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Re: Failure of Connection between Rafter and

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I think he is referring to the gravity component of the roof along the axis
of the rafter.  If you hold a rule between two hands and gradually lower one
hand the ruler will ultimately slide off your hands.  I think that is the
observation.

Thor Tandy  P.Eng  MCSCE
Victoria BC
Canada
vicpeng(--nospam--at)vtcg.com
-----Original Message-----
From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Friday, April 30, 1999 9:53 AM
Subject: Failure of Connection between Rafter and


>Is this a situation that defies statics?
>
>Assuming that the ridge beam is a beam (supported by columns or walls and
>capable of supporting gravity loads from the rafters) and is not a ridge
>plate, and the wall is incapable of resisting horizontal loads, how do you
>get horizontal loads at the ridge beam?  (Wind loads that act normal to the
>roof structure are a different subject.)
>
>Now, if you have a ridge *plate* that is not capable of supporting gravity
>loads from the rafters, then the only reaction possible at the ridge plate
is
>a horizontal load which has to be resisted by some other horizontal
>restraint.  If the wall is incapable of resisting horizontal forces, then
>collar or ceiling joist ties are necessary to provide the horizontal
>restraint.
>
>For the broader question of why some of these roofs don't collapse since
some
>of the collar ties are seriously inadequate and ceiling joist ties are not
>always connected to the rafters, it seems to me that the roof begins to
>perform as a folded plate structure, i.e., a deep inverted "V" beam, rather
>than a series of rafters supporting sheathing, etc.  This is also a
possible
>explanation for bow string truss supported roofs not collapsing when the
>tension chord fails --- the roof starts acting as a curved shell and holds
up
>the failed trusses.
>
>A. Roger Turk, P.E.(Structural)
>Tucson, Arizona
>
>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)RSEC.com
>
>
>