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RE: Proposed roof design

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I fully agree with you Dennis and my apologies to anyone who took my
comments to imply that we don't design our diaphragms here in the Midwest.
I didn't mean to imply that the system I described was simply sketched up
with no regard to maintaining a load path or recognizing the magnitude of
the shear loads.  Every project should be designed to assure proper transfer
of the lateral shear forces.  I wanted to state up front what conditions my
project was designed for: it was not Zone 4 (Zone 1 in fact) and the wind
loads in my case were relatively light (due to the site and size of the
building).  

That being said, when the diaphragm forces are low, the standard sheathing
on rafters system isn't applicable and you have an architect who doesn't
want to use structural insulated panels, a purlin/sleeper/sheathing system
similar to what I outlined is a valid option.  As I stated in my first post,
I feel the important thing when designing and detailing a somewhat "unusual"
system like this is the importance of adequate oversight during construction
to make sure that the system functions as you designed it.  When some
contractor decides to build it "his way", all the number crunching and
detailing in the world isn't going to do any good.

Troy A. Madlem, P.E.
mailto:tmadlem(--nospam--at)kenherceg.com



 -----Original Message-----
From: 	Dennis Wish [mailto:dennis.wish(--nospam--at)verizon.net] 
Sent:	Thursday, October 02, 2003 9:44 PM
To:	seaint(--nospam--at)seaint.org
Subject:	RE: Proposed roof design

I am somewhat surprised by the first sentence in this reply. Excuse me,
Troy, for picking on you but you raise an assumption that is widely
maintained by engineers - that in a high risk seismic zone, we design
the diaphragm differently or consider its ability to transfer shear
uniquely.

The fact is that every area of this country designs or should design
buildings (which are not prescriptively designed) to transfer shear.
Shear is not a unique concept to seismic zones, but occurs due to wind
load as well. Therefore, if you design to wind loads, you distribute
shear through the diaphragm to shear resisting elements or shearwalls. 

Therefore, the diaphragm should be constructed so as to adequately
transfer shear. If conceived to transfer a smaller amount of shear (as
Bill's example might show) then the question is whether or not it is
capable of transferring enough shear to the resisting walls.

With that said, I think the comments given are valid, but that everyone
needs to be cognizant that lateral design is to be considered in every
section of the world except possibly where there is no wind and no
seismic (or Hurricane) force to consider - maybe below ground :>)

Dennis S. Wish, PE

-----Original Message-----
From: Troy Madlem [mailto:tmadlem(--nospam--at)kenherceg.com] 
Sent: Wednesday, October 01, 2003 1:55 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Proposed roof design

Bill,

I did a roof similar to that that had exposed timber framed trusses but
not
in zone 4 and with fairly light lateral loads.  It had 2x8 purlins
spanning
between the trusses with the space between the purlins insulated, then
2x4
sleepers laid flat on top of the purlins (to form an air space to vent
the
roof) and the plywood deck on top.  I called for full depth 2x10
blocking to
be installed between the purlins immediately above the trusses and
transferred all my in plane loads thru this blocking.  The purlins were
also
design for the biaxial bending if I remember correctly.  The big thing
is
being on site when they start constructing the roof so you can emphasize
the
importance of how everything gets put together then oversee the first
section or two.

Troy A. Madlem, P.E.
mailto:tmadlem(--nospam--at)kenherceg.com



 -----Original Message-----
From: 	Bill Allen [mailto:T.W.Allen(--nospam--at)cox.net] 
Sent:	Wednesday, October 01, 2003 3:07 PM
To:	seaint(--nospam--at)seaint.org
Subject:	Proposed roof design

Ladies and Gentlemen;

An architectural client of mine is proposing an unorthodox (to me) roof
design and has asked me to review. I have some concerns, but I thought I
would solicit comments from the engineering community before I
responded.

When he first described the system to me, I thought the sheathing would
be applied directly on the roof rafters.

See: http://members.cox.net/ballense/roof.htm

Criteria:

UBC/CBC 70 MPH Wind, Exposure C
Seismic Zone 4 (although residential construction and diaphragm shears
should be small < 150 PLF).

Your comments would be most appreciated.

Thanks,

T. William (Bill) Allen, S.E. (CA #2607)
V/F (949) 248-8588
San Juan Capistrano, CA
http://members.cox.net/ballense/




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