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A Shop for Dan #4: Engineering a wall

Blog entry by Dan Lyke posted 08-15-2009 05:48 PM 2412 reads 0 times favorited 0 comments Add to Favorites Watch
« Part 3: Musings on foundations Part 4 of A Shop for Dan series Part 5: I can dig it! »

Any wall within 5 feet of a property line needs to be 1 hour rated. With a living roof, each end of the rafter will be supporting a thousand lbs. Furthermore, I live in earthquake country. Building a wall isn’t a trivial matter of slapping up some 2×4s and calling it done.

I’ve looked around for information on AAC and SIPs and those foam concrete forms, and even asked some people about information on them, and it sure seems like we’re coming back to stick-built for price and ease of construction.

My notes are missing the URLs of a bunch of the wall assembly notes that I’ve found from various sources, so I’m going to have to go back and find those, but if the seismic calculations work out for the roof load I’m looking at, I’m currently tending towards 2×6 studs on 16” centers, R-19 unfaced batting in the middle, gypsum wallboard on the inside, an exterior rated gypsum wallboard on the outside, with an air gap (probably with thin plywood lath), and ferrocement siding.

Using fairly similar materials on the inside and outside of the wall increases the shear strength dramatically (you can see that if you have OSB on the outside and gypsum on the inside, then the gypsum will fail before the OSB and you only get the shear strength of the stronger of the two). The other options for a 1 hour wall stick built seem to be either additional layers of gypsum, or 2×4s on 8” centers with staggered offsets, still resulting in a 6” nominal thick interior space.

Finally, on seismic design, I followed the procedure at this PDF about determining seismic design category and used the app at the USGS hazmaps design page to come up with some numbers (I need to verify my guesses at soil type):





Period (sec)Sa (g)
0.21.500 (Ss, Site Class B)
1.00.600 (S1, Site Class B)

I’m not quite sure how to interpret some of these numbers in terms of shear strength, but if all I have to worry about is that .6g number (because flex in the wall will hopefully absorb that 1.5g(!) at the much shorter period), then I can just look at the shear strength tables for my wall assembly and see what’ll give me ten tons (the weight of the living roof) of lateral support.

-- Dan Lyke, Petaluma California, http://www.flutterby.net/



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Dan Lyke

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