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RE: Details, Struts, Thrust, Etc.

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A third solution if the ceiling is to be sloped to at least 1/2 of the
roof slope is to provide the ties from the upper 1/3rd of one rafter and
back down to the rafter end at the plate of the wall. The same for the
other side. There is still a thrust to consider, but I've seen this
detail used in residential custom homes with a ridgeboard rather than a
beam. It seems to act like a scissor truss and because of the connection
of the diagonal ties, acts to resist outward thrust while providing a
peaked ceiling.

Dennis


-----Original Message-----
From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com] 
Sent: Thursday, July 17, 2003 10:18 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Details, Struts, Thrust, Etc.


Jake,

I think that what Bill Polhemus was referring to was support for the hip

rafters.  Normally, you don't have a ridge beam, but a ridge plate for 
rafters to bear against from each side of the roof and the horizontal
thrust 
is taken by either collar ties or by ceiling joists.  However, with the
hip 
rafter, you have a different problem as the two hip rafters meet at an
angle, 
and the horizontal force at the ridge is counteracted in one direction
with 
the opposing hip rafter and by the diaphragm in the other direction.

Taking the horizontal thrust at the top of the wall is a different
question, 
however as there are no ceiling joists in the plane of the hip rafter.
There 
are three solutions as I see it:

1. Have the hip rafter act as a hip beam, which will require a column at
the 
peak or a girder truss where the hip beams meet.

2. Use hip trusses parallel to the main roof rafters/trusses (they have 
varying length horizontal top chords) to a point where hip beams can
attach 
to them and jack trusses form the remainder of the hip.

3. Use the top plates on each wall to resist the horizontal thrust from
the 
hip rafter.

In actuality, the hipped roof acts as a shell when the roof sheathing is
in 
place, with the roof sheathing taking the uncalculated horizontal forces
from 
the hip rafter.

A ridge beam is generally out of the question.

HTH

A. Roger Turk, P.E.(Structural)
Tucson, Aridzona

Jake Watson wrote:

. > You may have answered your own question.  Use the hallway as a
support. :)

. > I use two models for ridge beams in general.  Collar ties or
standard 
. > beam. A 30 degree pitch is a 7:12 roof, not exactly shallow. Should
be 
. > relatively easy to get collar ties of some sort to work. Connections
(as 
. > usual) will likely still be difficult. For some people, the collar
ties 
. > are the ceiling joists.

. > HTH a little,

. > Jake Watson, P.E.
. > Salt Lake City, UT
  -----Original Message-----
  From: Bill Polhemus [mailto:bill(--nospam--at)polhemus.cc]
  Sent: Wednesday, July 16, 2003 8:18 PM
  To: Wood(--nospam--at)structuralist.net; seaint(--nospam--at)seaint.org
  Subject: Details, Struts, Thrust, Etc.


  "Dumb Question" Time.

  (FWIW, I once again posit my definition of a "Dumb Question": A
question to which everyone PRETENDS that everyone knows the answer, but
each one's answer is most likely different from everyone else's, no one
is really in agreement as to the "correct answer," and the collective
ignorance is perpetuated because no one wants to be heard actually
asking the question).

  Anyhow...

  You have a house with a hipped roof. The house has an "ell" or two
coming off the "main" part of the structure, and the roof heights are
different because the slopes are the same, but the widths of each part
of the building (in plan) are different.

  I cannot figure out how to get such a configuration to "work" without
a post at each end of the ridge beam. It just doesn't compute for me,
especially when the pitch is rather shallow (30 deg. or less).

  So, convince me that the thing doesn't collapse into the hallway of
the main house.

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