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RE: wind loads

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Mark,

Be careful to establish whether this is a "fastest mile" wind, a "3 second gust" wind, a "sustained wind", or a wind reported by the weather service that has not been normalized.  It will make a very large difference.  The ASCE 7-93 dealt with "fastest mile" winds as does the UBC.  The ASCE 7-95 deals with "3 second gust" winds.  If it is not normalized, call a wind engineering expert like CPP in Fort Collins, CO.

This being reported as a hurricane wind suggests a "one minute" wind or "sustained" wind, which is the standard of hurricane guys.  They will spout things like the Saffir-Simpson scale Category 5 which will give a range of sustained winds.  If it is a sustained wind, convert it to a 3 second gust using the Krayer-Marshall curve that is in the appendix of ASCE 7-95.  Then use the ASCE 7-95 to determine the wind pressures on your structure.  The conversion is not straight forward.  

If you chose to convert to a "fastest mile" wind to use ASEC 7-93, be careful.  Using the curves becomes an iterative process in which you close on a time of duration.  It takes about 3 or 4 iterations and is not for the uninitiated.  You can reference Emil Simu's book on wind design for good commentary and examples.  There is a paper floating around that does not use the iterative technique.  It is wrong.

Also be careful in how you use the wind pressures.  You will obviously use them to establish loads for your primary frame.  But lateral drift for building performance is up to the engineer.  The codes will not hold your hand on this one.  Currently the trend is to use a 1 year wind for human perceptibility due to lateral acceleration issues and a 10 year wind for deformation compatibility issues.  (Yes deformation compatibility is an issue in other than seismic design.)

Regards,
Harold Sprague
Krawinkler, Luth & Assoc.
4412 W. Eisenhower Blvd.
Loveland, CO 80537
Voice: 970 667-2426
Fax: 970 667-2493
Email: hsprague(--nospam--at)klaalov.com


-----Original Message-----
From:	Jessica Pemberton [SMTP:jmpember(--nospam--at)softcom.net]
Sent:	Thursday, March 05, 1998 8:36 PM
To:	seaoc(--nospam--at)seaoc.org
Subject:	wind loads

I have been involved with a panelized building system for areas subject to hurricane wind conditions.  The design speed was specified at 200 mph.  The 94 UBC does not have wind stagnation pressure coefficients for wind velocities above a maximum wind velocity (I think it was 110 mph).  I did however notice that these coefficients were propertional to the velocity squared and was able to derive a parabolic equation that would predict the coefficients for any wind velocity.  Is it acceptable to use this formula for wind loads as high as 200 mph?  Is there other provisions for such a high wind velocity?  How were the existing UBC coefficients derived?
Mark Pemberton, P.E.

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