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# RE: Commercial Sign structure

• To: "'seaoc(--nospam--at)seaoc.org'" <seaoc(--nospam--at)seaoc.org>
• Subject: RE: Commercial Sign structure
• From: hsprague(--nospam--at)aspen.klaalov.com (Harold Sprague)
• Date: Fri, 3 Apr 1998 08:48:42 -0700

```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.

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:	Alex Nacionales [SMTP:anacio(--nospam--at)skyinet.net]
Sent:	Friday, April 03, 1998 8:01 AM
To:	seaoc(--nospam--at)seaoc.org
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.

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

Elevation

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?