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Where did I go wrong?

AaronRosenthal | Posted in Construction Techniques on September 9, 2005 08:40am

Not the first time, but this time for my house, I had to put in a footing for an extension and the calculator gave me the wrong bag count.
Footing = 96″ x 18″ x 7 1/8″. Construction Master IV gave me 7.25 cu ft per footing, and at .5 cu ft per bag, that’s (rounded up) 15 bags.
In fact, each footing ran 20 bags.
My counterman at my lumberyard when I asked him how I got it wrong, drew a little sketch and said “22 bags” each.
How did I get it wrong?

Quality repairs for your home.

AaronR Construction
Vancouver, Canada

 

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Replies

  1. jimblodgett | Sep 09, 2005 09:18am | #1

    I usually convert all the measurements to feet.  That way the answer to the math problem ends up in cubic feet, which (even) I can convert to cubic yards, or sacks of redimix. 

    I'm guessing that "7+1/8" you used was because you used 2x8s for forms?  I'd use 2/3 for that dimension instead of 7+1/8.

    So for me, 96" x 18" x 7+1/8" would be 8 x 1.5 x 2/3

    multiply that out

    8 x 1.5 = 12

    12 x 2/3 = 8 cubic feet

     

     

     

    1. User avater
      AaronRosenthal | Sep 09, 2005 09:51am | #2

      Right. 8 cu ft should be 16 bags. But I used 20 bags, and that's a lot more than 10% for margin of error.
      Hi Jim.
      It's too late for guys our age who have to go to work in the morning.
      Good night.Quality repairs for your home.

      AaronR ConstructionVancouver, Canada

       

      1. jimblodgett | Sep 09, 2005 04:48pm | #4

        You got that right.  My nephew is here at Ft Lewis on his way home from Kuiwait so I was just one eyeing it last night, Aaron.  Gonna have to compensate with extra caffiene today, huh? 

  2. 4Lorn1 | Sep 09, 2005 11:20am | #3

    The math is mostly right. I think that the problem is that the bags of cement are 1/3 cu ft. not 1/2 cy ft.

    Dividing the 7.25 , actually I get 7.125 but the difference is not significant, by .333, 1/3 cu foot, gives a bit more than 21.5 bags.

  3. blue_eyed_devil | Sep 10, 2005 03:25am | #5

    Aaron, the first thing I would do is toss out that CM!

    I needed to do some simple math today. The closest calculator that I had was the CMIV that rides in the bottom of my tool box. After pulling it out and working for awhile I got extremely frustrated. The memory is hard to work and I finally tossed it back in the box and walked to the truck to get my $4.99 dual power. In about 30 seconds I had my answer.

    CMs suck as bad as speed (slow) squares.

    blue

     

    1. MikeSmith | Sep 10, 2005 06:22am | #8

      i notice you're still hoping for lightning to strike...

      carrying your CM IV around in your bag hoping you'll figure it out by osmosis

       

       

      i got the same problem... only i  carry mine in my purse... er... my soft-sided briefcaseMike Smith Rhode Island : Design / Build / Repair / Restore

      1. blue_eyed_devil | Sep 10, 2005 06:44am | #9

        Mike, I've worked enough with it to know how it works. It works slow. It's hideous.

        Ill race you anytime to do the calculations that I need onsite. You use your cm, I'll use my cheapie. Loser buys lunch.

        I'll have a PBJ, hold the J.

        blue 

        1. User avater
          Huck | Sep 10, 2005 10:16am | #10

          I'm with you there. Threw my CM out years ago. Seldom run into anything needing calc's now, anyway, with the type of remodel/repair stuff I do. But even when I was framing roofs, I found a cheap trig calculator faster and more accurate. As I recall the CM suffered from cumulative error of some sort... among other things.

          1. blue_eyed_devil | Sep 10, 2005 03:35pm | #11

            Huck, I don't use trig at all, I never learned it.

            All I do is very simple basic geometry. The key for me, to attain fast numbers, is the memory. For me, it is critical to have the four basic memory keys: (M+, M-, MRCL, and most important, a separate MCLR). Of course, I need the sqrt function.

            If I'm calculating rafter lengths, I automatically work with the diagonals instead of pitch. For instance, I know all the unit lengths of the whole number pitches. An 5/12 = 13, 6/12 =13.42, 9/12=15, etc. With the whole number diagonals, I punch them in direct when needed. But when a number like 16.97 comes up, I won't punch that in. If I did, it causes a very slight cumulative error, AND it takes five punches of the calculator! Instead, I punch in 244 SQRT. By doing that, I reduce the amount of punches into the calculator by 20% and get a much more accurate number.

            There are many more things wrong with the CM, I'm sure, but since I can't get over that very basic hump, there really isn't any reason to carry one. Maybe once a year I wish I had the knowledge to calculate an arc with it, but the thought quickly vanishes when I remember how unwieldly it is on the more important day to day issues.

            blue 

  4. brownbagg | Sep 10, 2005 04:18am | #6

    you screwed up by using bagged concrete. beside being a joke to begin with, it does not have the strength to hold anything up but fence post. It does not have the cement in it. by shorting the cement you short the cubic yards of the mix. that why you was short. you said 20 baggs per footer. How much a bag? round here about $3 75 a bagged. thats $75 dollars for 7 cu ft. A yard here is $85 with two yard minimum so $170 for two yard or 45 baggs for you.

    1. User avater
      AaronRosenthal | Sep 10, 2005 05:01am | #7

      Absolutely right, even though you are changing the topic.
      I have an existing kitchen which, because I was out of the country, the contractor placed on 2 piers, 6x6 rough cedar to a 24"x24"x8" concrete pad on the ground. Around here, code says minimum 24" deep. I have to work around that, becasue I do not want to demolish the kitchen extension.
      So, dig to 30", run 18"x7"x96" footing with rebar, do a wall, on both the east & west side. Then do the 14' long side.
      Small amounts of concrete here are expensive - for the footer and wall, east & west about 300.00. Because I'm 30' away from any access point the truck could park, even working with 2 wheelbarrows and people, it would take too long & there would be charges - and a line pump will be 350.00 above that, each time.
      By the time I finish, it's cheaper to rent a mixer, get the bags and do the pour myself. The mix is rated at 2800 psi, but I'm a careful guy and usually my mix on small jobs like this run at about 3600 psi when cured.Quality repairs for your home.

      AaronR ConstructionVancouver, Canada

       

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