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

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     The values for open frame towers come from ASCE 7, the source
document 
     for the UBC wind provisions.  The values reflect the density of
the 
     diagonal and cross-members of a typical tower, and are based on
wind 
     tunnel testing at the University of Western Ontario.  The values

     probably seem high because each individual member is capable of 
     receiving a positive pressure on its windward side and a
negative 
     pressure on its leeward side.  I also suspect that the structure
as a 
     whole interrupts the wind field more as a solid object than as
an open 
     structure.
     
     You might check out ASCE 7-95 to see if any of the wind
coefficients 
     provided are for structures more similar to yours.  Be careful,
ASCE 
     7-95 defines wind velocity in 3-second gusts, not the
fastest-mile 
     wind velocity in the UBC.  You'll have to convert to use ASCE
7-95.
     
     Rick Drake, SE
     


______________________________ Reply Separator
_________________________________
Subject: [SEAOC] Wind Pressure Coefficients (Cq)
Author:  seaoc::(SEAOCAB) at ~FABRIK
Date:    8/10/96 9:15 AM


From: seaoc(--nospam--at)power.net
Date: Sat, Aug 10, 1996 9:15 AM
Subject: [SEAOC] Wind Pressure Coefficients (Cq) 
To: seaoc
Can someone explain why the wind pressure coefficient (Cq) for
Open-Frame 
towers (UBC 94) is so much higher than other structures?  According
to UBC 94 
Sec. 1621, Open-Frame Towers are "Radio towers and other towers of
trussed 
construction ..."  I suspect shape factor of assembled trusses has
something 
to do with it.
     
Secondly, I am designing 60' tall x 15' sqaure open tower, assembled
with 
four steel tube columns at the corners and connected at the top with
steel 
tube beams.  No diagonal members in between the columns; just tall
free 
standing columns.  What Cq factor should be used?
     
Brad
     
...
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