Tech Today

The 2010 edition of ASCE 7, "Minimum Design Loads for Buildings and Other Structures," has been published and incorporated into the International Building Code, 2012 Edition. ASCE 7-10 contains significant revisions to the standard's previous edition, ASCE 7-05. Several of these changes significantly will affect the wind design of buildings' exterior envelopes, including roof systems. Therefore, you should be familiar with ASCE 7-10 and its changes.

Comparing standards

In ASCE 7-10, wind-load requirements are provided in Chapters 26-31. In ASCE 7-05, wind-load requirements were provided in Chapter 6.

ASCE 7-10 provides six primary design approaches applicable to roof systems, as well as a wind tunnel testing method, compared with three design methods contained in ASCE 7-05. ASCE 7-10 provides simplified design approaches for roof heights of 160 feet or less; ASCE 7-05 applied the simplified approach to roof heights of only 60 feet or less.

ASCE 7-10 contains three basic wind speed maps based on a 300-year, 700-year and 1,700-year mean recurrence interval, respectively. Specific building occupancy dictates map selection.

ASCE 7-05 (and ASCE 7-02 and ASCE 7-98) used one basic wind speed map based on 50- and 100-year return peak gust wind speeds. ASCE 7-05's calculation methods also included an importance factor and load factor not included in ASCE 7-10.

Wind-uplift calculation results for sample buildings comparing ASCE 7-05 with ASCE 7-10

Comparing ASCE 7-10's basic wind speed maps to ASCE 7-05's map shows the wind speeds of the ASCE 7-10 maps are higher.

The American Society for Civil Engineers (ASCE) claims these higher values are somewhat offset by changes in ASCE 7-10's wind-load calculation procedures.

The figure shows a comparison of calculated wind-uplift loads for identical sample buildings located in Kansas City, Kan., and St. Petersburg, Fla. From this example, ASCE 7-10's wind-uplift loads appear to be 33 percent to 64 percent higher than ASCE 7-05's wind-uplift loads.

In addition to changes in wind-uplift loads using ASCE 7-10, the changes in the basic wind speed maps also affect asphalt shingles, which typically are designed based on wind speeds. For example, in Kansas City, asphalt shingles with a Class D (90-mph) rating would be appropriate using ASCE 7-05's basic wind speed map. Using ASCE 7-10's maps, Class F (110 mph) or Class G (120 mph) would be necessary depending on the specific building occupancy.

Become more aware

Because ASCE 7-10 provides different—sometimes significantly different—design wind-uplift loads for buildings' roof systems, roof system designers should become familiar with the standard.

ASCE 7-10 is available from ASCE in print, CD and downloadable electronic file formats at www.asce.org.

To help designers use ASCE 7-10, NRCA is updating its online Roof Wind Designer application, which is accessible at www.roofwinddesigner.com. NRCA's updated application will be available later this year.

Mark S. Graham is NRCA's associate executive director of technical services.