Author Topic: Underground Concrete Root Cellar Roof  (Read 13091 times)

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Offline Squirl

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Underground Concrete Root Cellar Roof
« on: May 27, 2011, 06:58:15 AM »
Ok.  I am designing a root cellar for my property.  It is in upstate NY, so it would have a deep frost depth.  I am looking to have a lot of dirt (4ft deep at least) on top of it.  I plan on building into the side of the mountain and burying it.  I want it to last far past my lifetime with little or no maintenance.  This leads me to veer away from wood.  I was looking for concrete, but I can’t seem to find load and span tables for concrete and steel the way there is for wood.  Mostly I was looking for rebar size, placement, and distance with concrete thickness.  I assume a curved dome would probably handle the weight best, but I can’t find out where the calculations for something like this would be.  I know this is probably a more advanced question and I am willing to do the work and learn the area.  Can anyone point me in the right direction?

Something similar to this:

Offline glenn kangiser

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Re: Underground Concrete Root Cellar Roof
« Reply #1 on: May 27, 2011, 07:22:10 AM »
I don't know about tables Squirl, but on Mikey B's place it was engineered for 12 foot span with 5/8 at 6 inches on center across with 5/8 at 12 inches on center lengthwise and was a flat floor top, ceiling below and was to support concrete masonry walls 12" thick above.  Rebar on a ceiling needs to be about 1 1/2 inches in from the bottom surface as the top concrete is in compression and the rebar in tension.  

I don't see a vaulted ceiling as being as strong rebar wise ans the shortest length of rebar is straight as in a flat roof.  A vault has extra rebar so is relying on the compressive strength of the concrete to support it though the deformities in the rebar would still hold things together.
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Offline Squirl

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Re: Underground Concrete Root Cellar Roof
« Reply #2 on: September 12, 2011, 06:33:40 PM »
Ok.  I've been trying to find this information for a while. I also now need this to design a cistern. I finally went to the library to start reading technical manuals on ACI 318.  It took me a while because I don't know the terms.  Searching for information on concrete spans yields little result.  The technical name for this is a "biaxial suspended slab" or two way suspended slab.

I'm still researching, as I find out more information I will share it.  This is from the book.  

Concrete has a tensile strength of 8-15%. of compressive rating. Most calculations use 7.5%.

Concrete has a shear strength of 1/3 to 1/4 of compressive strength.  I would use 1/4 for any calculations.

Upping the psi rating from 2500 to 4000-6000 is cheap, and many times done.  I found this in the ICC code too. You can drop 2 inches off the wall by going to 4000 psi.  I think it only takes an extra 2 bags per yard or $20 or Portland cement.

The minimum thickness of any two way slab according to ACI 318 is 5 inches.

I am still researching rebar design.  The book is very expensive and the only one like it at the library.  They won’t let anyone take it out.

I assume creating an arch converts more of the shear force to compressive force and would only be stronger.

Offline rick91351

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Re: Underground Concrete Root Cellar Roof
« Reply #3 on: September 12, 2011, 06:41:58 PM »
Hey Squirl very interested in this project.  What is the book and I will see if I can find it?

 
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Offline Squirl

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Re: Underground Concrete Root Cellar Roof
« Reply #4 on: September 13, 2011, 05:14:47 AM »
http://www.amazon.com/Design-Reinforced-Concrete-Jack-McCormac/dp/0470279273/ref=sr_1_1?ie=UTF8&qid=1315918971&sr=8-1

Design of Reinforced Concrete
by Jack McCormac

Good book, but I don't think I'll use it enough to warrant $150. 

Offline rick91351

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Re: Underground Concrete Root Cellar Roof
« Reply #5 on: September 13, 2011, 06:31:01 AM »
Thanks...

When driving ready-mix truck I delivered to several fallout shelters and cellars.  Basically poured the footing, came back for the walls and then they did the roof.  Usually it was plywood with lots of shoring underneath.  Held atop the plywood about three inches was a lattice of rebar tied about at eight or twelve inch spacing.  A cellar would need some venting incorporated on to it.  I was never around a real root cellar to understand all there is to understand about one.   

You can find on Amazon lots of those books at about $20 used.  However they might not be the most current.  But if this is basic stuff it would not change all that much most likely until you get in to some more modern applications.  We are talking just basic stuff here.         
Proverbs 24:3-5 Through wisdom is an house builded; an by understanding it is established.  4 And by knowledge shall the chambers be filled with all precious and pleasant riches.  5 A wise man is strong; yea, a man of knowledge increaseth strength.

Offline Squirl

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Re: Underground Concrete Root Cellar Roof
« Reply #6 on: October 20, 2011, 12:30:50 PM »
I got some time to more reading and research.  I will share what I had found

Since the root cellar would be supported on both sides by a complete concrete wall the concrete roof spanning between the would be a "one way slab".  This would require rebar to be placed perpendicular to the walls to handle the tension and shear loads.  Rebar would also have to be placed parallel to the walls as shrinkage/thermal expansion bars.  IIRC from the Concrete Book I listed earlier, ACI 318 states that the maximum spacing of shrinkage thermal expansion bars is 18" O/C.

Thank You State University of New York Delhi.  A walkthrough for calculating.

http://faculty.delhi.edu/hultendc/AECT480-Lecture%206.pdf



In this example it is simply calculated as a series of 12” wide concrete beams.  There is a lot of published data on concrete beam calculations.

The tension rebar has a maximum spacing of 12” O/C.  It also has a requirement of no more than 3x the thickness.    I believe there is still a 4-5” minimum thickness.

Two way slabs are when the slabs are just supported by a post and have a less than 2 to 1 length to width ratio of span.
http://faculty.delhi.edu/hultendc/AECT480-Lecture%207.pdf

More calculations and design to come.

Offline Squirl

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Re: Underground Concrete Root Cellar Roof
« Reply #7 on: October 21, 2011, 07:57:40 AM »
I will try to post this in sections so as not to overwhelm a single post.

First I decided I am going to go with an 8 foot wide root cellar.  It would give me just over 2 ft to walk down the center and 3 ft of shelving off to each side.  Next I decided from my reading that it you can get a 25% increase in strength for a 10% increase in cost by going with a higher psi concrete.  I will be running my calculations for 4000 psi concrete.   I will be using 60,000 psi #4 rebar, because this seems to be the most common.  From my reading, reinforcement (rebar, wire mesh)  seems to be calculated by volume in X amount of concrete.
So let’s start out.

To calculate thickness there are charts.  These are expressed as Height (H) of the concrete in relation to the Length (L) divided by a factor (X).  H=L/X .  The X you get from various concrete charts in publications.  The chart from the Delhi publication say to use 20 and the length in inches.  So H=96/20. H = 4.8”.  ACI 318 states a minimum of 5”. So it will be a 5” slab.

Offline Squirl

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Re: Underground Concrete Root Cellar Roof
« Reply #8 on: October 21, 2011, 07:58:53 AM »
Now to determine the Moment factor (Load).

The calculation is Wu = 1.2D + 1.6L, with D = the dead load and L= the live load.  I suppose this is the safety margin the book talked about that is built into the concrete load calculations.  This increase the dead load by 20% and the live load by 60%.
I plan on this being a maximum of 4 ft underground.  From the soil charts in the IRC, the calculated average for the heaviest clay soils is 60 lbs per cubic ft.  So 4 ft of this would be 240 lbs/f dead load.  Also you have to factor in the 150 lbs per cubic foot of the slab for 5 inches.  For the live load, this would be buried in the yard.  I want it to be able to withstand driving over it with the tractor or the ground snow load.  Based on my readings here: http://www.skghoshassociates.com/sk_publication/design_load_parking_2005.pdf
http://www.ilevel.com/literature/TB-105.pdf
parking garages have a 40-50 psf design requirement for slabs.  This combined with my 50 psf ground snow load give me a 100 psf live load.

So the equation for the Moment factor would be:
 W= 1.2(240) + (5/12)(150) + 1.6 (100) = 510.5 Lbs or .5105 Klbs
To calculate the maximum moment force:
Mmax =(w*L^2)/8 = (.5105*8^2)/8 = 4.084 thousand pounds per square foot or 49,008 lbs per inch.

Offline Squirl

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Re: Underground Concrete Root Cellar Roof
« Reply #9 on: October 21, 2011, 08:04:16 AM »


Now to use all of this information to determine the amount of rebar required (P).  This is represented by.

Where Mu is the Mmax I just calculated.  Φ is .9.  B is the width of the “beam”, in this case 12” so you know how much rebar is required per ft, and D is the depth the slab is to the in the slab to the center rebar.  In this case D=4 because the rebar requires ¾ of an inch coverage beneth it and it is ½ inch rebar so that is another ¼ inch to the center so ¾ + ¼ = 1” out of the 5” = 4”.  You can see how extending the depth of the concrete reduces the PSI load calculated that the slab has to hold.  Taller depth equals more load handled.

So 49,008/(.9*12”*(4”^2) = 284 psi. 

The next part is the tricky part.  How to take this and find out how much rebar is required.  This is where the concrete book posted earlier comes into play.  I was looked forever to find these charts online.  The charts tell you the factor volume of reinforcement required per psi rating of concrete and psi rating of rebar.  When you click on the “look inside” feature of the amazon book, these charts just happen to be the few pages displayed.  They are Tables A.8 – A.13.  So from Table A.13 with a psi of 284 I get P = .0050. So I take this and put it into the equation Area=P(B)(D), or =.0050(12”)(4”)  or .24 square inches of rebar per square foot of slab.
Final conversion would be Rebar spacing = Area (12”) * (the area of a single bar (#4 = .2) divided by the area of coverage required.  So Rebar = 12” * (.2/.24)

Or #4 rebar 10” O/C.

Offline Squirl

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Re: Underground Concrete Root Cellar Roof
« Reply #10 on: October 21, 2011, 08:18:14 AM »
The last piece is to calculate the rebar running parallel.  (shrinkage bars)  The only equation I have to go on is the one published in the delhi problem.  It states that spacing is based on size and  total height, but there is no mention of span or weight.  I do not have the concrete book in front of me to confirm this.

So A = .002 (12”)(5”) = .12 square inches. So rebar spacing would be 12” *(.2/.12) or 20” O/C.  Since ACI 318 specifies a maximum of 18” O/C, so that it is.


Offline Squirl

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Re: Underground Concrete Root Cellar Roof
« Reply #11 on: October 21, 2011, 08:21:08 AM »
So in summary for an 8 foot span root cellar 4 ft below grade in a heavy clay soil I would pour a slab 5” thick, 4000 psi concrete, with 60,000 psi #4 Rebar 10” O/C perpendicular to the walls and #4 rebar 18” O/C parallel to the walls.

It may not be perfect, but I like taking this approach more than just “winging it.”


All comments/criticism appreciated.  One of the parts I have less surety in is the calculations of live load if a vehicle drives over it.  I expect the loading is the same as in a parking garage.

Offline glenn kangiser

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Re: Underground Concrete Root Cellar Roof
« Reply #12 on: October 21, 2011, 09:02:32 AM »
Sorry I have not had time to study this all over real well, Squirl, but it is similar to or heavier than lots of the second story building floors I work on with about a 9 foot span.  I am sure you should have no problem.  If you wanted to insure extra strength, you could up the size of the perpendicular rebar.  I assume you will be 1 1/2 to 2 inches off of the bottom of the ceiling with your bars?  The rebar is in tension and the concrete above it is in compression.

My comment about the arched roof above was meaning that the rebar would want to pull out the bottom of the arch if the walls were to give at all.  With the flat roof the rebar is in tension so it holds the sides directly together.  With the arch it can get longer since a failure of the concrete below the bar coupled with the walls pushing out from weight would not keep the arch together.  Hope that makes a bit of sense.  An arch - made properly with sides that do not move does not really need the rebar -except to hold chunks of concrete together possibly, since the entire arch is in compression.  A failing arch needs the rebar to hold pieces together - the failure would be due to the sides not being built properly - ie: buttressed and or properly designed footings.   My take on it - let me know if someone else sees it different.
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Offline CjAl

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Re: Underground Concrete Root Cellar Roof
« Reply #13 on: October 21, 2011, 09:41:22 AM »
Ever think about just buying a tornado shelter? Might not be something thats common up there but down here all the companies that cast septic tanks also cast shelters. Its all pre cast and you just bury it.

Offline Squirl

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Re: Underground Concrete Root Cellar Roof
« Reply #14 on: October 21, 2011, 10:18:10 AM »
Ever think about just buying a tornado shelter? Might not be something thats common up there but down here all the companies that cast septic tanks also cast shelters. Its all pre cast and you just bury it.

No.  This is upstate NY. No tornados. No shelters, just septic tanks.  I looked.

The question is not just burying it, but how deep.  That average septic tank is designed to be buried 12" deep, so you are talking a 300% difference in weight.  There are few additional factors that veer me away from a pre manufactured product.

Size. - Almost all pre-manufactured products are 4ft squares.  No room to put much of anything.
Weight-  Those 4 ft tanks weigh thousands of pounds.  Concrete, whether pre purchased or poured in place, will require the same thickness.  So at 5" thick that is 62 lbs per square foot.  So for an 8x8 room that would be 4000 lbs. Which requires a crane or backhoe rental.
Cost- Pre manufactured products cost around 50-100% more than doing it yourself.  And the few that I found didn't give any specs as to what load they could handle.
Safety – The manufactured products for sale around here are not designed for people, especially me.  I would rather be able to understand the concept if some tell me “sure, you can bet your life on it”

Part of the point of this is also an exercise of to know and understand how to do something rather than just paying someone to do it for me.  I could always just hire an engineer and be done with it.

Offline Squirl

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Re: Underground Concrete Root Cellar Roof
« Reply #15 on: October 21, 2011, 10:41:37 AM »
Thanks for the comments glenn.  The arch would more likely be slight.  The purpose is not just structural, but functional.  Root cellars require humidity and condensation can accumulate on the roof.  Most designs recommend a slight arch for the condensation to not drip on the food.

The examples run through by the Engineering Professor at the University were for 3/4 of an inch coverage of #4 rebar.  That is also the requirement in a footnote in the I.C.C. "Concrete cover for reinforcement measured from the inside face of the wall shall not be less than 3/4-inch."  I also remember a low requirement from the concrete design books, but don't have the exact figure in front of me.  From the equations, the closer to the bottom of the slab, as long as it has the minimum coverage, the more weight the slab can bear.  In reality it would be hard to keep the rebar at exactly 3/4 of an inch.

It is probably a little beefier than some of the floors you had seen, if I did it right.  A few of the examples I read for building designs didn't use close to the dead and live loads I used for this design.  I used over 500 lbs per square foot, and most examples I read were less than 200 lbs per square foot. 

I am also looking into additional fiber reinforcing.  It is more expensive, but from what I was reading the impact, crack, and failure properties were pretty amazing.

Offline CjAl

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Re: Underground Concrete Root Cellar Roof
« Reply #16 on: October 21, 2011, 11:36:53 AM »
An arched ceiling of a perfect 180 degree half circle distributes all the weight straight down to the footing it sits on. Its been an architectural detail since the beging of building just about.

A less arched roof will distribute the weight in an outward spreading manner to the top of the walls.

But i am no engineer.

In new york they found a room with an arched roof made of blocks with keystones down the center. The room had been sealed for hundreds of years and was in perfect condition. And its below the city and the subway system.

I know, i watch too much tv

Offline Grimjack

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Re: Underground Concrete Root Cellar Roof
« Reply #17 on: October 22, 2011, 06:24:56 AM »
Have you looked into "spancrete"

http://www.spancrete.com/single_family.php

apparently you tell them the area you need to "span" and the load it needs to carry and they do all the dirty work!

Offline glenn kangiser

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Re: Underground Concrete Root Cellar Roof
« Reply #18 on: October 22, 2011, 07:14:16 AM »
Thanks for the comments glenn.  The arch would more likely be slight.  The purpose is not just structural, but functional.  Root cellars require humidity and condensation can accumulate on the roof.  Most designs recommend a slight arch for the condensation to not drip on the food.

The examples run through by the Engineering Professor at the University were for 3/4 of an inch coverage of #4 rebar.  That is also the requirement in a footnote in the I.C.C. "Concrete cover for reinforcement measured from the inside face of the wall shall not be less than 3/4-inch."  I also remember a low requirement from the concrete design books, but don't have the exact figure in front of me.  From the equations, the closer to the bottom of the slab, as long as it has the minimum coverage, the more weight the slab can bear.  In reality it would be hard to keep the rebar at exactly 3/4 of an inch.

It is probably a little beefier than some of the floors you had seen, if I did it right.  A few of the examples I read for building designs didn't use close to the dead and live loads I used for this design.  I used over 500 lbs per square foot, and most examples I read were less than 200 lbs per square foot. 

I am also looking into additional fiber reinforcing.  It is more expensive, but from what I was reading the impact, crack, and failure properties were pretty amazing.

Thanks for the explanation on the arch, Squirl.  That makes sense.

Yes - the floors actually had a composite slab with decking keyed to the bottom of the concrete through indentations but half of the concrete displaced by the form of the decking.  Yours is stronger.
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