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RE: Bent Beam/Frame

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Mark hit the nail (or is it post in this case) on the head.  It is a function of the lateral stiffness of the supports.  If the bent beam is TRULY a simply supported beam (i.e. one end if completely free to laterally deflect), then there will only be vertical reactions.  If there is any lateral resistance at the "roller" support, then there will be horizontal reactions.  The magnitude of those horizontal reactions then be come a function of how much lateral stiffness the supports have (if they are REALLY flexible, then the ends can move very easily...if they are rather stiff, then the ends cannot move so easily and more reaction will be present) as well as how stiff the bent beam is (i.e. how much does it want to deflect laterally under load).
 
The tought part is modelling the lateral stiffness of posts in combination with the wall.  It is pretty hard to get an accurate model of that.  Thus, you will likely "fudge" it along the lines of what Mark suggested.
 
Regards,
 
Scott
Adrian, MI
-----Original Message-----
From: Mark Deardorff [mailto:mark(--nospam--at)rstavares.com]
Sent: Friday, September 07, 2007 1:54 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Bent Beam/Frame

Assume one support is a slider. Check the horizontal deflection at the slider and assume half goes into each wall. There is no horizontal reaction unless you model in the stiffness of the posts and then they would have to be cantilevering off of the foundation for any horizontal force to develop. Just make the frame stiff enough to limit the pushout at the top of the posts to a reasonable and visually undetectable amount.
 

Mark E. Deardorff, SE
R & S Tavares Associates, Inc
9815 Carroll Canyon Road
Suite 206
San Diego, CA 92131
Phone: 858-444-3344
Phone: 209-863-8928

mark(--nospam--at)rstavares.com

www.rstavares.com

 

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From: erik gibbs [mailto:erik.gibbs(--nospam--at)gmail.com]
Sent: Friday, September 07, 2007 10:04 AM
To: seaint(--nospam--at)seaint.org
Subject: Bent Beam/Frame

We are providing a steel bent beam/frame in a custom home to support the ridge beam within the ceiling instead of the typical king post to ceiling beam. The Steel frame is sized to fit within the depth of the R.R. and it is a 3/12 pitch, made with 2 square steel sections and a full penetration weld connection is provided at the apex, where the 2 beams meet. The bearing of the frame is at a 4x post at each end, hidden within the wall. 
 
Now my question is, if you determine the reactions using a simple FBD there will be an x & y reaction at the bearing points at the 4x posts, but if the steel frame is designed as stiff as possible to limit the deflection to negligable amounts then I am assuming that there will only be vertical reactions in the y direction. Therefore a wood post can be used without designing the connection for a very large reaction in the horizontal direction. 
 
Am I right in this assumption?  
 
Erik Gibbs