I'm near the Loma Prieta fault line, and am thinking about some sort of earth sheltered home. Would the structural posts in a wofati provide adequate earthquake protection from the thermal mass on the roof?
It's not the posts themselves I would be concerned about, but the joints between structural members. In stable situations, pretty much any joint that looks strong will do, but seismic stresses can make certain kinds of soil act like liquid and push/pull things in all kinds of directions. I would build in redundant bracing so that one failure could not cascade, and if I valued my life I would consult a professional engineer to design the wood framing sizes, spacings and connections.
Location: USDA zone 8b
posted 2 years ago
Thanks Glenn. I guess I do value my life enough to invest in a pro. If I ever had to go for it by myself, I would overbuild it. I would keep it small too.
Location: Salt Lake Valley, Utah, hardiness zone 6b/7a
posted 2 years ago
Morgan, might I suggest the you simply delete the mass on the roof? With a well insulated roof, you will still have the walls and floor in contact with the moderating temperatures of the earth and your engineer will be much happier. You could still retain a green roof, if you like that aesthetic. It would require a layer of soil, but not nearly as much (and your engineer won't be quite as happy).
Another consideration about green roofs is that dirt attracts burrowing critters that can shred liners and insulation. A solution from the PAHS/umbrella-house discussions is to separate the roof from the rest of the structure with a two foot vertical gap and an overhang, all the way round, or dispense with the green roof altogether and use conventional roofing material or pave a flat roof for a patio.
Even without the mass on the roof, you still need to make sure that your posts will not shift in the event of an earthquake, or just with the slow motion movement of the dirt over time. Your roof frame needs to brace those posts in place against lateral loads., and you may need to brace the bottom ends as well, against soil liquefaction, since you probably are not going to pour a concrete floor.
I had a cousin who lived in a beautiful woodland home in the coastal hills near Santa Cruz . It had two-story window walls that shattered spectacularly during the '89 Loma Prieta earthquake. Fortunately, no injuries.
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