Simple math question...

Discussion in 'All Things Boats & Boating' started by bntii, Oct 16, 2012.

  1. troy2000
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    troy2000 Senior Member

    I'm on my way to bed after being up all night working, and didn't have enough ambition to read all the posts. So forgive me if someone else already said this. But as an ex-contractor, it looks pretty simple to me: since one corner goes to zero, add the height of the other three corners and divide by three to average them out. Multiply the average height by the length and breadth of the pour. Divide the result by two, and it should give you the volume.

    Disclaimer: not only am I tired; I also cracked a bottle of double-aged Jim Beam for breakfast this morning. So I take no responsibility for anything I may allegedly have said until tonight, when I've had a chance to double-check it....:D
     
  2. DStaal
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    DStaal Junior Member

    I think you halved the volume one to many times - the averaging of the corners would already have done it for you.

    I'd use the average-height-cube method that's being suggested myself, and consider it a decently close rough estimate. If better is needed:

    Work out the volume of the pyramid with the apex at the zero-height corner, and the base of the two medium height corners. (Take an imaginary plane through across the diagonal as your base.) Then, work out a flat plane at the height of the shortest corner, and which will form another pyramid underneath it. You then have a flat triangular prism
    at the top. Add the volumes of the two pyramids and the prism, and you have the volume of the whole space. (Assuming all surfaces are flat...)

    The volume of a pyramid is 1/3*height*area of base. (It doesn't matter how it leans, as long as the apex is over the base.) The volume of a prism is height*area of one base.
     
  3. philSweet
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    philSweet Senior Member

    Oh good grief, thats a shed for a buzz box. I thought you meant a welding shop. Around here, usually 80X150 40' highbay on twenty acres or so. You couldn'd get a decent drillpress into that. Sorry for the distraction.

    If the existing lip is where you want the finished grade to be, Bust the old slab back until you have 5 inches or so, then fill with ten yds of compactable road base, then set up a little slab in the usual way. You can't feather an edge like that in concrete nomatter how you bond it- not in a shop envoronment, anyway. Compare 5.5 yd concrete and 10 yd ABC to 9 yd concrete.
     
  4. bntii
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    bntii Senior Member

    Sniff sniff- don't tell our fabricator that..
    (& I amended post #13 so as to avoid any confusion...:))

    He is going to shove a 14" lathe, Bridgeport mill, a couple of Do-All BS's, benders, benches, all his welding gear etc etc etc into this shed.
    In the end he will make good money for us though admit-ably the quarters are cramped for him.

    Around here shop space on the water like this is hard to come by & this is all we could do for him just now.
     
  5. daiquiri
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    daiquiri Engineering and Design

    DStaal, you have made it more complicated than it actually is... ;)
    The correct (and exact, no need for rough approximations) formula for the volume is, simply:
    V = 0.5 L B H
    as noted several times in previous replies.
    Cheers
     
  6. DStaal
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    DStaal Junior Member

    I am well aware of that. :cool: Just giving the 'this is the system for an irregular volume' answer. Only reason I'd actually use it in real life is if the the floor was significantly other than flat. (At which point it would be more complex yet.)
     
  7. troy2000
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    troy2000 Senior Member

    I only halved the area volume once, because it tapers from each of the other three corners to zero.....
     
  8. Petros
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    Petros Senior Member

    Sounds like you could be making a big mistake if you pour this concrete. Before you actually set up the forms and pour that you need to ask yourself a few question. ONe of the things I do as an engineer is design foundations and soils investigation. If you have an existing slab that settled like that, pouring more mass on top will not stop it from sinking, but could accelerate the settling by adding weight to it.

    What caused the original slab to settle? If it was poorly compacted fill, than the question is how deep is it and has it fully consolidated? Is it clean fill, or full of old construction debris, rotting wood, and tree stumps, etc. than it will just continue to consolidate as the organic material rots away. If you have water movement below the surface (seepage), it could be pulling out the fine silt and causing it to settle, this process may not stop and it will continue to sink no matter what you do.

    I would advise you remove the old slab, excavate out any old poor quality fill, than fill it with crushed rock in 8 to 10 inch lifts machine compacting up to your desired grade and than pouring a reinforced concrete slab 6" think, with footings around the edges at 12" thick.

    No point in wasting money building a shop if it is also go to sink and go out of level in a short time. You will have twice as much concrete to break up and haul off if you have to level it again later, add more weight to unstable subsoil conditions only makes it worse. The cheapest way to build it is to build it once!

    If the slope was poured as a boat ramp and the underlying soil is stable but just not level, I do not think I would fill the whole area with concrete. It would be less costly to pour the inner area with "controlled density fill" (CFD), a weak mixture of concrete and sand. than pour perimeter walls and a 6" thick concrete slab over the top to seal it off. it would save you some cost to do it this way. Most ready mix concrete suppliers will sell it to you at half the cost of structural concrete. Much cheaper than filling the whole volume with costly concrete.

    PM me and I can draw you something up if you would like. I will be happy to help you work out what to do on this project.
     
  9. Petros
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    Petros Senior Member

    BTW, I get 8.38 yards based on your measurements.
     
  10. jonr
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    jonr Senior Member

    If it's on unstable soil and stable soil is too far down, then put enough foam under it so that the whole building floats. This also resolves any frost issues you might have.
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    - speaking of water closets - don't forget drainage.
     
  12. Petros
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    Petros Senior Member

    Except you might find the building has floated away on the last high tide. Actually if you have deep soil instability, the cheapest thing to do is to drive 2 or 3 inch pipe down with a jack hammer until they stop on something solid, than set your reinforced slab on top of the pipe piles. these are called pin piles, they are usually far cheaper to drive them than digging out lots of deep squishy, slimy dirt and peat. I have designed many projects using pin piles, we use them on water fronts all the time. Sometimes we have to drive them 40 feet or more, we use 10 ft lengths and either weld or install some kind of connector at each length as it is driven down.

    sounds exotic but you are just pounding pipe into the ground until it stops on something solid. have used it on site with deep old fill that cost too much to remove, on unstable hill sides, and on water front buildings, piers and bulkheads. Usually the cheapest way to stabilize deep unsuitable soil.
     

  13. bntii
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    bntii Senior Member

    Thanks all for your ruminations on my problem.
    The issue really was limited to just getting my head around the volume between two planes which threw me there for a bit.
    West really had it & I have ordered high and have a secondary use (build a small culvert cap on site) if I end up with a couple of yards of hot mix which needs to be dumped somewhere.

    Just for documentation- the job is capping a very thick and sound slab which was poured on grade years back to set a spray tent over. It's one of the best stabs on site as it has not cracked up yet & I simply needed to get the grade out of it so the fabricator can jig work off his benches down to floor and will not have to piss and moan too much as he tries to level the machine tools..
    The concrete supplier was appraised of how we are using the concrete and specified a mix specifically for capping use. I think it had to do with fiber flocking and pea gravel plus & being a 6 pack or something..
    So the truck is rolling in this morning and we are good to go.

    Thanks

    [​IMG]
     
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