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RE: IRC Braced Panels

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Dmitri,

Many of us feel your pain.  This has been discussed some in the past.  I
myself have brought it up in the past year as I have found myself working in
an area where the IRC is "king".  I have seen so many projects that in my
opinion should never be considered for the prescriptive designs of the IRC.
I've seen custom residences where no design is done before the "designer"
submits for a building permit.  Then the project will be taken to an
engineer with a request to design a couple of beams and "just this one
sheared wall".

I believe that part of the problem is there is no definition of "unusually
shaped" in the IRC.  I participated in a seminar last September on the ASCE
Wind Provisions.  Only a few minutes were spent on the "simplified
approach".  I raised the question to the speakers as to what structures fall
into the "usual" or "unusual" categories since there is no definition.  The
answer was extremely vague, with the phrases such as you'll need to use your
judgment there.  There is an example in the "Guide to the Use of the Wind
Load Provisions of ASCE 7-02" where a 2500 square foot residence is the
example the house is "L" shaped with the short leg being only an 8'
projection from the main area of the house.  In the example the simplified
approach is not used and there are over 6 pages of calcs just to get the
loads let alone doing any design with them.

If that one definition alone had some guidelines as to what is or is not
usually shaped would be a big help.

J. Grill

Joseph R. Grill, P.E. (Structural)
Shephard - Wesnitzer, Inc.
Civil Engineering and Surveying
1146 W. Hwy 89A Suite B
Sedona, AZ  86340
PHONE (928) 282-1061
FAX (928) 282-2058
jgrill(--nospam--at)swiaz.com
 


-----Original Message-----
From: Dmitri Wright [mailto:dmitri(--nospam--at)pciengineers.com] 
Sent: Sunday, January 09, 2005 7:18 PM
To: Seaint
Subject: Re: IRC Braced Panels

Ed,

Thank you for confirming my views on this subject.  I think the main 
problems with the IRC braced panel requirements are as follows:
1)  The IRC allows "parts" of the building to be engineered, while the rest 
of the building is prescriptive.  This as opposed to a prescriptive "path", 
or engineered "path", where the entire building is designed one way or the 
other.  This results in a hodge-podge and incoherent design.
2)  I have tried to understand the braced panel requirements in the IRC, and

I found them to be very,very ambiguous.  Perhaps I am missing some necessary

information, but it appeared that a design that would clearly not meet the 
intent of the code, could be allowed by a literal interpretation of the 
requirements.
3)  Even if the code is interpreted correctly, designs that clearly do not 
meet the code are routinely permitted.  I see this all the time, and I know 
the code reviews in this part of the country (Portland, OR metro area) are 
more thorough than in other parts of the country.
4)  You have now confirmed my guess that if all the requirements were 
correctly followed, the resulting design would still not be equivalent to an

engineered design.  One great example of this is that the IRC allows braced 
panels with a height to width ratio of 4:1 in certain situations, whereas 
the IBC restricts any braced panel to a maximum ratio of 3-1/2:1.

As a design engineer "on the ground", as opposed to an academic or code 
official, these discrepancies are very frustrating.  I spend more time 
trying to make excuses to owners and contractors, about why the codes are so

inconsistent, than I spend in design.

I would like to see this problem addressed in future codes.  Because change 
is difficult and slow, a compromise may be the way to start.  Perhaps the 
prescriptive method could be better clarified, and some of the provisions 
brought in line with IBC in high wind and seismic areas (such as the 4:1 
ratio restriction).  The IBC could perhaps reduce safety factors in single 
family residential construction 3 stories and less.

Well, I think I got about 2-1/2 cents in on this one.  I would love to hear 
what other list members think about this subject.

Dmitri Wright, PE
Portland, OR




This is a a retry - I don't believe it made it first time.

Curiosity has killed my cat, so for fun I "engineered" a simple 2 story
28 ft x 64 ft house that for lateral bracing would work prescriptively
per the IRC.  It is in Category D(sub)1, or 110 mph, per IRC table
R602.10.1.  I used the 2003 IBC for engineering, with ASD load
combinations per 1605.3.1.

Using the minimum quantity of prescribed exterior braced panels (48"
wide or more), I came up with holdown forces that varied from approx
1000 to 2500 lbs.  The IRC 48" braced panel requires no holdown.

The most shocking result were a couple of interior braced panels, 8 ft
long, that can be gypsum faced (no wood sheathing), with no holdowns,
and support below the wall consisting only of a double floor joist.  The
unit shear was 464 plf, and the holdown force 3500 lbs!  I feel like I
should be wrong, but it's a pretty simple calc.

Do you think that this will work?  Based upon performance of similar
structures in design-level events, I suspect it would probably do okay.
Of course, I would never engineer such a thing, deferring to the IBC.

Does anyone know where the source documents can be found for developing
the current IRC bracing requirements?

Going back to a discussion a couple of weeks ago, when we engineer
residences, what should be the cutoff value for the holdown/no holdown
decision?  500#?  1000#?  3500#????  I will be using this set of
calculations as a defense if some plan reviewer ever questions my cutoff
value (I formerly used zero, but that's going up a bit for residences).
This all makes the flexible vs rigid diaphragm issue look like a tiny
speck.


Ed Tornberg
Tornberg Consulting, LLC
503-551-4165




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