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RE: Exposed Beam Roof Design

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Scott,
I went off the digest mode because there are some topics that I wish to
discuss and want to be able to receive replies quickly - the reason I am
responding so quickly.
I have concerns about the use of what I described as a SIP panel even though
the lower plywood sheathing would be what transfers shear while the sandwich
was intended to support and distribute the weight of the heavy clay tiles
without causing damage or too much compression to the rigid insulation. 

The real problem with this is not so much in shear transfer as it is in
appearance. The lines at the end of the roof should be such that the tile
appears to sit above the heavy exposed beams and any framing (as I did
suggest) would raise the exposed perimeter of the roofline and give the
appearance that I had multiple roofs upon each other. I don't think this
would play well to most architects.

Years ago I tried using rigid insulation from below the structural diaphragm
when I had smaller beams (4x) spaced at say 32" on center and the client
wanted to give the appearance that the roof was T&G. What I did here was to
create a modified panelized roof with the rigid insulation below the
structural sheathing and secure T&G from below that would hide the
insulation and not increase the visual thickness from the exterior of the
home. 

The trouble with this is that it becomes difficult to work electrical and
mechanical into the room and the labor becomes very costly. This is the
reason I was looking for a more affordable solution.

I'm not knocking SIPS - I've seen the details for long screw installations,
but the lateral movement between layers of plywood inducing bending in the
screws places the tile at risk during seismic (cyclic) motion and can create
a hazard to anyone on the ground being a target for tile. While I understand
that SIP's are glued and laminated, I am not aware of any testing for
eccentricity or deformation of the skins due to the weight of the tile
above. I would not have the same fear in lightweight roof coverings, but
this would also change the mode of lateral control to wind (most likely).

Best Regards,
Dennis
-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: Wednesday, June 21, 2006 6:48 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Exposed Beam Roof Design

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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