22.5: Design Example: Calculating Safe Diameter for Wind-Exposed Disc

Assessing safety in wind-exposed installations is crucial to preventing potential failures. This example explores the calculation and design adjustments needed to mount a circular disc on a building facade, where wind forces are a primary concern. A 4-meter diameter disc was initially designed as an aesthetic feature facing winds at a velocity of 25 meters per second, with an air density of 1.25 kilograms per cubic meter. Given these conditions, the drag force on the disc was determined using the drag coefficient of 1.1, which measures the resistance an object experiences in a fluid such as air.

When calculated, the drag force exceeded the reaction force capacity of 3,250 Newtons that the wall mounting could safely handle, indicating a potential failure risk. Since drag force is directly related to the disc's surface area, which increases with the square of the diameter, the engineer focused on reducing the disc size to lower the drag force. By recalculating, it was determined that a reduced diameter of 3 meters safely brought the drag force within the wall member's capacity. This example demonstrates the critical role of structural adjustments in ensuring safe designs, particularly in wind-exposed installations, where external forces must be carefully balanced with material and structural limitations.