All ETDs from UAB

Advisory Committee Chair

Fouad H Fouad

Advisory Committee Members

Ian E Hosch

Jason T Kirby

Document Type


Date of Award


Degree Name by School

Master of Science in Civil Engineering (MSCE) School of Engineering


Variable Message Signs (VMS) are becoming an integral part of transportation infrastructure on US interstates and highways. VMS structures are vital in ensuring the safety of motorists by relaying messages concerning potential road hazards such as fog, traffic congestion, highway construction, and lane closures. These structures are larger and heavier than typical flat panel signs, and thus behave differently when subjected to wind loads. The design of VMS structures is specified in the 2013 edition of the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals. The AASHTO Supports Specifications currently provide a single drag coefficient for all VMS applications: "A value of 1.7 is suggested for Variable Message Signs (VMS) until research efforts can provide precise drag coefficients." However, the actual drag coefficient varies depending on a number of factors, including: wind speed, wind direction, structural geometry, and vibration characteristics. The drag coefficient for VMS structures provided in the AASHTO Supports Specifications is an estimate which does not account for the variability of wind loading and structure properties. Accurate drag coefficients for VMS structures are not available to designers, potentially leading to unsafe or inefficient designs. The objective of this study was to develop accurate drag coefficients for VMS to be incorporated in the AASHTO Supports Specifications. Experimental testing was performed at the FIU Wall of Wind testing facility to develop a table of drag coefficients for VMS organized according to aspect and depth ratios. The accuracy of the drag coefficients was verified at UAB using experimental data and FEA dynamic modeling. The analyses showed that the current VMS drag coefficient of 1.7 specified in the AASHTO Supports Specifications is overly conservative. In addition, a sensitivity study was conducted to investigate the impact of the new VMS drag coefficients on the design of sign structures. It was determined that the new VMS drag coefficients significantly reduced the design stresses for both extreme event wind and fatigue level wind. The effects of corner modifications were also investigated and found to cause additional reductions in the VMS drag coefficients.

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Engineering Commons



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