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Re: Use of Collar Ties in Light Frame Wood Construction

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>From the modeling we have done here, collar ties just don't work,
particularly the connections to the rafters.  (We usually have 30 psf snow
load and up.)  If the collar ties do work, the rafter tails usually end up
overstressed in bending.  If you can use the ceiling joists to form a bottom
truss chord, this takes some of the curses out of it.  I would try a RISA
model of a single rafter/ceiling joist/collar tie (I think you mentioned
that you use RISA) with the appropriate members, and see if the members even
pass a wood code check.  I would neglect the ridge, since it has failed.  If
they work, you will then have the forces you need to design ALL the
connections.  Ends up being a site built truss.

HTH

Conrad Guymon, P.E.

----- Original Message -----
From: "Structuralist" <dennis.wish(--nospam--at)gte.net>
To: "SEAINT Listservice" <seaint(--nospam--at)seaint.org>; <aec-residential(--nospam--at)polhemus.cc>
Sent: Tuesday, June 05, 2001 1:27 PM
Subject: Use of Collar Ties in Light Frame Wood Construction


> I have a client with a home in escrow. There is a cracked ridge board in
the
> garage and it appears that the ridge has dropped a few inches. The framing
> is rafter tied by the 2x6 ceiling joists to the 2x8 roof rafters, the
ridge
> board is a 2x12 DF. The crack in the ridge board is parallel to grain
which
> runs diagonally through this section of the board - an obvious defect in
> material not found during original construction.
>
> I intended to jack up the roof and install 1x6 collar ties at 1/3 the
> distance down from the peak of the roof. The attic is accessible to
install
> the collar ties and all other existing framing looks pretty good.
>
> The 2000 IBC lists the use of 3-10d nails to secure the 1x to the rafters.
> However, this 2000 IBC is not used in California and collar tie nailing is
> not indicated on the 97 UBC Nailing Table. The only reference to collar
ties
> I could find was in Appendix Chapter 23 - Prescriptive Masonry
Construction
> in High-Wind Areas where Section 2112.10.2 states: "Gabled and sloped roof
> members not supported by the ridge shall be tied by ceiling joists or
> equivalent lateral ties located as close to where the roof member bears on
> the wall as is practically possible, at not more than 48 inches on center.
> Collar ties shall not be used for these lateral ties."
>
> Now I am at a loss. In my opinion (as this was a common form of
construction
> for many years) collar ties are ideal to resolve this situation but this
> condition requires them to act as lateral support. The existing ceiling
ties
> obviously help, but are not restraining all of the outward thrust of the
> rafter tails since the ridge board cracked. Installing a Ridge beam seems
an
> inappropriate solution here as it would redistribute the roof load and
> require the possible addition of two foundations at each end - requiring
> that we cut through one slab and underpin the exposed slab edge.
>
> Any suggestions or information that would help me justify the use of
collar
> ties?
>
> If I still use collar ties (assuming the building department accepts the
fix
> as I suspect they will) I intended to install a 1x6 on each side and face
> nail each end to the rafters with (3) 10d nails - spacing the collar ties
> every other bay or 48" on center.
>
> Regards,
> Dennis S. Wish, PE
> Structural Engineering Consultant
> mailto:structures(--nospam--at)engineer.com
> (208) 361-5447 E-Fax
>
>
>
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