Pressures are not always lower with ASCE 7-10. While many of the locations shown on slide 10 have lower pressures for Exposure C, slight increases in pressures result for Exposure D in some locations. The changes to mapped wind speeds represent improvements in the prediction capabilities of high wind events in hurricane-prone regions. The wind speeds in ASCE 7-10 are on a 700-year return period basis while the wind speeds in ASCE 7-05 are on a 50-year return period basis. To adjust from the ASCE 7-10 to ASCE 7-05 wind speed basis, the wind speeds are divided by SQRT(1.6). Due to the change in presentation of wind speeds in ASCE 7-10, the load factor for allowable stress design (ASD) wind pressures in ASCE 7-10 changed from 1.0 to 0.6. The 2012 IBC adjustment to calculate VASD
in ASCE 7-05 from Vult
in ASCE 7-10 multiplies the Vult
wind speeds by SQRT(0.6) to ensure the ASD design wind pressures are on the same basis, not the wind speeds. The ASCE 7 load combination committee does not want load factors to have more digits than to the nearest tenth, not hundred, thus a load factor of 0.6 and not 0.625. This automatically causes some inaccuracies in the conversion.
See the following FAQ for more background:
"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?"