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RE: Commercial Sign structure

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Your best bet is to use a 3 D model.  Refer to the ASCE 7-95 and look at Case 4 to check the tendency to rotate.  You can also model in 2 plane frames, connect the frames with link members, apply the unbalanced load per ASCE 7-95.  Determine the force in the link and resolve that force into the diagonal horizontal brace.  This is what we did before computers were in wide use and before 3 D programs were widely available.

With all that in mind, you will probably have a dynamically sensitive structure.  Check the appendix of the ASCE 7-95 to address this issue.

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

-----Original Message-----
From:	Alex Nacionales [SMTP:anacio(--nospam--at)]
Sent:	Friday, April 03, 1998 8:01 AM
To:	seaoc(--nospam--at)
Subject:	Commercial Sign structure

I am designing a  commercial sign where  a steel open frame tower
10'x10' with four legs and 90' tall supports a 30' high x 50'wide
billboard  like a cantilever beam.  The billboard  is framed with
several horizontal trusses connected to the four legs
of the tower. The wind load on the  billboard  produces  a large
torsional force on the tower.

               30'     n
n                                                  50'
n                                            10'   x      x     x
x      x     x
                         n    n 60'
n                                                                 Plan
                     __n__n __


I am using RISA 2D only.  How do you create a 2D model for the open
frame tower taking into account the torsional forces from the billboard?
Is it possible to do this or do you need 3D analysis?

Thanks in advance.

Alex Nacionales, CE