Understanding the Updated ASCE-7 Standard
The American Society of Civil Engineers and the Structural Engineering Institute have updated ASCE/SEI 7, Minimum Design Loads for Buildings and Other Structures–the go-to technical reference for assigning maximum wind speeds to various U.S. localities and thus for determining fenestration design wind load and performance grade (PG) when using the North American Fenestration Standard. ASCE/SEI 7-2010 is already approved for referencing within the 2012 International Building Code, but won't come into effect until after the 2012 IBC comes online and is adopted by the various states (not likely to happen immediately).
Among the notable changes in ASCE/SEI 7-10, the wind speed map and the importance factor for wind loads have been replaced with three wind load maps based on different and newly-defined “return periods” and for structures with different Risk Categories. which replace the former Occupancy Categories. The new wind loads are specified at the Strength Design level, rather than the Allowable Stress Design level.
The Allowable Stress Design approach seeks to maintain serviceability by using safety factors to ensure that applied loads do not exceed the elastic limit (i.e., cause permanent deformation). With Strength Design, the approach is to avoid failure by sufficiently increasing service loads through the application of load factors to obtain the ultimate design load (the point at which the structural element fails). A load factor of 1.6 for Strength Design has been rolled into the design wind speeds specified in the ASCE/SEI 7-10 maps, so the wind load factor to be applied to the wind speed map values in ASCE/SEI 7-10 for strength design is consequently now 1.0.
The return period is the average frequency of occurrence of a wind event of defined strength. For example, an event with a return period of 50 years (commonly referred to as a "50 year storm") implies an annual probability of occurrence of 2 percent. In going from a 50 year model to 300-, 700- or 1700-year recurrence models, ASCE/SEI 7-10 defines more powerful events that have much lower probability of occurring. This implies a design load based on Strength Design rather than per Allowable Stress Design.
The change from “Occupancy Category” to “Risk Category” acknowledges the intent to regulate the acceptable risk of failure for buildings (again implying the Strength Design approach), based on factors other than just the structure’s occupancy, such as the number of persons endangered by a structural failure.
Another important change is that wind load chapter (# 6), has been replaced by six new chapters, #26-#31. The map wind speeds, based on 10 years of more exacting wind speed measurements obtained through improved monitoring technology, vary according to Risk Category and length of return period. It turns out that wind speeds farther from a given storm's core actually tend to be lower than the previous model had assumed. So, the likelihood of destruction at various parts of the shoreline is now predicted to be less than in past models.
That result shows up in the new wind maps. In many regions of the Gulf and south Atlantic coasts, the wind pressures are actually lower than in ASCE 7-05. One implication of this is that the amount of coastline where buildings need impact-resistant glazing or storm shutters will likely be reduced.
The end result of these changes is minor when comparing the design pressure as calculated with ASCE 7-10 vs. ASCE 7-05, but there will most certainly be confusion about proper interpretation of the changes.
To help manufacturers and architects understand the implications of the changes inherent in ASCE/SEI 7-10 and how the design loads relate to exterior fenestration products, the AAMA ASCE 7-10 Review Task Group has drafted a Technical Bulletin entitled Relating ASCE/SEI 7-10 Design Wind Loads to Fenestration Product Ratings. Undergoing balloting, the bulletin will be issued jointly by AAMA, the Window & Door Manufacturers Association, the Fenestration Manufacturers Association and the Door & Access Systems Manufacturers Association.