The American Society of Civil Engineers (ASCE) Structural Engineering Institute (SEI) publishes Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10), which describes wind load design methods and is the basis for most building codes in the United States. ASCE 7-10 includes wind speed maps for locations throughout the country.

Engineers who were previously accustomed to the wind speeds in ASCE 7-05 discovered that the wind speeds in the newest revision were substantially higher. Why? The answer comes down to two things: return period and relative importance.

• The return period is a statistical measure of the length of time expected to pass between similar events. When climatologists talk about a 100-year flood, they mean a flood whose severity is such that one expects to witness it only once every 100 years or so. A 500-year flood would only occur twice in a millenium. Similarly, wind engineers speak of return intervals for wind events. In this case, it’s the highest wind speed that is sustained for 3 seconds or more, measured 33 feet (10 meters) above the ground. The wind speeds in ASCE 7-05 all have 50-year return intervals. The wind maps in ASCE 7-10 reflect 3-second sustained wind speeds with return intervals of 300, 700, and 1700 years.

• Relative importance refers to the idea that the design and construction of a hospital or power plant needs to be more robust than that of, say, a small outbuilding. Engineers using ASCE 7-05 calculated a single wind load based only on the geographically determined wind speed and then applied an importance factor to reflect the relative importance of the building. In ASCE 7-10, the importance factor is eliminated in favor of three distinct wind maps reflecting different wind speeds for various risk categories.

These two ideas are intimately related, and in fact, relative importance is expressed according to return period: The more important the building or structure, the more robust it should be.

Many engineers may have been shocked to see the new wind speeds in ASCE/SEI 7-10. But peering beneath the surface and discovering how those wind speeds translate into actual wind loads reveals that not much has really changed. It’s more or less the same wind load, but expressed in different ways that more accurately reflect the intent of the code authors.