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tlhfirelion
New Member
Posts:35
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| 18 Oct 2008 10:44 AM |
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Posted By dmaceld on 10/18/2008 12:22 AM
I've never really looked at roof or building tie down methods so don't know details or have links to offer, but here is one idea. How about straps over the entire roof anchored into the concrete of the walls? A method similar to this is used for anchoring manufactured homes. The manufacturer installs metal straps that go from one rim board all the way over the house to the opposite rim board. The installer then attaches these straps to ground anchors or anchors embedded in the the footing.
That is an excellent idea, I never had even though of that. It's rather straight forward and a diy'er could do that. Good post thank you.
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Raul Hinojosa
New Member
Posts:40
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| 19 Oct 2008 12:49 AM |
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There's other way also to built the entire house to withstand 200+ mph, using a reinforced concrete roof, and both icfs walls and the insulated concrete roofs can be structurally designed for that kind of wind. A lot of full concrete houses have been built in south Florida using this concept.
I hope that this can give you answers and options to make you house a fully insulated concrete shell.
Regards. |
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dmaceld
Veteran Member
Posts:1465
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| 19 Oct 2008 10:39 AM |
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I got to wondering this morning how many homes that are damaged by tornadoes aren't actually damaged by the wind or funnel cloud but rather explode from inside due to sudden extreme air pressure drop around the funnel. This would be exacerbated by the air tight nature of an ICF house because there's no way for the air inside to suddenly escape. The force on the roof of a typical house could easily be way over 200,000 pounds if the atmospheric pressure dropped 1" water column, or about 1 psi.
How about designing gable walls to blow out on purpose? Make them of two to four sections fastened around the perimeter to the wall and roof and weakly fastened together at the seams. Sudden air drop and poof! The walls blow out, air inside escapes, and the roof is left intact.
Does this make sense to anyone? I'm not a tornado scientist so this is somewhat speculative on my part.
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| Even a retired engineer can build a house successfully w/ GBT help! |
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dmaceld
Veteran Member
Posts:1465
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| 19 Oct 2008 10:43 AM |
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Oops, that should be about 12" water column, or about .5 psi. Atmospheric pressure is about 32', or 384" water column. |
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| Even a retired engineer can build a house successfully w/ GBT help! |
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Alton
Veteran Member
Posts:2164
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| 19 Oct 2008 12:04 PM |
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Dmaceld,
The text below is from this web site:
http://science.jrank.org/pages/6868/Tornado-Tornado-characteristics.html
When houses or other structures are destroyed by a tornado, they are not simply blown down by the high winds: they appear to explode. For many years it was believed that the low pressure of the tornado vortex caused such explosions. According to this theory, if the pressure outside a building drops very quickly the air inside may not escape fast enough (through cracks, holes, and the like) to equalize the pressure difference. The higher pressure inside the building then pushes out windows or whole walls, and the structure looks like it had exploded. Studies of tornado damage have shown that buildings do not actually explode in this manner. Instead, high wind passing over a house roof acts like the air moving over an airplane wing: it gives the roof an upward force or lift which tends to raise the roof vertically off the house. Winds also enter the building through broken windows or doors pressurizing the house as one would blow up a balloon. The combination of these forces tends to blow the walls and roof off the structure from the inside out giving the appearance of an explosion. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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dmaceld
Veteran Member
Posts:1465
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| 19 Oct 2008 11:11 PM |
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Posted By Alton on 10/19/2008 12:04 PM Instead, high wind passing over a house roof acts like the air moving over an airplane wing: it gives the roof an upward force or lift which tends to raise the roof vertically off the house. Winds also enter the building through broken windows or doors pressurizing the house as one would blow up a balloon. The combination of these forces tends to blow the walls and roof off the structure from the inside out giving the appearance of an explosion. Thanks for the explanation. I was aware of the above in relation to hurricanes having spent 15 yrs in the New Orleans area and going through several severe weather training sessions. I just didn't think of it in terms of a tornado, but it makes sense. I imagine a tornado will rip brick off a wall just like a hurricane will because of the airplane wing effect, i.e., wind going past a corner of the building causing a low pressure on the back side of the corner. Probably the best tornado proof house would the same as a hurricane proof house, a concrete dome.
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| Even a retired engineer can build a house successfully w/ GBT help! |
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Dug
New Member
Posts:45
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| 20 Oct 2008 12:48 AM |
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We are in the process of building in the midwest and tornadoes were a topic of concern. We have an excellenct location for a walkout basement (no, Kansas is not entirely flat...) and plan to go with 6 inch ICF on the basement and first floor. We will finish the basment and have a combination movie room/safe room in the southwest corner (no windows and an exterior steel door). I had planned to cap the "safe room" with concrete, however, my engineer advised against this uneeded expense. He designed the walls to withstand 200+ mph winds and with the concrete exteneded 20 feet high, the saferoom would be untouched, however, I would loose my roof and windows. Given the statistically near zero chance of a direct hit from a tornadoe, I can live with the threat of loosing my roof. ; -)
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Farmboy
Basic Member
Posts:356
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| 21 Oct 2008 12:13 AM |
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Dug. May I ask how the walls are engineered? What size, strength rating and spacing of the vertical rebar? Let's assume for the 10' wall above grade, the Logix table I referred to before says #4 (1/2" dia.) rebar, 60ksi, spaced on 24" centers handles up to 140 mph winds. If you use larger #5 (5/8" dia) rebar, 60ksi strength you can increase the spacing to 48" on center. Is your vertical rebar spacing more like 12" or 18"? The table call for 3000 psi concrete and a minimum of #4 horizontal rebar every other course or 32". Just curious!
Regarding your "fraidy room" ceiling, if you don't cap it with concrete, be sure your floor system is sturdy enough in that area keep any pianos or cast iron skillets from penetrating it. Cheers Dave |
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DesignGreen
New Member
Posts:9
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| 21 Oct 2008 08:45 AM |
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I would caution increasing the spacing horizontally beyond the height of one (1) course of block. Nintey percent of the block out there is 16" in height (Nudura is 18".....I think it's a Canadian thing...lol). If you increase the spacing beyond that height, then you will have a course that is un-reinforced horizontally. Now I know that this is a monolithic pour and it should not matter. But it has been my experience that the pattern and frequency of the steel mat in the wall definitely plays not only to increased strength but it makes for a better install because of the rigidity of the wall during the pour. I also specify an ICF mix to the concrete companies in our area. Nothing really special about it other than 3500 psf with 3/8 aggregate. It helps to prevent voids and is defintely easier to work. |
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Farmboy
Basic Member
Posts:356
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| 21 Oct 2008 11:02 AM |
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DeGree, I agree and will place horzizontal bar every course, but was only stating what I found in the Logix installation guide as a minimum. Being in Kansas myself, want to build strong.
Do you stay with 3/8" aggregate for all wall thicknesses?
Aggregate Story. Know someone who had a crew pour 10' ICF walls 6.25" thick. They had problems getting concrete to flow under window sills and it congregated on the horizontal rebar overlaps at the top of the wall. There were stones up to 1-3/4" x 5/8" x 1". I would imagine the flow didn't improve down deep and there is some honeycombing. AND they didn't vibrate internally! Lesson learned Dave |
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slenzen
Basic Member
Posts:434
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| 22 Oct 2008 05:57 PM |
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My business got hit by an F-3 in MN and lost much of the steel roof panels. Commercial freezer roof/wall panels were found miles away. Had to throw out over $1million in frozen food finished goods. Two double block buildings across the street were blown down to their foundations. Nothing left but concrete slab and pallet racking. Still a miracle my double block building stood. I am considering using a structural concrete insulated panel SCIP for a monolithic passive solar home. http://www.metrockscip.com/ There are a number of these systems such as tridipanel, m2. |
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Dug
New Member
Posts:45
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| 22 Oct 2008 08:20 PM |
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Hi Dave,
I went with #5 on ground contact walls and #4 above ground every 16 inches both vertically and horizontally. Concrete was 3000 psi with 3/8 aggregate.
The fraidy room will be capped with 11 7/8 joists on 16 inch centers (spanning 15 foot) annd 3/4 inch subfloor with 3/4 inch hardwood. It's not bullet proof, but close enough. ; -)
Dug
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