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Re: IRC Braced Panels

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You are missing the intent of the IRC.  It is not based on testing.  The IRC is one big book of rule of thumbs that has worked for quite a while.  Those rules are similar to the prescriptive design that was contained in the UBC, and have weathered near design level earthquake and wind events.  No, you can't make the IRC pencil out.  There are a hundred other variables that go into the actual force resisting system in a house that we don't consider in an engineered design, force transfer around windows and doors, friction between all the shear resisting elements, interior walls that are connected but not counted, and the list goes on.  That is why I am comfortable with the IRC for mostly symmetric, two story or less wood framed houses.
No about the flexible versus rigid diaphragm question.  It isn't a tiny speck, it is real.  There has been testing and that testing suggests that we ought to rethink our concept of what a flexible diaphragm really is.  Full scale tests have been performed that strongly indicate that a  wood diaphragm can often distribute load like a rigid diaphragm would.  This testing was not done on your IRC house however, they were houses with lots of irregularity, one that would not meet the IRC.  Anywho, watch out for this rigid vs. flexible issue to come up in a code near you.
Lastly, the cutoff limit for me on holdown force is a force above which I could not physically resist myself.  I figure that force to be 300# or so.
Ted Ryan
----- Original Message -----
Sent: Saturday, January 08, 2005 2:35 PM
Subject: IRC Braced Panels

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