stress of the truss structures caused by unsteady wind loads associated with vortex shedding behind the DMS cabinet and thermal load on the truss supporting the DMS cabinet due to diurnal temperature variations. The last 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 research plan describes how each objective will be accomplished.