Wind data has been collected in many different ways, varying over time and by location. Wind data collection varies by its averaging time. Many sites collect wind data on an “hourly mean” basis. Obviously, an instantaneous wind within any given hour can be substantially higher than the hourly mean. The question then arises, what is the time interval that quantifies an “instant”?
The American Society of Civil Engineers publishes its Minimum Design Loads for Buildings and Other Structures designated as ASCE-7. This publication is a National Standard for structural design loads including wind. This standard has been adopted by the majority of U.S. Building codes, including the widely adopted International Building Code
The older U.S. wind standards (ASCE 7-93 & prior) used the “fastest mile” wind speed which is the average speed an air particle would travel over a one-mile distance. Current U.S. wind standards have since changed to a “three second gust” wind speed which is the highest average speed measured over a three second time period. This means that for a given wind event, the wind speed reported for a three second gust will be higher than the fastest mile and both will be higher than the hourly mean.
Wind speeds are also dependent upon the Mean Recurrence Interval (MRI) which uses a probability of occurrence. The MRI will vary based on the wind standard being utilized and the structural category within that standard. The higher MRI, the lower probability of the occurrence which yields a higher design wind speed.
Previous U.S. wind standards (ASCE 7-05 & Prior) utilize an MRI of 50 years, then adjusts based on an Importance Factor to account for structural category. This results in a Service Level wind speed which is then utilized with a load factor of 1.6 for Strength Design (LRFD) and 1.0 for Allowable Stress Design (ASD).
Recent U.S. wind standard (ASCE 7-16) utilize an MRI varying from 300, 700, 1,700 and 3,000 years for Design Categories I, II, III & IV respectively. This results in an Ultimate Level wind speed which is then utilized with a load factor of 1.0 for Strength Design (LRFD) and 0.6 for Allowable Stress Design (ASD).
It is important to note that the resultant wind speed per the older standards (ASCE 7-05 & Prior) were Service Level versus Ultimate Level in the more recent standards. The Service Level wind speed will be lower than the Ultimate Level wind speed; however, both speeds will yield similar results once the appropriate load factors are applied. As well, the MRI will be consistence when accounting for the adjustments in the design category (Importance Factor) and the load factor.
Various coefficients are used to modify the basic wind pressure. These coefficients can include Gust Response Factors, Exposure Factors, Velocity Pressure Coefficients, Shape Factors (or Force Coefficients), Topographic Factors, Directionality Factors, Ground Elevation Factors and Importance Factors. All of these are dimensionless factors that adjust the magnitude of the basic wind pressure for use in structural design.
Most states and some municipalities have adopted Building Codes that establish minimum design criteria for many aspects of construction including wind loads. Some Building Codes such as the International Building Code reference ASCE-7 in it’s entirety. Some local governments adopt ASCE-7 as a criteria basis, but arbitrarily increase certain quantities established by that Standard. It is important to note that the American Society of Mechanical Engineer’s ASME STS-1 Steel Stack Standard adopts ASCE-7 but makes some modifications to the load distribution based on the structure’s dynamic response to wind for a flexible structure.
