In comparing the 2010 edition to the 2005 edition of the ASCE 7 we see that there are significant changes to the layout, format, load factors used for wind and basic wind speed maps. These changes affect how you determine wind design wind pressures.
ASCE 7-10 Minimum Design Loads for Buildings and Other Structures. Found here
ASCE 7-05 Minimum Design Loads for Buildings and Other Structures. Found here
ASCE 7-05 uses a single basic wind speed map. For each building risk category an importance factor is applied. Note that these importance factors only depend on the type of building, not where the building is located. The wind-load factor is then applied to determine the design wind pressure. For this edition (05), the ASD wind-load factor is 1.0 and the strength design wind-load factor is 1.6.
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 for strength design (LRFD) and 0.6 for service level loads (ASD), while ASCE 7-05 uses a single map with an importance factor and wind-load factor of 1.6 for strength design (LRFD) and 1.0 for service level loads (ASD).
Why the Change?
The commentary in ASCE 7-10 (section states 26.5.1) a few reasons for basic wind speed changes:
- A strength design wind speed map brings the design approach used for wind ‘in-line’ with that used for seismic loads.
- Multiple maps remove the inconsistencies inherent the importance factor approach. With multiple maps a distinction may be made based on location (i.e. hurricane prone vs non-hurricane prone which also changes the recurrence interval).
- New maps establish a more uniform return period for the design-basis winds.
- The maps more clearly inform owners and their consultants (that’s you) about the storm intensities for which designs are preformed.
- We have justify our pay check somehow
ASCE 7-10: 3 wind speed map based on 3-sec gust at 33ft above ground. The different maps are ‘calibrated’ to strength level design (LRFD LF=1.0) and also include building classification and location.
ASCE 7-05: (1) wind speed map based on 3-sec gust at 33ft above ground. Importance factors and Load Factors are used to increase design pressures.
EIA-TIA-222 Rev G: Wind speeds are similar to 7-05 with different definitions of classification of structures and gust effect factors.
EIA-TIA-222 Rev F: Wind speed maps based on fastest mile. These are not directly comparable to ASCE 7-05 or 10, as the ASCE 7 uses 3-sec gust. The 3-sec gust represents the peak gust wind speed where as the fastest-mile wind speed represents the average wind speed over the time required for one mile of wind to pass the site. The design pressures are derived using different adjustments for height/exposure and gust effects than that of Rev G and/or the ASCE 7 standard
How ASCE 7-10 Wind speed were developed – return periods:
Risk Cat I which is based on 25-yr return period equates to 300yr return period
Risk Cat II: 700yrs or 0.0014 annual exceedance probability
Risk Cat III and IV which are based on a 100-yr return period (thus there importance factor was greater in -05): 1,700yrs or 0.000588 annual exceedance probability
Interestingly enough new research gathered since 2005 indicated that design wind speeds should be reduced (they also note that the overall rate of ‘intense’ storms increased). Therefore it is likely that you will noticed reduced wind pressures along coastal regions.
For most of the US of A the wind load remains basically unchanged. A quick look at the basics –
ASCE 7-10 (eqn 27.3-1) or ASCE 7-05 (eqn 6-15) wind pressure:
Assuming that and V = 90 mph then we have
ASCE 7-05 => (ASD)
ASCE 7-10 => (ASD)
A nice paper by AWC (American Wood Council)