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Re: Residential construction-hip beams

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I've watched this thread with interest.

There's one approach that I haven't seen mentioned yet (back to the old
free-body diagram).

The roof is loaded vertically with dead an live loads (let's not discuss
wind, etc., for the momemt).

If you take a free body diagram of the rafter (typically a pair framing
into the hip beam at right angles in plan view), the reactions are
vertical...  But "you ain't home yet".  

The vertical reaction on the hip beam must be supported by something.  If
you use a 3-hinged arch analogy, you can exchange the vertical reaction at
the hip for a horizontal reaction.  Using this approach, there are a few
things that must be dealt with for each rafter:

	1.  The horizontal reaction at the eave.
	2.  The horizontal reacton at the hip beam.
	3.  Connections to make sure everything works.

1.  The reaction at the eave can be handled several ways (assuming you can
get it built the way you detailed it).

	.  Use the roof sheathing as a beam/girder/diaphragm to span to the
sidewalls of the house.
	.  Tie the lower end of each rafter horizontally along ceiling joists, and
transfer loading into the ceiling.  Or if you aren't 	comfortable with that
approach, you can construct a horizontal diaphragm on top of the ceiling
joists and  span it between 	sidewalls.
	.  I once (because of vaulted ceilings, and a host of other
"enhancements") hid a horizontal steel beam (plate) at the eave all 	the
way around the top to work as both a beam and a tension member.
	.  etc...  You get the idea.

2.  The rafter reaction at the hip beam is pretty straightforward.  You
simply have horizontal reactions from applied load, and a small vertical
reaction while you're setting the rafter(s).

'Bottom line...  If you can't resist the loads (Technique is your choice;
you're the engineer) it won't work.  If you can demonstrate that the loads
can be resisted, there shouldn't be a problem.  Except...  The odds of the
contractor's building it the way you intended are fairly long against you
unless you're working a custom home, and have a contractor who is willing
to do it *right*.  The KISS principle is king here.

'Apologize for long-windedness.

Fountain E. Conner, P.E.
Gulf Breeze, Fl. 32561




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