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[SEAOC] Re: [SEAOC] Re: [SEAOC] Wind Pressure Coefficients (Cq)

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The "Wind Commentary to the UBC" by the Structural Engineers Association
of Washington can answer your questions.

Per page 170: (1991 edition)  "....wind loads on an open frame tower are based
on the normal projected area of the members located on one of the sides of the
tower no matter what the wind direction.  The plane on which the area is 
projected is a vertical plane that is parallel to one side of the tower.  The
UBC provides two Cq factors:  a value of 3.6 for winds normal to a side and
4.0 for winds along a diagonal  Each of the Cq factors is adjusted to 
account for the wind drag against the tower members on the other sides of the

When the wind direction is along a diagonal the same projected area as
calculated for the normal winds is used the Cq factor is increase from 3.6
to 4.0 to account for the greater area of the members actually directly
exposed to the wind.".......

The SEAOW commentary also discusses the ANSI/EIA 222E (now 222F) standards, 
which is recognized as a standard by Section 1603.2 of the UBC and they 
comment that this is more appropriate code for tower structures than the UBC.


Neil Moore