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Old 09-09-2013, 12:12 AM   #1
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Small storm shelter poured concrete roof how to??


Hi Folks: My wife and I live in a VERY tornado proned part of Alabama, and after the last cp of years (and MANY near misses) we have decided to dig out a hillside on our property and build a tiny (6ft by 6ft) tornado shelter of our own. I have my plans both in my head and drawn up on paper, except for the rood, which will be 4 inch thick reinforced concrete that I plan to pour myself. My question is the best way to do a roof like this. My idea was to lay the 8inch block walls, filled with concrets and rebar as I go. Then, on the top course of blocks, use "L" blocks, which have a 4 inch by 4 inch spaced notched out in them. Then, I'd put both reinforcement wire, along with, say, 1/2" rebar wired into a mesh of about 6inch by 6 inch squares. Then, tie those into the rebar coming out of the wall blocks. Then, build a "form" using 2" by 4";s and 3/4" plywood, spaced about every 12 inches on center. Then, put temporary 2" by 4" "posts", right about the center of the span of each of the rafter 2" by 4"'s. Then, pour in the concrete and tie it into the wall blocks and that notched out area. Then, when the concrete is cured, take down all the form studs, plywood etc. My question is this. I did my math, and that much roof of solid concrete, plus the wire and rebar will weight about 3700 pounds.

So, since the inside deminsions are right at 6ft by 6ft, and I read where concrete weighs about 150 pounds per cu. foot. How do I know if I have enough of a framework to stop the concrete from just breaking down my studs, plywood etc??? I figured at 4" thick, I need the studs, plywood etc to hold up about 50 pounds per square foot, being 4" or 1/3ft thick.

Can anybody point me to the right place, or tell me exactly how to SAFELY know how to pour a solid concrete reinforced ceiling, and KNOW that it's not going to come "crashing down" on up? Thanks a LOT, Chuck

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Old 09-09-2013, 07:52 AM   #2
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Small storm shelter poured concrete roof how to??


I think your math is a little "heavy", I come up with closer to 2000#'s of concrete and steel.

There's only one way I know of to ensure that the formwork won't collapse, and even that has a margin of error: Consult a structural engineer to draw exactly what you need. It's fairly simple for them to apply the weight of the plastic concrete to the limits of the forming lumber.........

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Old 09-09-2013, 08:17 AM   #3
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Small storm shelter poured concrete roof how to??


I would just leave 6 feet of rebar sticking out the top of the block.
then bend it over so it is inside the slab. Let the slab cover the entire top of the blocks.
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Old 09-09-2013, 11:25 AM   #4
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Small storm shelter poured concrete roof how to??


If you've got a hill, you don't need a concrete roof. You just need something strong enough to hold up some sod. Way back when I was a small sprout, we used to play in my great aunt's tornado shelter. It wasn't anything more than a surplus school bus from the county that they parked next to the hill and used a welder to install some extra ribbing in the roof, then covered over the whole thing with dirt.

No, I'm not suggesting that you do anything similar! But, I am suggesting that you are perhaps overthinking the type of shelter needed to ride out a tornado. Just being a couple of feet underground is enough protection without the bunker roof. So, maybe in the modern age that translates to a block structure covered in Ice and Water Shield, with some LVL's across the roof span (sloped, just like a "flat" roof would be.) and then more Ice and Water over some plywood. Yeah, you'd probably want an engineer to still calc the loads as wet soil and water can be every bit as heavy as the concrete you're thinking of doing.
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Old 09-11-2013, 05:27 AM   #5
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Small storm shelter poured concrete roof how to??


think the weight's off a bit but defer to mama we use 150# cf for figgerin' weight so i figger there's 2,700#,,, didn't incl steel as thats supported by ' chairs ' on top of the walls - i'd place the roof on top of the walls so used 7'4" x 7'4"

IF it were mine ( ONLY IF ! ), i'd nail 2x4 pieces to the wall & fit in 3/4" cdx w/plywood jnts supported by 2x4's,,, roofing paper will prevent conc from sticking to ply form,,, then place slab on top of the block walls,,, when its cured, knock out the 2x4s & down the ply will come down,,, use a rope on the support's btm part & you won't even need a hard hat,,, of course, my pe thinks i wear a belt AND suspenders

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Old 09-11-2013, 07:56 AM   #6
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Small storm shelter poured concrete roof how to??


Make sure your door swings inward to allow it to open and you to get out if there is any debris that prevent an outward swinging door from opening.

Almost any method of temporarily supporting the plywood roof form will work. - There nothing wrong with a few 2x4 just wedged in and knocked out a few days after pouring.

In India, they use freshly cut 2" saplings for support of the floor slab forms on buildings up to 7 or 9 stories and then often often remove/knock out them after a couple of weeks and reuse them on later floors. If there is financial snag it was always funny to see a project temporarily stalled and the saplings start to sprout because of the heat and water from the flooding of the slabs for curing. Despite the crude methods, they work well and building are very substantial and durable.

They have all the technology to used fancier methods that may be cheaper, but there is moral desire to provide work for people. - The women on the jobs that carry concrete in baskets on their heads are probably wearing there wealth in the form of gold jewelry.

Dick

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Old 09-11-2013, 02:40 PM   #7
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Small storm shelter poured concrete roof how to??


for what it's worth, FEMA has engineered design drawings available for free download

http://www.fema.gov/media-library/as.../26215?id=5788 for safe rooms.

they also have grants and other funding opportunities for the construction of safe rooms

http://www.fema.gov/safe-rooms/residential-safe-rooms

good luck!
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Old 09-11-2013, 06:58 PM   #8
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Small storm shelter poured concrete roof how to??


Gary: Thanks a lot. I'll check those sites out. Our volenteer Fire Dept, which is only 1/2 mile from my house JUST finished having a storm shelter put in, with matching funds from FEMA. It's an above ground, based on a 12-18 inch thick slab to anchor it to, 1/4" solid steel, with a round roof. They say it's built for 65 ppl sitting or up to 200 ppl standing in there. That's close to me, but I just do NOT like crowds, but would use it if a tornado comes at us before I get mine built. Chuck
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Old 09-11-2013, 09:31 PM   #9
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Small storm shelter poured concrete roof how to??


The FEMA site is great for details and ideas that you ordinarily come up with.

There are two different sets of criteria for the small, family safe cell and a larger group cell because absolute protection is not practical because of the size and exposure.

The FEMA designs are not a code item unless your local authority requires it. The small cell suggestions are bullet-proof with zero penetration of debris and the same thicknesses and materials apply whether it is below ground, in a basement, built into the living area of the house or even out in the open, so they will work everywhere.

The larger group shelters follow most of the same guidelines, but there are differences and the varied materials for larger cells do not have the lengthy testing history since they are normally done through a professional for each building. There is a difference between sheltering a family for a short time than for a large group of relative strangers. I imagine they even need special door treatments/sizes since it is a public building.

Dick

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Old 09-13-2013, 01:54 AM   #10
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Small storm shelter poured concrete roof how to??


Dick: You're right, they (FEMA) does have different requirements/guidelines for small family shelters, vs large public ones. I was reading the FEMA site last night, and you have to have it pretty much like any public store. Our big one for example that our Fire Dept just had built and for example, it has a restroom, and the stall has to be wide enough to get into in a wheelchair, and has to have the adjusted toilet height. The door has to be at least 36" for the same reason. It has to have ramps in order to get into the thing of course if there is any kind of obstacles. Has to have hand rails. It just follows along with the "Americans with disability acts". Chuck
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Old 09-13-2013, 12:52 PM   #11
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Small storm shelter poured concrete roof how to??


I would consider using a structural steel pan and placing the concrete over it
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Old 09-13-2013, 01:52 PM   #12
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Small storm shelter poured concrete roof how to??


Marvel if does not meet the FEMA suggestions unless the concrete is 8" thick and reinforced at 8" o.c. because it has not been tested for pentration by debris. - It took the steel and wood people many year of testing lamination combinations to come up with something.

Dick

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