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RE: Details, Struts, Thrust, Etc.

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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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