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Re: Exposed Beam Roof Design

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Dennis:

While not intending to be a "company hack" just pushing a product, I would
be remiss if not saying that SIPs would be ideal for such a situation.

The only difficulty is that you don't have a ton of information as to how
well SIP panel screws or panel spikes (big nails) will deal with the
seismic diaphragm loads in a "desireable" way (i.e. ductile response
similar to nails through structural sheathing in typical diaphragms).

But, in theory, they should work.  You just may (or may not) have a tough
time gettings code official to allow their use in high seismic zones.

Regards,

Scott
Adrian, MI


On Wed, 21 Jun 2006, Dennis Wish wrote:

> 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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