In terms of determining the risk category to a building component mounted to a building, the component ‘inherits’ the design properties of the structure. So a sign attached to a risk category II structure would be risk category II.
If the sign is freestanding and not part of a main structure, while matching the risk category is a safe approach, an argument could be made that the structure meets the criteria if risk category I and could be designed as such. The sign could be so designed to ‘list’ or ‘break over’ in the event of a design wind event if there is no chance of loss to property or injury at a lower design wind speed than that which a building would theoretically fail. This should be considered with extreme caution and used for economical reasons as the code permits.
Risk Categories are defined based on the importance of a structure and are tied into relative wind speeds. For example, a structure with risk category II may be designed with a 170mph wind velocity while the same structure in the same place as a risk category I structure would be designed using a 156mph wind velocity.
To determine wind velocities for various areas using available risk categories, click here for the ATC wind Council website.
For more information on risk categories, click here.
To understand how wind velocity relates to wind pressure, click here.
To calculate wind pressures based on wind velocities, use our interactive wind pressure calculators on our calculator page by clicking here.
Last Update: March 26, 2021