"New" ASCE 7-10 Wind and What It Means
- by Tom Albani -

The wind speed maps have changed dramatically from ASCE 7-05. The most significant change is that the wind speed values represent “Ultimate” wind speeds. Therefore, with the new maps the 3 main changes are:

1.       Instead of one map we have three maps to use. Each one represents a different Risk Category (300 year return period, 700 year return period and a 1700 year return period.

2.       Strength design level wind speeds replaces the ASD level wind speeds.

3.       Re-introduction of Exposure D for coastal areas to the new Florida Building Code.

The strength design or ultimate winds speeds are new and while the wind speeds on the maps are much higher in ASCE 7-10, the load factor is now 1.0 instead of 1.6 as in ASCE 7-05. Therefore, the actual design pressures are close to the same even with the higher wind speeds. Prior to ASCE 7-10 the maps represented a 700 year event after multiplication of the 1.6 load factor.

Importance Factors were used in previous versions of ASCE 7 to adjust the velocity pressure to different annual probabilities of being exceeded. For example, residential buildings had an importance factor of 1.00, while emergency facilities had an importance factor of 1.15. The 1.15 factor actually doubled the return period in non-hurricane areas from 50 to 100 years and was an approximate means for adjusting the return period. The return period increases the distance inland where hurricanes can influence wind speeds, and using importance factors did not adequately address this.

In conclusion, the new maps used with 1.0 load factor for strength design and the .6 factor for allowable stress design, results in a 20% decrease in design wind loads in Hurricane–Prone areas. However, for parts of Florida due to the re-introduction of Exposure D for coastal areas the loads are approximately the same. An example of the wind velocity change is in areas where the wind velocity in ASCE 7-05 was 100 mph now in ASCE 7-10 it is 126 mph. 




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