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IRC Braced Panels

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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