Tuesday, September 10, 2019

Some features of disaster resistant houses

Disaster resistant buildings don't need to be expensive.

Natural disasters include hurricanes, earthquakes, wild fires and termites.
In many countries, cinder blocks or the local equivalent are the least expensive building material. From a structural dynamics standpoint, the bending, rending and twisting motions on the house are dominated by stiffness at the bottom of the structure and are dominated by mass at the top.

The ideal structure would be stiff at the bottom and light-weight at the top. It makes economic and structural sense to use a cinder block half-wall and to design around the material's limitations.
 A ring beam is a continuous, or near continuous beam along the top of the cinder block wall. Relatively short lengths of concrete rebar are used in the corners to countermeasure cracks that want to "grow" from the interior angles.

---added after RBD noted that I did not specify the material for the ring beam---While the beam can be virtually any material as long as it is available in long lengths and it can be joined to efficiently pass the loads from one length to the next, in most of Latin America poured concrete would be the material of choice. It is inexpensive and and can be poured to make a continuous beam.

The ring beam also serves to mitigate against termites working their way up the insides of the cinder blocks from the soil line.

Threaded studs are cast-in-place to attach the stud wall's sill plate.
Stud and sheet metal construction is light weight and fire resistant. Sheet metal ties the studs together to resist shear in the plane of the wall, i.e. racking.
We are counting on the ring beam to tie the cinder block wall together. The best way to preserve the integrity of the ring beam is to position the bottoms of the window casements on top of the ring beam.
Hip roofs are the roof shape of choice in areas with high winds. Contrary to intuition, a steeper roof is better in a high wind because "separation" is physically identical to an airplane wing stalling at a high angle of attack. That is, they both lose lift because the air flow lifts off the surface and back-circulation causes a loss of lift.

Incidentally, metal roof for strength and fire resistance.
Hip roofs present the minimum amount of sail area to the wind for a given pitch. They are also not sensitive to wind direction.
Use awnings if you want shade. The hurricane may rip off the awnings but you will still have a roof over your head. In areas of wild fires, skip the eve troughs.
Functional shutters. Also windows that open and screens that fit. You might live without power for many weeks. Your air conditioner will not run without power. Windows that open allow cross breezes and screens mean you will not be eaten alive by mosquitoes and flies.

If the roof is painted white (steel), then the solar loading will be minimized as you wait for the power to come back on.
 Cinder block walls are vulnerable to bending in-and-out of the plane. Angles stiffen them but not if the angled buttress is too short.
Small and simple to avoid vast expanses of cinder block wall without support perpendicular to the plane. If the structure gets big, then interior walls need to be cinder blocks as well to buttress those expanses.

None of these features are incredibly expensive but they do require forethought and do add a little bit to the cost.

Hat tip Old NFO

10 comments:

  1. What is a ring beam made of? Wood or do you use a metal beam?

    Do you suggest leaving the cinder blocks hollow or do you fill them with cement?

    Attic - is it ok to have a floor up there? Maybe not living space but storage?

    I'm not an engineer (far from it) but I have been thinking about the "safe" house concept. In eastern Nebraska, high winds and hail are the primary issues we face related to natural disasters. I would not be building near a flood zone.

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    1. Good catch. The picture in my head was concrete although it could be steel or triple two-by with angle steel in the corners. The two-bys would offer no termite resistance.

      I would stuff the hollows with rubble to minimize the amount of concrete draining into them. Some sagging in as good as that helps lock the structure together. Too much is wasted money.

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    2. I missed answering the question about the attic.

      I am not sure most of these cabins have enough foot-print to make much attic. One option would be to use scissors trusses and use the extra space for a storage loft...perhaps wide, wire mesh shelves suspended from cables. Not a full attic but extra room for bedding and other light weight, bulky goods.

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  2. Thanks, and it's shutters, and they do make 'hurricane' shutters that are actually worth having. Re the cinderblocks, I'd fill them with rebar and concrete for strength. Also awnings/porches should be 'separate' from the roof structure itself, not a continuation, as you discussed WRT the wind angles/forces.

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  3. Hmm, if I were building from scratch, I'd take the block ALL of the way up, and rebar/grout multiple cavities for protection as the NFO suggests. For the roof structure I'd use either steel or prestressed concrete panels. Fire damage goes to zero.

    If cost wasn't an issue, the whole thing would be concrete. Minimizing weight up top is only an issue it can blow away. You can then clad it however you want for aesthetics - they make anchors to hook brick courses.

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    1. It is not my intention to dispute you, but many places where hurricanes/typhoon risk is high are also high for earthquakes. (Pacific Rim, Central America, islands around Cuba, etc)

      Even steel reinforced masonry does not fare well in earthquakes, especially where interrupted by window openings. https://www.nap.edu/openbook/0309090644/xhtml/images/p200087ceg47001.jpg

      While better for winds, I think you lose too much to earthquake risk by going all concrete.

      Just my humble opinion -Joe

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    2. I was mainly thinking wind. I'll leave the earthquake zone to Aesop.

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  4. Er, NFO, unless "the" NFO is appropriate. Apologies if I got it wrong.

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  5. There are always local considerations. My suggestion is a higher than average ceiling height, transom windows and an even higher ceiling in the entry area. All concrete would sweat in the south. All concrete cracks. Termites fit in cracks. Keep water away from the structure to minimize termite interest. Also, had a house once that had the footprint of the "S" as in the SS script used in WW2. Porches in the angle of each side of the house caught wind from whatever direction and shot it through the house. Very pleasant.

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  6. Another thought is the ring beam needs rebar. Concrete does well in compression but is much less capable in tension, pulling forces.

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