ASCE 7-10 Wind Speed Map Updates
- The wind speed map for all locations has been revised & Importance Factors have been removed.
- There are 3 new wind speed maps. Wind speed maps are provided for each Risk Category as opposed to a single map with importance factors (300 year period, 700 year return period, 1700 year return period). The difference is that all of the previous ASCE 7 Standards had referenced only one wind velocity map for all Risk Categories.
- New wind speed maps have replaced the existing maps that are directly applicable for determining design wind pressures using the strength design approach. Different maps are provided for different Risk Categories instead of a single map with importance factors to be applied for each Risk Category.
- Wind speed values are now represented in the ASCE 7-10 as “Ultimate” wind speeds.
- Strength design level wind speeds replace the ASD level wind speeds.
- Comparative hurricane wind speeds are lower than those given in the ASCE 7-05.
- The wind speeds in the maps are much higher than those in previous editions, the Load Factor on “W” in Section 2.3.2 is now 1.0 instead of 1.6 as established in the ASCE 7-05.
- Importance Factors have been used in previous editions of the ASCE 7 to adjust the velocity pressure to different annual probabilities of being exceeded. However, the use of Importance Factors was an approximate means for adjusting the return period because the slope of the wind speed vs. return period curves differ. The distance inland where the hurricanes can influence wind speeds increases with the return period. This situation was not adequately addressed by using Importance Factors from a table.
Updates for allowable Wind Speeds That Supersede Wind Speeds Provided In The ASCE 7-10
- The basic wind speed shall be increased where records or experience indicate that the wind speeds are higher than those reflected in Fig. 26.5-1 of the ASCE 7-10.
- Mountainous terrain, gorges, and special wind regions shown in Fig. 26.5-1 of the ASCE 7-10 shall be examined for unusual wind conditions. The authority having jurisdiction shall, if necessary, adjust the values given in Fig. 26.5-1 to account for higher local wind speeds. Such adjustment shall be based on meteorological information and an estimate of the basic wind speed obtained in accordance with the provisions of ASCE 7-10 Section 26.5.3.
- For areas outside hurricane-prone regions, regional climate data shall only be used in lieu of the basic wind speeds provided in the ASCE 7-10 Fig. 26.5- when (1) approved extreme-value statistical-alaysis procedures have been employed in reducing the data; and (2) the length of record, sampling error, averaging time, anemometer height, data quality, and terrain exposure of the anemometer have been taken into account. Reduction in basic wind speed below that of Fig. 26.5-1 shall be permitted.
- In hurricane-prone regions, wind speeds derived from simulation techniques shall only be used in lieu of the basic wind speeds given in the ASCE 7-10 Fig. 26.5-1 when approved simulation and extreme value statistical analysis procedures are used. The use of regional wind speed data obtained from anemometers is not permitted to define the hurricane wind-speed risk along the Gulf and Atlantic coasts, the Caribbean, or Hawaii.
- In areas outside hurricane-prone regions, when the basic wind speed is estimated from regional climatic data, the basic wind speed shall not be less than the wind speed associated with the specified mean recurrence interval, and the estimate shall be adjusted for equivalence to a 3-second gust of wind speed at 33 ft (10 m) above the ground in Exposure C.
ASCE 7-10 Importance Factor Update
- In the equation for Velocity pressure (qz) the importance factor has been removed, and the coefficient 0.00256 (0.613 in SI) shall be used except where sufficient climatic data are available to justify the selection of a different value of this coefficient/factor for a design application.
ASCE 7-10 Category “D” Update
- Surface Roughness Category “D” now applies to all water surfaces including water surfaces in hurricane prone regions.
ASCE 7-10 Updates for the State of Hawaii
- Entire State of Hawaii has its own special wind region.
- The reasoning behind this is due to the highly complex three-dimensional topography in the State of Hawaii. This conclusion was reached by numerous studies. The topography has speed-up effects that cannot be adequately portrayed by a single statewide value of wind speed nor at the macro-scale of a national map. The State of Hawaii has addressed this issue with the development of wind maps for each local jurisdiction.
- For these new “special region maps” for the State of Hawaii, you must reference the Hawaii State Building Code.
- Although the probabilistic reference wind speeds are provided for Hawaii in the ASCE 7-10, the intent is that the actual design wind speeds are to be further modified as determined from the authority having jurisdiction. Wind speeds are identified simply to provide the reference wind speed for each Risk Category, and also ensure that the wind-borne debris region criteria in the ASCE 7-10 is appropriately triggered by the net value of net effect wind value.
Reference: General Requirements for Determining Wind Loads & ASCE 7-10