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Re: IRC Braced Panels[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "Seaint" <seaint(--nospam--at)seaint.org>
- Subject: Re: IRC Braced Panels
- From: "Dmitri Wright" <dmitri(--nospam--at)pciengineers.com>
- Date: Sun, 9 Jan 2005 18:18:28 -0800
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 ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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