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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.


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
anacio(--nospam--at)skyinet.net

Ed Haninger wrote:
alex,

     With a 2D analysis program you will be very limited in what you can
do
      here.  You could try to approximate the torsional forces on the
     structure by hand and apply that to 2D frames representing the
sides
     of the tower.  If you have bracing on 4 sides that will be very
rough
     (probably conservative)

     You really have a 3D problem.

     Ed Haninger
     Fluor Daniel
     Irvine, CA

Ed,

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