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Explanation of ASCE 7-05 and ASCE 7-10

ASCE 7 provides minimum load requirements for the design of buildings. Loads and appropriate load combinations (load factors), which were developed to be used together, are set forth in two design methods: allowable stress design (ASD) and strength design. Designers are given the option to use either ASD or strength design. Generally, roof systems are designed using ASD.

Comparing the 2010 edition to the 2005 edition, there was a significant change to the load factors used with wind and basic wind speed maps. These changes affect how you determine wind-uplift resistance.

ASCE 7-05 uses a single basic wind speed map. For each building risk category, an importance factor and wind-load factor are then applied to determine ultimate wind loads. For this edition, the ASD wind-load factor is 1.0 and the strength design wind-load factor is 1.6. To determine wind-uplift resistance loads for a roof system, you would multiple the ultimate wind loads by the ASD wind-load factor (1.0) and the appropriate safety factor.

ASCE 7-10 uses three different basic wind speed maps for different categories of building occupancies. These maps provide basic wind speeds that are directly applicable for determining pressures for strength design. Consequently, the strength design wind-load factor was changed to 1.0 in this version. Simply put,, ASCE 7-10 uses three maps based on strength design in conjunction with a wind-load factor of 1.0, while ASCE 7-05 uses a single map with an importance factor and wind-load factor of 1.6.

Because ASD results sometimes are used to determine wind-uplift resistance values, a designer using ASCE 7-10 may want to adjust the strength design values to ASD values. In doing so, applying a load reduction factor of 0.6 and the applicable safety factor may be appropriate.

If a user chooses to use ASCE 7-10, ASD and strength design results will be identified in the Roof Wind Designer report.