# More math

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#### olds

##### Elite Member

You are going to build a concrete wall with a 30" diameter pipe going through it. The wall is 8'H X 10'W X 3'D.

1. Find cu ft of wall. 240 cu ft
2. Find surface area (or would it be volume?) of pipe. 3579.6 cu in or 2.071 cu ft
3. Subtract pipe from wall.
4. Order 237.929 of concrete

Thanks.

#### L0ne

##### Member
For total volume of concrete needed, it would be (total volume of wall)-(volume of pipe/cylinder)

#### olds

##### Elite Member
For total volume of concrete needed, it would be (total volume of wall)-(volume of pipe/cylinder)
I knew I was making it harder than it was.
Thanks.

#### DrPizza

##### Administrator Elite Member Goat Whisperer
Is that outer diameter or inner diameter of the pipe.

And, a tip. Generally (at least on the few occasions I've needed any), concrete is sold by the cubic yard, not cubic foot. There are TWENTY-SEVEN (27) cubic feet in a cubic yard. (It's funny to hear stories about the people who think there are 3 cubic feet, or 9 cubic feet in a cubic yard; it's not like they can take it back.

Also, THREE FEET THICK WALL?! wtf are you building? I've seen supports on bridges that aren't that thick.

#### Sunny129

##### Diamond Member
area of cross-section of pipe = (3.14)(1.25')^2 = 4.90625 sq. ft.

volume of pipe = area*depth = 4.90625*3 = 14.71875 cu. ft.

volume of concrete needed = volume of total wall - volume of pipe, just as LOne mentioned...so total concrete needed = 240 - 14.71875 = 225.28125 cu. ft.

#### olds

##### Elite Member
Is that outer diameter or inner diameter of the pipe.

And, a tip. Generally (at least on the few occasions I've needed any), concrete is sold by the cubic yard, not cubic foot. There are TWENTY-SEVEN (27) cubic feet in a cubic yard. (It's funny to hear stories about the people who think there are 3 cubic feet, or 9 cubic feet in a cubic yard; it's not like they can take it back.

Also, THREE FEET THICK WALL?! wtf are you building? I've seen supports on bridges that aren't that thick.
It's just an example.
I have to take a promotional exam for work. They ask crap like this that we don't even do. I'll never use this stuff again. Yes I work for the government. I teach defensive drivers training and safety.

8.34 cu yd.

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#### olds

##### Elite Member
area of cross-section of pipe = (3.14)(1.25')^2 = 4.90625 sq. ft.

volume of pipe = area*depth = 4.90625*3 = 14.71875 cu. ft.

volume of concrete needed = volume of total wall - volume of pipe, just as LOne mentioned...so total concrete needed = 240 - 14.71875 = 225.28125 cu. ft.
Thanks, that confirms what I got after LOne's comment.

8.34 cu yd

#### HeXen

##### Diamond Member
Is that outer diameter or inner diameter of the pipe.

And, a tip. Generally (at least on the few occasions I've needed any), concrete is sold by the cubic yard, not cubic foot. There are TWENTY-SEVEN (27) cubic feet in a cubic yard. (It's funny to hear stories about the people who think there are 3 cubic feet, or 9 cubic feet in a cubic yard; it's not like they can take it back.

Also, THREE FEET THICK WALL?! wtf are you building? I've seen supports on bridges that aren't that thick.
lol, when we order concrete, we just tell them the L-H-W and they figure it out for us. I always sucked at math anyway, believe it or not i only had 8th grade level math and still graduated with 3 credits over, guess they let us get by with it back then.

#### lxskllr

##### No Lifer
It's just an example.
I have to take a promotional exam for work. They ask crap like this that we don't even do. I'll never use this stuff again. Yes I work for the government. I teach defensive drivers training and safety.

8.34 cu yd.
Get this book...

http://www.amazon.com/Pocket-Ref-Edi...ket+ref+glover

It's the awesomest book that ever awesomed. It has anything you'd ever want to know for practical math, conversions, or bits of physical trivia. It's also small, so it easily fits in a computer bag, or briefcase.

#### Broheim

##### Diamond Member
Also, THREE FEET THICK WALL?! wtf are you building? I've seen supports on bridges that aren't that thick.
maybe "stuff" has been getting out of the basement?

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