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Re: Sloped Roof Diaphragm Discussion/ APA wish list

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Tom,
One difference is that there is often a ridge vent at the ridge. In this
case, the gap is usually 2" wide or more, compared to the typical 1/8"
gap between panels. Maybe this is a problem, maybe not. Who knows?

I had this question from a plan reviewer once, and I could not find
anything published that answered the question of what happens at a ridge
vent as far as force transfer is concerned.

Since you are with APA, maybe you can get then to look into this and
publish something. 

On a related subject, I was interested to see your message the other day
about the out-of-print folded plate article where you said:
"Surprisingly enough, our help desk was inundated with requests for APA
Research Report 121."

Personally, I was not surprised that a lot of people were interested.
This is a good example of one of the gray areas that engineers have to
deal with all the time, and where we could use some help from the trade
associations.

In fact, there are 4 areas that seem to fall within the mandate of APA,
which come up over and over on this list, and which are not addressed
adequately anywhere as far as I know:


1. Do folded plate roofs really work like the flat diaphragm assumptions
that are used to design them? Can a roof have hips and valleys and
gables all over the place with no problem?

2. Is a 2" or more gap at a ridge vent any concern for force transfer in
a diaphragm, and if so, what details can address this?

3. Building codes require roofs to be ventilated. When blocking is also
required at the eaves to transfer wind or seismic forces from a roof
diaphragm down into a shear wall, how are these two competing
requirements handled? I have seen answers on this list like drill a few
holes, leave out alternate blocks, etc., but it would be nice to see
something from APA addressing this.

4. For architectural reasons, wood trusses often have a heel height of
2', 3' or more at the exterior bearing wall. How can the roof diaphragm
forces be transferred down to the shear wall/bearing wall? Again, some
ideas have been suggested on this list, such as little 22.5" wide
special "blocking trusses" made to fit between the reqular trusses at
24" spacing. Is this a good way to handle this problem?


Well, that is my Top Four wish list for APA. I have a big thick notebook
of design aids and other stuff I got at an APA seminar, and I don't
think any of these common items were really covered in there. Something
with some example calculations and sample details would be nice. Someone
may respond to this and say "use your engineering judgement", or "run a
finite element analysis", but it seems to me that on frequently
encountered problems like these it doesn't make sense for a million
engineers all over the country to have to reinvent the wheel
independently.

If some of this stuff is already addressed out there, please let me know
where and I will be happy to order the book or publication.


On a side note, I want you to know that industry participation (whether
it is official or not), such as by APA (you and John Rose APA
(retired)), and AISC (Charlie Carter and Heath Mitchell)  help make this
list one of the best engineering resources anywhere. 

Thanks,
Rick Burch
Columbia, SC




>Interesting discussion.  So how does a "slit" at the ridge of a >diaphragm differ from unblocked sheathing edges on the same roof?
 
Tom
----------------------------------------------------------------------- 
Thomas D. Skaggs, Ph.D., P.E. 
Senior Engineer 
APA - The Engineered Wood Association 
P.O. Box 11700 
Tacoma, WA 98411-0700 
ph: 253/565-6600 
fx: 253/565-7265 
-----------------------------------------------------------------------

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