Roadside dynamic message signs with breakaway components handle wind-induced vibrations adequately for an almost 24-year fatigue life. Rural intersection conflict warning signs vibrate noticeably under wind forces and require design changes to resist fatigue damage.
These are the results of a new research study sponsored by the Minnesota Department of Transportation that investigated the effects of wind loading on roadside message and conflict warning signs.
What Was the Need?
Signs alongside MnDOT roadways must meet safety standards for collision response. If posted within the safe or clear zone, sign structures must feature supports that break away when struck by a vehicle. Safe or clear zones may extend to 20 feet from the shoulder or 8 feet from a guardrail, depending on the slope on which the sign stands.
Wind-induced vibrations in sign structures can lead to fatigue in bases, bolts and other components. National standards address fatigue design for overhead and high-mast signage, but not for traditionally small roadside structures. Newer roadside sign designs have been getting larger and heavier. The impact of wind-induced vibrations on larger and breakaway-enhanced structures is not well understood.
Dynamic message signs (DMS) installed alongside highways warn drivers about traffic, emergencies and work zones. These signs have large surfaces and can weigh 1,500 pounds or more. Breakaway support systems for these signs, which are often mounted within clear zones, typically entail a slip plate at the base and friction plates below the sign face. When a signpost is struck, the slip plate pivots and the friction plates split the signpost and break away. Together, these features allow vehicles to pass under the sign with little collision impact.
“This study was very helpful. It eased our concern with DMS and breakaway structures in roadsides, which clearly are performing adequately,” said Jihshya Lin, bridge evaluation and fabrication methods engineer, MnDOT Bridge Construction and Maintenance.
Rural intersection conflict warning signs (RICWS) are smart signs placed at intersections and flash to alert drivers of traffic on the highway ahead. These signs are typically installed within the clear zone and require breakaway components. The standard breakaway system for these roughly 400-pound signs entails a slip base sleeve that allows posts to fracture, bend on impact or pull free from the base, limiting damage to colliding vehicles.
For both types of signs, MnDOT lacks strong data about the impact of wind-induced vibrations on the durability and performance of the breakaway systems.
What Was Our Goal?
MnDOT sought to evaluate the impact of environment-induced vibrations on the support systems and breakaway features of standard DMS and RICWS used in the state. The agency also needed recommended design changes to allow the signs to better accommodate wind forces and fatigue.
What Did We Do?
Researchers began by reviewing available instrumentation for monitoring the response of sign structures to wind. The team then fitted selected devices to a DMS on Highway 169 North in the Brooklyn Park area of Hennepin County and to an RICWS structure at Highway 7 and County Road 1 in McCloud.
Using field data, the research team developed numerical models to simulate and investigate wind loading on DMS and RICWS, and to evaluate the impact of this loading on the fatigue life of the signs.
Scaled models of RICWS were used to test drag and other properties of standard and modified RICWS designs in water tanks and wind tunnels. Further design changes were made based on numerical modeling and analysis.
What Did We Learn?
Field data and modeling indicated that vibration-induced stress concentrates in portions of the DMS support system, particularly in the friction fuse plate that connects posts just below the sign face to the posts anchored at the base. This may impact fatigue. Analysis showed this sign had a fatigue life of 23.8 years, at which point the support system would need repair or replacement.
“The breakaway post connections in RICWS are pretty loose. The sign height, fixity or mass could be adjusted, or the sign area could be decreased,” said Lauren Linderman, assistant professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.
In RICWS, field data and modeling indicated that vortex shedding was the prominent wind action. Vortex shedding causes signs to shed wind force alternately on one side and the other, creating strong vibrations in the sign and support legs. Researchers proposed three solutions that may mitigate vortex shedding vibrations: fixing bases more firmly to stiffen supports, increasing sign weight with pendulum-style dampers and removing background shields from lights to reduce drag.
For RICWS, stiffer posts would reduce crash safety, and the damping weight required is too great to be feasible. Instead MnDOT is considering single poles of aluminum or steel to support RICWS panels and lights, and using lights without background shields.
Research suggests that there would be value in instrumenting and analyzing other DMS designs used in Minnesota. The fatigue life of DMS like the sign instrumented and evaluated in this study is reassuring and suggests that this design is sufficient. MnDOT will continue to install DMS along roadsides, which can cost 30% to 40% less than mounting overhead electronic signs.
MnDOT may choose to fund vehicular impact studies of DMS and RICWS to gather more data on their crash performance.