Why are the wind speeds in Minimum Design Loads for Buildings and Other Structures, ASCE 7-10 so much higher than those in ASCE 7-05?

Wind speeds in ASCE 7-10 differ from those in ASCE 7-05 and earlier. Wind speeds in ASCE 7-05 are based on a 700-year return period divided by a factor (1.265) to adjust design values to a 50-year basis with adjustments for other return periods, importance factors, etc. Wind speeds in ASCE 7-10 are based on a specific return period and there are different return period maps based on the importance of the structure.

The Applied Technology Council (ATC) has developed a web-based tool that provides a way for users to find a site-specific wind speed using the GPS coordinate system (www.atcouncil.org/windspeed). Spatial resolution of wind speed maps currently displayed in ASCE 7 are not sufficient to determine a site-specific wind speed and there are no reference city or town locations on the ASCE 7 maps. While county boundaries are shown, the resolution is degraded when maps are expanded large enough to distinguish boundaries and approximate city locations. This new tool eliminates the resolution problem by using GPS coordinates to locate a site. Wind speeds are interpolated to the nearest one (1) mile per hour based on wind contours on the maps. The platform is Google Earth; either street maps or satellite images can be used to locate a specific site.

Wind speeds are provided in 3-second peak gusts for three (3) Risk Category maps provided in ASCE 7-10 in a one-page report. It is expected that users will be potentially using building codes that reference other versions of ASCE 7 for some years to come and therefore, the wind speed report will include wind speeds for ASCE 7-05 and fastest mile wind speeds from ASCE 7-93 for those who need a comparative speed between averaging times. In addition to wind speeds for the three risk classifications, wind speeds are provided for serviceability purposes for 10, 25, 50, and 100-year return periods.

The wind speed site will be free for users. Users are encouraged to also provide ATC with feedback about the site so that enhancements can be planned.

To determine the net effect of changes to ASCE 7-10 on calculated design velocity pressures and see comparison tables for select geographic locations, download ASCE 7-10 Wind Provisions and Effects on Wood Design and Construction. It also compares select provisions of ASCE 7-10 with similar provisions in ASCE 7-05, and discusses implementation of ASCE 7-10 in model codes and the Wood Frame Construction Manual for One-and Two-family Dwellings (WFCM).