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Re: commercial sign structure

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     ASCE 7 is available from ASCE at 1-800-548-2723 or www.asce.org
     
     On the analysis part of your question, torsion in a 4 sided truss 
     tower can be approximated by resolving the moment portion into 
     reactions on the 4 sides rotating about the center of stiffness, 
     probably the tower center.  Considering the global torsion action in 
     this case will probably result in lower forces than the simple 2D 
     frame type analysis (simple reactions on 2 sides).  You roughly have 
     the 4 sides of the tower resisting the torsion vs the 2 sides you are 
     looking at.  It is difficult to assess your situation accurately over 
     email, but in this case it sounds like you need to consider the 3D 
     effects.
     
     Ed Haninger
     Fluor Daniel  


______________________________ Reply Separator 
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Subject: commercial sign structure
Author:  anacio@skyinet.net_at_-FDInternet at FDINET
Date:    4/4/98 7:47 AM
The tower has bracings on four sides.
I made a similar computation. First a made a plane truss model for the 
horizontal truss
with  its 4 hinged supports representing the 4 legs of the tower. I then 
applied the reactions to the plane tower  model frame neglecting the 
bracing on the two sides.
The forces on this frame is more than twice in magnitude than the forces 
for the wind load alone without the torsion.  The resulting members are 
quite big so I  advised my client
to place the tower at the center of the billboard to avoid the torsion. 
Actually a similar structure is already being constructed here with the 
tower at the center. The second structure is for another city where the 
site has an  existing small shed that will be affected by the foundation 
if the tower is in the center. It will be cheaper to demolish the shed 
and rebuild it at about 500 US dollars.
     
A 3D analysis will be probably give a more economical design but I only 
know 2D analysis and do not have a 3D software. I can't afford it yet .
     
Thanks to all who replied to my post.
     
Ed Haninger, Harold Sprague, Bruce Bates and Jim Warne.
     
Harold,
     
How do I get a copy of ASCE 7-95. Thanks again.
     
     
Alex Nacionales
A.C. Nacionales Construction
Iloilo City, Philippines