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Exposed Beam Roof Design

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I am looking for details showing how other engineers design typical exposed
beam ceilings / roofs with shear transfer. I have done a variety of
different designs over the years, but the cost of labor is beginning to make
me think more seriously if I am providing the best design. Here are some
specifics:

Zone ? California Mojave Desert with summer climates that can reach 130
degrees at an average of between 4% and 10% relative humidity and the
minimum roof insulation requirement of R38 according to the last time I
checked.

In the past I would construct the roof in he following manner:
Exposed beams 6x material at 4-feet on center
2x10 roof joists perpendicular and hung from the beams with Simpson hangers
Roof sheathing above (typically 5-ply 15/32? minimum OSB or Plywood)
unblocked
Clay tile roof where slope exceeds 2-1/2: 12 or 3:12
5/8? or ½? gypsum ceiling at bottom of 2x10 joists (depth of 2x10?s
determined for uncompressed R38 batt insulation

Alternative:
2x Joists parallel to exposed beams with 2x3 or 2x4 nailer secured to side
of exposed beams for nailing or securing of gypsum ceiling.

Full depth blocking at double top plate set by flat cut (depth greatest at
exterior of bearing wall) beams and ripped 2x continuous blocking w/ A35F or
new flat clip ? 2 per block at 4? lengths or 2 per block where spacing does
not exceed 24? on center and joist rather than exposed beams are used.

2x6 exterior DF Kiln Dried studs with double top plate not cut for exposed
beams. Birds cut or Corbel cut ends of exposed beam overhang with 1x T&G at
exterior  eave for southwest look. Boundary nailing of structural diaphragm
to solid blocking based on analysis or minimum 8D common nails @ 6? o.c. ?
10D common @ 6? o.c. if blocking is 3x or wider.

Lately I have been reconsidering the cut and flush nailed roof rafters
because of cost (shrinkage no a factor where KD lumber is used). If I stack
the roof 2x above the exposed beams, I can save on the beam cost but will
need to block the rafters above the beams. The alternative may be to use a
T&G ceiling with a rigid insulation above, but I am not crazy about the idea
of using either a sandwiched OSB sheathing with a Rigid Insulation (similar
to an SIP panel).

The latest design I am considering, the client will have a vaulted living
room ceiling with a second floor gallery for his legal library that over
looks the great room below. This means stacking a 38-foot support for the
gallery (open one side) and the use  installation of a heavy timber Ridge
Beam with exposed beams flush cut. The backside needs to change slope to
accommodate the 18 or 19- foot plate height to accommodate the book cases
flush to the wall and this can be flush framed to the ridge beam so the roof
diaphragm sheathing is flush to the top of beams and rafters. It does pose a
problem on the lower sloped backside if the lumber rafters are stacked above
the exposed beams.

I would like some ideas and possibly if you wouldn?t mind sharing some
details to show the means of detailing these types of ceilings, I would
appreciate the help.

Finally, I am also considering the use of manufactured lumber for the studs
in order to minimize the labor necessary to shim the interior radius of the
crowned studs for a plumb and flush fit of interior drywall with a skim coat
of plaster. For the high wall (on the exterior side viewed from the gallery,
the stud wall will be platform framed since an exterior patio cover at least
12-feet deep will brace the wall at 9-feet from out of plane buckling. At a
38-foot clear span through the living room (minus the framing for the stairs
to access the second floor octagonal tower, library (gallery) and exterior
balcony, the designer has provided sufficient locations of interior walls on
the first floor to stack shear. The ends of the ridge beams will be
sufficiently solid to allow for shear transfer down to foundations and the
depth of the room will be such that the aspect ratio of this roof diaphragm
will not exceed a 2 to 1 aspect ratio. Shear transfer is pretty straight
forward, but some creativeness will be required for the octagon shaped tower
with exposed beam ceiling so as to flush frame the floor joists for
performance grade to minimum bounce and to provide for a sub-floor necessary
to create a 1-inch step from interior to balcony with a ¼? per foot slope
for drainage at the balcony.

I hope the explanation is clear. I am seeking some typical shear transfer
and framing details to see how others have address the ceiling framing to
accommodate a minimum of R38 roof insulation (batt or rigid) so that the
material and labor costs provide the best solution. Also I can use some
thought to the use of manufactured lumber in the exterior platform framed
walls to see if the cost saving in labor to straighten the wall justifies
the use of manufactured lumber which generally saves waste and does not need
as much labor and shimming.

I?m working in AutoCAD and can read any DWG, DWF or DXF format. You are
welcome to send a PDF file as well and can attach directly to this list
replies as long as you purge your details to keep the size of file
considerate for those with slower Internet connections.  If the list refuses
the drawing format, zip the file but do not use EXE or self-expanding
formats. 

If responding from the SEAINT list, do not post attachments. Please send
directly to me at dennis.wish(--nospam--at)verizon.net . 

Thanks in advance for your assistance on this question.

Dennis S. Wish, PE
California Professional Engineer
C-41250 Exp. March 31, 2007
Structural Engineering Consultant.
 


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