OK Math Problem/ Concrete Figuring HELP!!!!

Anonymous-0 · Aug 10, 2012

putting up a 10ft x 35 silo. The footing needs to be 10ft dia. x 2 ft thick x 4ft deep. so the outer is 11ft the inner is 9ft = the 2ft thick. How much concrete?

I got 10yards...

what did you get?

bjb in TX from Ne · Aug 10, 2012

I get 9.3 cubic yards

State of Jefferson · Aug 10, 2012

Circumference of a 10' diameter circle is 31'5"x2'x4'=251cu/ft / 27(cu/ft per yard) =9.3 cubic yards.

I'd order 10 to be sure.

Ben

Bill(Wis) · Aug 10, 2012

You're close but if you're having it delivered by mixer truck they'll figure it out to within a shovel full. BTDT.
What kind of a mix are you specifying?

JakeR. · Aug 10, 2012

I ended up with a completely different number, 4.65 cubic yards. Using the formula pi(3.142) x r squared(5.5 x 5.5=30.25) height(4) for the outer cylinder I got 380.182 cubic feet. Now do the same with the inner cylinder 3.142 x 20.25(4.5x4.5) x 4=254.502. Now subtract the inner from the outer(380.182-254.502)= 125.68 now divide that by 27 and you get 4.65 cubic yards. Hopefully somebody else will respond and maybe we'll get some answers that correspond!

Angle Iron · Aug 10, 2012

Thats real close to what I got. This is a footer not a slab.
Angle Iron

State of Jefferson · Aug 10, 2012

Better recheck your math...

Ben

Jake Collie · Aug 10, 2012

The volume of a cylinder is found by multiplying pi times radius squared times depth. Circumference has nothing to do with volume with this formula. The correct answer is 4.65 cubic yards.

Traditional Farmer · Aug 10, 2012

See what the normal load is probably 8 yards and then refigure before ordering a balance I guarantee you the measurements will be off some
unless its going into a steel form better to have too much as sending out a yard to makeup a shortage will cost big $$$.

550Doug · Aug 10, 2012

pi x height x (5.5 x 5.5 - 4.5 x 4.5) / 27
= 3.142 x 4 x (30.25 - 20.25) /27
= 12.568 x 10 / 27
= 125.68 / 27
= 4.65 cu yds

Note: When you build a straight wall, then bend it into a circle you end up crunching out concrete that you don"t need. That"s why you cannot use the circumference approach.

Cliff Nelson · Aug 10, 2012

4.65 yards is what you need but you should add at least 10% for safety factor.

willie in mn · Aug 10, 2012

Using the info given, 4.65 yards, order 5 to allow for uneven base, spliiage. etc.
However 2 items missing. How thick is the silo wall, & is the 10 ft dimension inside, outside, or centerline. Normal construction gives dimensions as outside measure. IIRC, silo staves are 4" thick, so centerline of wall would be 9' 10". Would being offcenter of the footing by 2" be enough to make a difference? Too expensive to have an OOPSIE later.
Willie

Ferd · Aug 10, 2012

The formula for circumference is pi x diameter so 3.14 (pi) x 10 x 4 x 2 / 25 (not 27) which will give about 10% waste = 10.048 cy.

The formula pi r squared gives the area of a circle.

Shetland Sheepdog · Aug 10, 2012

Ferd, I think that we want the [u:04f40feb75]area[/u:04f40feb75] of the outside circle with the [u:04f40feb75]area[/u:04f40feb75]of the inner circle subtracted, times the depth to achieve the volume! Circumfrence has nothing to do with it, except to determine feet of forms needed!
JMHO, Dave

dstates · Aug 10, 2012

[b:210b576465]IF IT IS 2 FT THICK THEN THE INNER DIAMETER IS 7.[/b:210b576465]

Volume of a ring is the volume of a cylinder with the outer diameter minus the volume of the "hole"

Volume of a cylinder = depth*pi*radius^2

Therefore, volume of a ring = (depth*pi*outerradius^2)-(depth*pi*innerradius^2)

volume = (4*3.14*5.5^2)-(4*3.14*3.5^2) = 226 cu ft = [b:210b576465]8.4 cu yd[/b:210b576465]

I double checked this in my CAD software. If the inner diameter really is 9, then the 4.65 cubic yard number is right.

Ferd · Aug 10, 2012

Sure hate to see you order short - gets expensive. Here's another way to look at it. A footing 2 x 4 is 8 cubic foot / lineal foot or 1/3rd cubic yard allowing for about 10% waste. So it takes about 1 cubic yard for 3 lineal foot of footing. So, 4 cy will pour about 12 lineal feet of footing, and 10 cy will pour about 30 lineal foot of footing.

Shetland Sheepdog · Aug 10, 2012

Ferd,
I'm on board with the logic of both ways of figuring it! I just can not understand why there is such a discrepancy in the outcome!
Somebody a whole lot smarter than I am is gonna hafta 'splain it to me! :? :roll:

Dave

dpendzic · Aug 10, 2012

4.65 cy is correct

Paul from MI · Aug 10, 2012

Pretty easy. Figure the volume of a 11' dia x 4' thick cylinder, then subtract the volume of a 9' dia. x 4' deep cylinder. then divide by 27 to convert to cubic yards. Answer 4.65 cu. yds.

Angle Iron · Aug 10, 2012

Go to www.concretenetwork.com and click on calculater, then click on columns. Input the measurement for the outside. Then the inside subtract and it will give you what you need. As others have said always order a little extra.
Angle Iron

Shetland Sheepdog · Aug 10, 2012

:? :? :? Somebody still gotta 'splain to me why a straight wall the [u:bce1e0ecab]same[/u:bce1e0ecab]length, depth and thickness as the circular wall takes [u:bce1e0ecab]exactly[/u:bce1e0ecab] twice as much concrete! Do the math using both approaches, I did! :shock:
Dave

State of Jefferson · Aug 11, 2012

I sure can't figure it out. I actually went out and scratched a couple circles in my driveway. Still doesn't seem right, but the 4.somethin seems like the right answer.

I goss publik skuuls dodn't tiech mi en mithmetiks...

Ben

Bill(Wis) · Aug 11, 2012

You have some questions to answer.
1. How did you arrive at an inner diameter of 9' and an outer diameter of 11' if the wall(footing) is 2 feet thick? A two foot thick wall(footing) all the way around would have a 4' larger outside diameter. Some of the guys figured the diametric numbers you provided and did their math based on that which would supply enough concrete for a 1' thick wall(footing). that answer was 4.65 cu yds which would be correct.
2. If the wall (footing) is to be 2' thick and 4' high then twice as much concrete would be needed or 9.3 yds which would be correct.

This reminds me of an absent minded neighbor who told me he was "putting up a silo". All he did was build a straight wall. I thought he'd really become absent minded and just forgot to go up. He then packed silage against the wall in a half hearted attempt to duplicate another neighbor's bunk silo.

SOO... MF1155????? Is this going to be a "TOWER SILO" or some other kind of "SILO". We need to know before we can accomplish any further calculations.

Shetland Sheepdog · Aug 11, 2012

Bill,
Okay, [u:d942b69a31]now[/u:d942b69a31] I get it! :roll: You have to add/subtract a foot to the diameter on [u:d942b69a31][b:d942b69a31]both[/b:d942b69a31][/u:d942b69a31] sides of the circle!

Ya gotta look at the [b:d942b69a31][u:d942b69a31]whole[/u:d942b69a31][/b:d942b69a31] picture! :shock: :roll: :wink:
Dave
PS: better order 10 yards of concrete just to be safe!

OK Math Problem/ Concrete Figuring HELP!!!!

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