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RE: Ridge beam/joist analysis

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I have a terminology comment that might help to clear up this discussion.

In the wood design that I'm familiar with doing, "collar ties" refer only to
a 2x horizontal member that is nailed to the upper 1/3rd of a pair of
sloping rafters. The purpose of the "collar tie" is only for a construction
aid and/or to help prevent separation of the rafter tops during wind uplift
(suction) events. As has been noted previously, they are negligibly
effective in restraining the rafters against spreading at the top plate.

"Rafter ties" are a separate bit of framing that are nailed to the bottom
(or at least below the bottom 1/2) of the rafters.  In typical framing,
these members would also act as the ceiling joists.  Only the "rafter tie"
connection would be designed for the tension created by the lateral
spreading force at the base of the rafters.  Two separate terms for two
separate functions...

Eli Grassley
PSM Engineers
Seattle, WA

-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com] 
Sent: Thursday, July 01, 2004 6:46 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Ridge beam/joist analysis

Actually, I only edited out a reference to a figure in that sentence. The
figure shows the collar tie some distance below the ridge. However, it does
go on to say that "This solution is reasonably feasible only for very short
spans (such as single car garages) and should use rafters of reasonable
stiffness." 

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

> -----Original Message-----
> From: Daryl Richardson [mailto:h.d.richardson(--nospam--at)shaw.ca] 
> Sent: Wednesday, June 30, 2004 2:32 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Ridge beam/joist analysis
> 
> > "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."
> 
> William,
> 
>         This seems to me to be a mistake.  Perhaps you edited 
> out more than you intended.
> 
>         It would seem (to me at least) that the collar tie 
> would effectively turn the rafters into a three hinged arch 
> with the lateral restraint force taken internally by the 
> collar tie.  The only moment that I can see being transferred 
> through the ridge joint would be limited to the axial load in 
> the rafters multiplied by the small eccentricity between the 
> effective centre of the contact area and the centrelines of 
> the rafters.
> 
>         I agree with everything else in your posting.
> 
> Respectfully submitted,
> 
> H. Daryl Richardson

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