Drag Coefficient Transition for Multisided Pole Cross Sections

The drag coefficients that are provided in the 2001 AASTHO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals address round and multi-sided shapes, which are typically used for tubular tapered poles. The drag coefficients do not appropriately address the condition of a tapered pole with a multi-sided cross section that has a large bend radius or is small in size such that the cross-section approaches a round section. A method was developed to determine the drag coefficient for multi-sided shapes that are nearly round. The proposed method uses linear interpolation between the drag coefficient for round cross sections and the drag coefficient for multi-sided cross-sections. This proposed change would impact wind loads on highway support structures composed of multi-sided tubular members, such as street lighting poles, cantilevered traffic signal mast arm structures, and high mast lighting poles. A step-by-step sample calculation is provided to illustrate the procedure for calculating the drag coefficient using the proposed method.