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bench footings to extend slab

tom21769 | Posted in General Discussion on December 14, 2006 03:34am

Anyone here experienced installing bench footings?

How expensive should it be to do this?

Problem:  I want to add a workshop to my existing 2-car garage.  At one end, I want to put a bathroom. I do not want to bury plumbing in a slab.  I understand I cannot simply dig out an 8′ deep cellar directly adjacent to the existing garage walls (though I’m not sure I can even dig out for a crawl in the same place).  The contractor I talked to yesterday recommends installing a bench/shelf about 8′ out from the existing slab.  My understanding is that the “area of repose” only needs to be as wide as the existing footing is deep (maybe 3-4′ in our mid-Atlantic location?).  I would assume a wide footing gets pretty expensive and would like to limit its size.

 

 

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  1. dovetail97128 | Dec 15, 2006 12:44am | #1

    My understanding of what you are calling the "area of repose" is that in all cases it extends down and out from the bottom edge of the footings at a 45 deg. angle.
    I am not sure this info is what you are looking for, but it is what I got from your question.
    Putting anything within that area requires dealing with the lateral loads exerted by the existing footing.



    Edited 12/14/2006 4:46 pm ET by dovetail97128

    1. tom21769 | Dec 15, 2006 03:28am | #2

      Thanks, I think you are confirming what I was trying to say.
      For example (as I understand it), if the new addition has a foundation 4' deeper than the bottom of the existing garage foundation, then (without underpinning) there needs to be a 4' wide buffer, beyond the existing foundation, where the new foundation (or undisturbed soil) continues to support the existing foundation.

      1. dovetail97128 | Dec 15, 2006 05:28am | #3

        That is my understanding, some soils will bear more and the angle of repose is greater, some less and the angle angle is therefore less but in general 45 deg. is what I was taught as a rule of thumb.

        1. [email protected] | Dec 15, 2006 07:14am | #4

          The best solution if you have the space is to stay out of the cone of influence of the existing footing. 

          If you need to get closer the design for the wall becomes one for a retaining wall, which will be need to be engineered to carry the impressed load of the adjacent structure. 

          If you are going to install a basement, it should be a pretty simple to design it as a single structure to help resist the soil load. 

          1. tom21769 | Dec 15, 2006 12:41pm | #5

            "...as a single structure ..." means, with all parts (walls, floor, joists, columns etc.) working together to resist the soil load?

      2. dovetail97128 | Dec 15, 2006 06:04pm | #7

        At 4' deep you need a 4' buffer, at 5' you need a 5' etc. etc.
        I would look at your bias against the plumbing in the slab, why not?
        From what I can read into what you have posted here you are going to great lengths and costs just to avoid the slab plumbing.
        You want to add a shop/ bathroom to your garage, so do so . Stay on the same level and put the plumbing in the slab (actually under the slab is the correct way usually, pipes go in the fill and rise up out of the slab) Again as I read what you are posting it would seem to be a lot cheaper.

        1. tom21769 | Dec 16, 2006 02:22am | #8

          Thanks, you make some good points.
          I have kept the slab option open all along.
          However, the plumbing is not the only issue steering
          me towards the other options. First, I'd like at least a small area under my workshop addition to put a dust collector and compressor. Second, a wood floor over joists would be more comfortable on my feet & easier on dropped tools Third, a slab would be harder to heat. A compromise I'm considering is to put flooring and insulation on sleepers over a slab.

          1. dovetail97128 | Dec 16, 2006 02:31am | #9

            Tom,
            Ok a better understanding of what you wanted to achieve. About all I can say is that unless you have a daylight basemant situation that already requires tall concrete walls then you will be paying a lot for the sq. footage by digging in and pouring tall walls. Might be worth it to you in which case you might look at only a partial basemant so you stay in clear of the loads from the garage footer.
            Best of luck.

  2. Catskinner | Dec 15, 2006 03:44pm | #6

    All good comments so far.

    Here is another way to look at it. You might be able to put that basement in after all.

    I was recently involved in a job whee the GC did put a full basement next to a house that had shallow footings. Pretty unstable soils, too.

    They installed three helical piers under the existing footings to bear the load of the house, and then dug out the basement as usual.

    It worked.

  3. IdahoDon | Dec 16, 2006 05:23am | #10

    You can dig a full basement as close to the house as the soil will allow, pour your basement walls and backfill the gap between the garage and house.  The most conservative is following the "angle of repose."  In many soil types you can be a few feet away and be ok, but it's hard to tell in some areas that have soil types that vary a lot or have unknown backfill trucked in for the existing garage.

    How lucky do you feel?

    It's probably better to rely on the experience of a well respected excavation contractor familiar to your area with regards to how close to dig.

    We run into this situation when putting additions with 9' basements up against crawlspace walls.  The next one that has difficult soils, I'll probably go with a few concrete filled holes bored deeper than the new footers and just enough to the side to allow forms to be set.  It would help to tie it to the crawlspace walls/footer with expoxied rebar to stabilize the top side.

    By using ICFs, they can be placed without worrying about how to strip the forms. 

     

    Beer was created so carpenters wouldn't rule the world.

    1. tom21769 | Dec 16, 2006 02:37pm | #11

      Pardon my ignorance, but would it be safe to assume that if I go with a crawlspace instead of a full basement, the risk to the existing gargage foundation/footings should be minimal? (assuming the excavator is competant, not too unlucky, and sober).In other words, do the footings for a crawl space go deep enough, compared to a slab on grade garage foundation, that there is still a risk of bringing the crawl up to the edge of the garage without underpinning? I would want a crawl space tall enough for a grown man to crouch in.

      1. dovetail97128 | Dec 16, 2006 09:45pm | #12

        The answer depends on your soils, as I stated earlier, the load from the slab is considered to spread down and out at a 45 deg. angle from the bottom edge of the footing for the slab.

      2. IdahoDon | Dec 17, 2006 06:13am | #13

        I'd ask an excavation contractor about your local soils.  From where I sit it's hard to tell. 

        Beer was created so carpenters wouldn't rule the world.

  4. user-158769 | Dec 17, 2006 09:00am | #14

    "The contractor I talked to yesterday recommends installing a bench shelf about 8ft out from the existing slab...."

    Tom,

    If I'm  reading  you correctly, your current 2 car garage is built on a slab?  If so, what is the problem?  You can form and pour another slab right next to your existing...drill some holes in the existing slab and epoxy in some rebar so as to tie both slabs together to act as one.  We do this all the time...this is no big deal at all.

    As for running underfloor duct routes, make your slab a little thicker ( say 6- 8inches) and form the ducts when you pour the slab.  For less leg fatigue, either invest in some good rubber (anti-fatigue) mats and place strategically, or install 2X4 sleepers to the slab and plywood overtop.  Unless you want a totally dust free environment for doing finish work, a forced air furnace unit ( gas or oil fired) will heat that space in no time. Plumbing up through a slab is straightforward, and done everyday in many tract homes built in North Carolina and other southern states.

    Everything that you want can be accomplished by building on a slab foundation. For a warm floor, install 4 inch ridgid insul on the pad site and then pour your slab overtop...or with the sleeper floor method, install 1-1/2 inch thick insul between the sleepers, or use PEX water heat and embed in the slab during pour and heat your floor and building using a hot water tank.

     That noisy air compressor...put it outside your new workshop in a shed enclosure to keep out the noise if you like. Ditto for a dust collector unit. Extend your slab a bit and errect these sheds on the slab; tied into your new garage framing

    Nomatter how you cut it, a slab is much cheaper than a basement foundation.

    Davo

     

    1. tom21769 | Dec 17, 2006 02:06pm | #15

      Thanks. Everything you and others are telling me about the slab option confirms what my builder advised me yesterday. In my area (Maryland), I'm just not familiar with slab on grade foundations in other than garages. I'm a lot more comfortable with this option now that I've learned what can be done with radiant floors, etc.

      1. user-158769 | Dec 19, 2006 07:48am | #16

        Tom,

        Many, many buildings (mostly single story) are built on slabs. My region, Pittsburgh PA/OhioValley fabricate slab buildings quite a bit...though the majority are for garages, or detached Mother-in-Law guest homes. Again, you can accomplish just about anything on a slab.

        Just make sure your contractor prepares the ground adequately before pouring. And, using 1/2 inch rebar for slab reinforcement is also a very good idea.

        My garage/workshop is now 9 years old. It is an 8 inch floating slab. I laid 2 rows of concrete  block around the perimeter of the slab and then framed on top of this. I drilled holes down into the slab approx 5ft on center and epoxied 1/2inch threaded rod which I used to bolt down my mudsills. I also mortered in each block containing the threaded rod.  My heat is an 80,000 BTU gas fired (York brand) 90% efficiency furnace.  I really could have got by with a 60,000 BTU but couldn't find one at the time.

        My furnace  "duct runs" consist simply of a tall plenum about 3-ft above the furnace unit. (My furnace is situate in a corner) with several heat cut-outs and metal grills attached. It takes less than 5 minutes to heat my shop from 50 up to 70 degrees, even with outside temps in the teens.  My shop is approx 36ft X24 ft.  

        My slab has a 1/2inch rebar mat running clear thru it. This "mat" is simply rebar placed 1FT on center running length wise inside the form, and another layer of rebar lyng perpendicularly on top of the first layer. This mat sits up off the ground approx 3 inches using concrete "chairs" which are designed for this purpose.

        To date, no cracks in my slab.  My garage / workshop has served me very well.

        I hope yours will to.

         

        Davo 

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