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RE: wind pressures

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Richard,

 

It sounds like LA County needs to follow Hawaii’s lead. They commissioned a research project using wind tunnel studies and CFD modeling to develop a statewide series of effective basic wind speed maps incorporating the local topographic effects. There is supposed to be a guide coming out from the Structural Engineering Association of Hawaii and ICC with the maps. In some cases, I understand the resulting basic wind speed is actually less than the 105mph cited in ASCE 7.

 

The compilation you suggest shouldn’t be that difficult to assemble. Assuming you can figure out the appropriate stations, NOAA/NWS has all sorts of weather data including local wind speeds; it’s just a matter of poking around their websites until you find the right information. I did it recently looking for maximum snow depths for a bunch of cities in the Southeast.

 

In the 2009 IRC, a jurisdiction must have some sort of documentation that wind speed-up is an issue before it can require consideration of topographic effects. We demanded that as the compromise for allowing provisions to be added, for just the reasons you describe.

 

Regards,

Gary

Gary J. Ehrlich, PE
Program Manager, Structural Codes & Standards
National Association of Home Builders (NAHB)
1201 15th Street, NW, Washington, DC 20005
ph: 202-266-8545  or 800-368-5242 x8545
fax: 202-266-8369
gehrlich(--nospam--at)nahb.com

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From: Richard L. Hess [mailto:RLHess(--nospam--at)HessEng.com]
Sent: Thursday, August 20, 2009 9:15 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: wind pressures

 

Casey,

This general increase for the entire southern half of Los Angeles County appears to have been from an incorrect reading of the ASCE 7-05 wind diagram, Fig. 6-1, which shows that area  as a Special Wind Region.  That figure shows all of CA, OR and WA to be in the 85 mph rather than a 100 mph zone. Whereas the special wind regions are those where the engineer should check for areas where designated special topographic features require the use of higher values; an example being L.A. City Document No. P/BC 2008-016.

Both ASCE 7 and the UBC have had identical wind maps since the early '80's and the UBC had similar maps since 1961.  Therefore, the basic wind speed in this area is 85 mph and higher speeds are called for only where terrain features and established local records indicate higher wind speeds exist.  I do not think that such features and records exist in the flat areas of the Los Angeles basin where the new ordinance applies and it is pretty unreasonable to expect the builder of a residence or a small commercial or industrial building to pay for a specific wind study by a meteorologist to justify what is in ASCE 7 and what has worked well in the past.  All that will do is add significant cost increases to every modest structure that is built.

What is needed is a compilation of areas with special topographic features such as mouths of canyons or mountain tops where very high winds have been recorded so that the public can be informed and not exposed to unnecessary and costly restrictions.

 

Richard Hess, S.E.