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Re: Failure of Connection between Rafter and Ridge Beam

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Have you contacted the joist manufacturer about using a design requiring
compression in the I joist.  A few years ago I looked at using rafters that
transferred compression forces.  Trus Joist MacMillian indicated that their I
joists are not designed to be used in that manner due to buckling.  If the ridge
beam is supported by posts the joists should simple span between the walls and
the ridge.  No tie should be required in this application.
Jill Shuttleworth, P.E., S.E.
Sunnyside, WA

Mark Oakford wrote:

> This post relates to sloped wood roofs framed with sawn lumber or
> manufactured I-joist rafters supported by a ridge beam. Does anyone know of
> a case where the connection between the rafter and the ridge beam failed due
> to axial load in the rafter?
> The axial load, which equals the vertical load times the sin(slope), can
> exceed 2,000 pounds for I-joist rafters. The typical details provide by the
> I-joist manufacturer calls for a Simpson LSSU variable angle seat hanger,
> which has an allowable tension of zero pounds. The staff engineer at the
> I-joist manufacturer believes the roof diaphragm carries the axial load from
> all the rafters into the roof diaphragm and over to the end walls.
> I would like to know if other engineers size the tie strap at the ridge beam
> based on the calculated rafter axial load and if anyone knows of a failure
> resulting from an inadequate connection. If you would like a spreadsheet for
> determining this axial load request it from the e-mail address shown below.
> Thanks
> Mark Oakford, P.E., oakfordm(--nospam--at)
> RSE Consulting, Federal Way, WA  98093-1417
> T 253-927-6169   F 253-838-3823