Pilot Study of a Special Wind Region at Select Site in California

By James S. Lai, S.E., F.SEAOC, Chair SEAOC Wind Committee

What is Special in the Special Wind Region

ASCE 7-10 wind hazard maps Fig. 26.5-1A through 26.5-1C show the 3-second gust basic wind speed. A footnote states unusual wind conditions need to be examined in special wind regions, which are shown as “gray” areas on the map. The special wind regions are known for abnormal wind speed.  ASCE 7 Commentary section C26.5.2 states that “when selecting basic wind speeds in these special regions, use of regional climatic data and consultation with a wind engineer or meteorologist is advised.” 2013 CBC section 1609.3 states that “the ultimate design wind speed Vult for special wind regions indicted near mountainous terrain and near gorges shall be in accordance with local jurisdiction requirements.” The 2015 California Residential Code imposes a requirement for Building Officials to identify “Climatic and Geographic Design Criteria” for the public. Some State agencies, such as the DSA, rely on the engineer-of-record to obtain basic wind speed information from local Building Officials.

The California Department of Housing Community Development (HCD) has previously published information bulletins (1981 and 1994, respectively) listing minimum design wind pressure based on the legacy code for each county jurisdiction. The design wind pressure had a minimum of 15 lbs. per square foot and varied up to 30 lbs. per square foot. However, the information could not be extrapolated to basic wind speed without confirmation by county building officials. After considerable discussion on this topic, the SEAOC Wind Committee agreed that it would be a worthwhile effort to collect data from AHJ within California and to make available the design information of basic wind speed for each county jurisdiction within the State of California. The Committee submitted an inquiry to each of the 58 county building officials within California, plus a few selected cities. The response rate was less than 20% from county building officials. More than half of the respondents were unable to furnish meaningful information to refine the draft tabulation. A number of building officials sought help from SEAOC to assist in the selection of wind speed for enforcement. Based on the result of the inquiry, it was the general consensus of the Committee that if wind speed in the Special Wind Region were to be tabulated, the data must be from technical literature, statistical analyses or informed sources from one of the meteorologists or wind engineering companies.

Proposal for Pilot Study

James LaiAt the SEAOC Board meeting in March 2015, Wind Committee Chair James Lai presented a proposal for the Wind Committee to carry out a pilot study program, funding for which was approved by the Board. The Committee intends to engage the assistance and service of Cermak Peterka Petersen (CPP), Fort Collins, Colorado. A member of the SEAOC PV Subcommittee, Dr. David Banks of CPP, has provided extensive data and technical insight into the development of SEAOC PV2.  Based on information from Dr. David Banks of and their analyses of extreme wind data, they are aware of at least two locations in California where a local downslope wind pattern would require a 700 year design wind speed above 110 mph. These locations are not within the boundaries of the high wind speed regions identified for California in the wind map. The extreme wind speed largely lies on mountainous regions north and east of Los Angeles and along the high points of the Sierra Nevada range. Dr. Bank expects there are other high wind speed regions not included in the current special wind region map as well.

In this pilot study, CPP will carry out an extreme wind analysis of one of these locations, near Santa Ana and Irvine. CPP will examine and interpret extreme wind data from airport anemometers in this region. This analysis includes an examination of the quality of the data from each airport. The results will be presented as a 700 year wind speed map for the Santa Ana area. This pilot study will help to gain knowledge on ascertaining questions from members of the Association as well as the building officials. We expect that the result of this pilot study will be available this summer. The Board was unanimously in favor of this pilot study.

The Committee hopes that this initial effort will help establish guideline for site-specific studies of wind speeds. Furthermore, the effort will help to attract funding from other sources to perform a thorough study and to lay the ground work for possible future micro-zonation of wind speed contours in California.

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