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RE: Ridge beam/joist analysis

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According to the book "Design for Lateral Forces" by Ambrose & Vergun: "When
a ridge beam is used to support the upper ends of the rafters ... the
outward thrust at the lower ends is essentially removed." (Ridge beam must
be supported at ends.) Also, "Another means for restraining inclined rafters
is through the use of a collar tie ... this turns the top joint into a rigid
connection, and stability becomes dependent on the bending of the rafters."
If the ridge beam is not vertically supported and a rigid connection is not
developed, you have a three hinged arch, which requires thrust restraint at
the sides. 

Unfortunately, I was looking in this book for a good description of how
lateral wind forces are transferred thru a gabled roof via diaphragm action
- but despite the name of the book, the section on gabled roofs only seems
to address vertical load effects. I once convinced myself that the two
plates act effectively as one diaphragm with shear transferred at the ridge
beam, but now I am having trouble reconfirming that conclusion using
free-body diagrams. Is there a reference that explains in detail how forces
are transferred in a gabled roof diaphragm? 


William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com
 

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