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foundation design

Discussion in 'Residential Foundation Codes' started by elwood, May 21, 2010.

  1. elwood

    elwood Member

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    Hi everyone,

    I am designing a footing for a 16’x20’ addition to my home, and was hoping to get some feedback from anyone who might have a little more experience with this in a professional setting. I have been mainly referencing IRC 2009, but being new to structural concrete, I want to make sure that I am not missing anything – or going overboard with the design. This is my first time here, if this is not the appropriate forum for these types of questions, my apologies. Here is my info:

    -Eastern PA

    -Seismic category B

    - Soil – I was told by the local building code office that soil in the county is assumed to be at 3000psf. Looking at the soil maps for the area though, seems that my property (and many other parts of the area) is type DfC which apparently has class SM, ML, GM, and MH at a depth of 53-72” (this terminology is all new to me so understand if I am misinterpreting this). To my understanding this is basically silty loam, loam and clay, which my reference says can be assumed to be 1500-2000 psf. I would think the code office knows what they’re talking about though.

    So here is what I have come up with, just want to know if it seems adequate or overkill or what – any input is greatly appreciated. Thanks.

    [​IMG]
     
  2. globe trekker

    globe trekker Sawhorse

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    .

    elwood,

    Welcome the codes forum! :)

    1. Are you planning to fill the hollow CMU's with concrete? 2. What's the straw

    for? 3. Is the reinforcement mesh [ in your drawing ] the "welded wire fabric

    / welded wire mesh" type of product, or something else? How do plan to keep this

    [ or other types ] of steel reinforcement centered vertically when the concrete is

    placed on top of it, and the concrete finishers are walking all over it? 4. I would

    recommend that you have 90 degree bends made into the #4 steel rebars extending

    down into the 2' wide footing [ approx. 14" - 18" in length ], so that they can be

    tied to the horizontal steel reinforcement in the 2' wide footing.

    .
     
    #2 globe trekker, May 21, 2010
    Last edited by a moderator: May 21, 2010
  3. Mark K

    Mark K Platinum Member

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    The proposed detail does not make any sense unless there is a place for the water behind the wall to drain.

    I would recommend you hire an engineer if for no other reason than to help you to put this information into context. You have a lot of facts but it is necessary to put them into context and to decide whether what has not been stated will be a problem. It is not feasible to give you advice about your particular situation from a distance.

    We do not know about the variation in soil around your addition nor do we understand how the addition sits on this basement. The amount of vertical reinforcement may not be adequate.

    You might be lucky and have no problem but if there are major issues the savings by not hiring an engineer can be very expensive.
     
  4. mark handler

    mark handler Sawhorse

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    I agree you need to hire an engineer.

    Reinforcement may not be adequate.

    Footing looks small as well
     
  5. pwood

    pwood Platinum Member

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    agree with others about the engineer. you will have about 6' 10" of headroom in the basement which is not a legal ceiling height here anyway. why not add another course of block and have a legal room?
     
  6. elwood

    elwood Member

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    To answer some of these questions: I am planning to fill the blocks with concrete where there is rebar, but not elsewhere, the slab reinforcement will be a wire mesh which will be held by rebar chairs or something similar - same for the rebar, the drainage pipe will drain to the surface at a lower point in the property. As for the straw - I had seen that this could be used in place of a layer of roofing paper on top of the stone. The low ceiling height in the basement is to allow for more ceiling height on the 1st floor - but if it's not allowed then it's not allowed.

    As for the engineer - I know this is a touchy topic. All I am asking here is if what I have designed meets code given my seismic category and assumed soil. I understand that the 'assumed' soil typing is a BIG issue, and I am not against getting a proper soil test. For now, I just want some kind of idea how the thing will be built. I was under the assumption that the codes are solid guidelines that can be used to guarantee a minimum level of safety. And I believe the beginning of the IRC foundation section says basically 'follow these guidelines and you don't need an engineer'. So the room for error is 1)understanding/interpretation of the code and 2)your particular variables - such as soil type, unusual loads etc. If it was not safe to design this without and engineer, I would hope the code would say this - as it does for many other areas. Not to say having an engineer isn't the number one best idea! I can be talked into it, but I'd also like to understand some of this a little better.
     
  7. Mark K

    Mark K Platinum Member

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    The building code does not guarantee that you will have no problems. Just because the code doesn't prevent something doesn't mean that you will not have problems. You have an unrealistic expectation regarding the codes and what the building department will/can do to protect you.

    If you want to understand get an engineer to help you. There are some real limits on the ability of the Forum to help you in this situation.
     
  8. peach

    peach Sawhorse

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    Provide a drain plane against the block that will direct any water to the stone and foundation drainage. The straw will degrade over time and cause settlement issues.. use sand or another drainable material..
     
  9. elwood

    elwood Member

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    Thanks everyone for the advice.
     
  10. TimNY

    TimNY Platinum Member

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    To be clear, there is no requirement to increase the ceiling height unless the space is to be occupied. Good idea for sure, but not required.

    Based on the tables, the footing seems more than adequate. Of course I am making certain assumptions. If you are building a 3 story brick clad building you could have an issue with the footing.

    Seems like a 12" plain masonry foundation would also work.

    Vertical rebar should not extend though the bottom of the footing.
     
    #10 TimNY, May 28, 2010
    Last edited by a moderator: May 28, 2010
  11. peach

    peach Sawhorse

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    as designed, it's a tall crawl space; which is ok...just not habitable.. (either make it a basement, or eliminate some of the issues and lower the wall to create an undeniable crawl space). If you're still unsure about the soil, have it tested. Clay is pretty stable, until it gets wet; loam is bad soil.

    In most jurisdictions you'll find areas with really great soil, and next door - muck... or really great soil down to 6', then a layer of organics. The soil test is worth the money spent
     
  12. Yankee

    Yankee Sawhorse

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    I thought 6'8" was the break between crawl and basement?
     
  13. peach

    peach Sawhorse

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    typical CMU is 8" x 16".. 7 blocksX8" = 56" (it's a crawlspace)
     
  14. Yankee

    Yankee Sawhorse

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    You were well ahead of me, I was going on the 6'10 info that was provided
     

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