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Re: Use of Collar Ties in Light Frame Wood Construction

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Dennis
I think collar ties will do the job as they have been installed in
conventional framing, meets code, for many years.  I would venture to say
that if you try to calculate the thrust force in the collar tie using 3- 16
d nail you might just get by.  I know that the prescriptive nailing
schedule in the code, table23-II-B while accepted by code provisions hardly
calc's out.  The thing that is puzzling in your case is that the ridge
board should be a compression member only that supports the rafters and
when considering a 2 foot span of the ridge itself, it should not have a
bending failure.  It seems to me, may be the owner or prior occupants must
have hanged a heavy load, like a car engine, car top or simply a boxing bag
to have cuased this failure.  At any rate, even if you have to design a
ceiling beam as it appears in your post, try to justify the existing
foundation under concentrated load without having to monkey with the
existing foundation.  My experience leads me to believe that it is doable.
Good luck
Samir Ghosn, P.E.
Harris & Associates
Plan Check engineer
At 12:27 PM 6/5/2001 -0700, you wrote:
>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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