Billboard Wind-Load Example - Part C

The subject of this example is a 50’ wide by 20’ high interstate billboard sign located in rural Nebraska on a site characterized by flat and open grasslands. The billboard is mounted on two 16” diameter steel posts, with the top of the sign reaching 80 feet above ground level. Part A of the Example is Available Here: https://youtu.be/jCIGUeYygMo Part B of the Example is Available Here: https://youtu.be/_gx8ikSEcco While Parts A and B established the criteria for the sign face, Part C focuses on determining the wind loads acting directly on the round steel support posts themselves, addressing the calculation of the additional force and bending moment contributed by the exposed posts from the ground up to the bottom of the sign (60 feet). The calculation follows ASCE 7-22 Section 29.4 for "Other Structures," incorporating the Variable Velocity Pressure, Directionality Factor, Gust Factor, and Force Coefficient. Unlike the sign face, which uses a single velocity pressure at the top of the structure, the posts require the evaluation of the velocity pressure at various elevations because wind speed increases with height. For this example, the posts are divided into three zones: 0–15’, 15–30’, and 30–60’. By combining these results with the governing load from the sign face established in Part B (Wind Case B), the final design values for the critical support post show that the total force and base moment increase by less than 2%. However, the example illustrates important differences that exist in the procedures for calculating wind loads for different structural elements. 0:00 Introduction 3:00 Wind on Chimneys, Tanks, and Similar Structures 3:40 Directionality Factor 4:40 Force Coefficients 7:08 Velocity Pressure Calculations 13:39 Force Calculations 16:50 Summary