Rebar in a 3 foot by 18 inch poured foundation

Discussion in 'Bricks, Masonry and Concrete' started by JHopkins, Apr 13, 2019.

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  1. Apr 13, 2019 #1

    JHopkins

    JHopkins

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    My house is two stories on a slab in Pennsylvania. My house is L-shaped. I am putting on an addition of a 14' by 14' room on the second floor and a bathroom that is 14' by 5 1/2' on the first floor (the bathroom is smaller due to a previous kitchen addition on the 1st floor). I jack hammered and removed the side concrete porch where the bathroom will be placed. I have dug out (by hand) a 3 foot deep by 18 inch wide trench on the outside edge of that space. The drains for the bathroom will be put in Monday. I want to setup to pour the foundation in the trench and a 4 inch slab which will be connected to the existing foundation - I believe that is called a mono-slab. I intend to drill into the existing foundation and cement in rebar above grade to connect to the new foundation. Also I plan to use wire mats in the 4" slab and wire tie that to the rebar connected to the existing foundation and the rebar in the new foundation/trench. My question is, how exactly how to place the rebar in the trench to prepare for the pour? I intend to use 1/2" / #4 rebar placed on 2" by 2" wire dobie concrete bricks. I was thinking that I would create 12" wide by about 32" tall squares and place them vertically every 2 - 3 feet on the dobie bricks, and these squares would also tie onto the wire mat. I would then tie 2 horizontal runs at 1' and 2 horizontal runs at 2 1/2'. I attached a picture of my house and my plan for the rebar placement. Please let me know if you feel my plan will work or if there is a better way. Thanks for your help.

    Rebar_design.jpg
    house.jpg
     
  2. Apr 14, 2019 #2

    Snoonyb

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    If your addition is encompasing the low slope and covered porch, how are you going to address the foundation supporting the low slope area, to conform to 2 story requirements?

    The rebar cage you are creating in the 3' X 18" footing is overbuilt.

    Rebar joints need to be lapped 24 bar dia., and tied.

    You can use #3 bar @24"oc instead of mesh, however, regardless you need to dowel into the existing 6" and tie, either, to the existing.
     
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  3. Apr 14, 2019 #3

    JHopkins

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    Snoonyb,

    To answer your question about the 2 story requirements - the exterior wall under the low slope is cinder block and the foundation below that wall is at least 2 feet thick. I believe that will support the second story.

    You said that my rebar cage is overbuilt, please tell me what amount of rebar or design would be appropriate for the 3' by 18" footer/foundation. I intend to build a 2x6 2 story exterior wall on top of the footer/foundation.

    In my research, I did read that rebar joints need to be lapped - what I read said to lap them 40 bar dia. and tie, so I figured 20 inches. Admittedly, my drawing is not good, but I do intend to drill into the 2 foot thick existing foundation and tie those dials to wire mesh or #3 bar used for the part that is the 4" slab.

    Thank you for your help
     
  4. Apr 14, 2019 #4

    JHopkins

    JHopkins

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    Snoonyb,

    I have watched many videos where 2 horizontal runs of rebar where placed in a footer that is about 1 foot thick and then block is laid on the footer. I am not planning to use block, and since my footer/foundation is 3 feet deep, I assumed that more rebar was necessary. That is why I thought I might need 2 runs at 1' and another 2 runs at 2 1/5'

    Thanks again!
     
  5. Apr 14, 2019 #5

    Snoonyb

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  6. Apr 14, 2019 #6

    JHopkins

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    Snoonyb ...... It's not the number of bars, it's maintaining the "cost" away from soil, which you had addressed with the spacers at the bottom.

    I am not certain what maintaining the "cost" away from the soil means. I think I understand - you are concerned that the rebar may be exposed on the side wall? With the width of the trench at 18" and the cage at 12", that should give me 3" on both sides. Is there better way to suspend the horizontal runs?

    Thanks again!
     
  7. Apr 14, 2019 #7

    Snoonyb

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    "Cost" is a term engineers use to describe that free space which is filled with aggregate.

    It's not necessarily better, just less laborious. We suspend them with tie wire from the top of the forms.
     
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  8. Apr 14, 2019 #8

    nealtw

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    The reading you have been doing is likely correct but I think you have been reading about 2 or 3 different systems and taken the most of all of them, then you get to over kill.
    You can buy plastic chairs to set the rebar on, we put the rebar in the footing to hold it together if it cracks or if you have a soft spot in the soil the rebar adds to the tension it can take so we put it 2" from the bottom.
    never let it get with in 2" of an outside edge, if air and water gets to steel it will break concrete.
    weight loads go down thru concrete and spreads at a 45* so if we need a big foot print but don't want a 3 ft high footing we put a grid of rebar in the footing to make that load spread out. It is really hard to get mat to sit center of a slab. I would do the small rebar in the slab if anything.
    In footings and foundations we use nothing but 5/8.
     
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  9. Apr 14, 2019 #9

    JHopkins

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    Thank you Snoonyb - you have been very helpful!
     
  10. Apr 14, 2019 #10

    JHopkins

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    You said that we don't want a 3' high footing and that you put in a grid of rebar. Could you please describe the grid that you use. The inspector told me to use #4 rebar, that is the only reason that I was planning to use that in the 3' footer/foundation. My dad had 3 wire mats left over from his garage project, but I am now leaning toward using the #3 rebar in the 4" slab part of the pour. Of course, I want to set it all up to pour both the foundation and the slab at the same time.

    Thank you for your help!
     
  11. Apr 14, 2019 #11

    JHopkins

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    Nealw

    I have to go to work. I greatly appreciate your help and I will be sure to read your response when I get home tonight. Thanks
     
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  12. Apr 15, 2019 #12

    nealtw

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    upload_2019-4-14_18-53-7.jpeg

    This is like what you have with big footing
    we usually get the same effect with 10 or 12" thick footing with steel in it to spread the load. [​IMG]
    But both want to have undisturbed soil so you won't be changing anything now. If you need an inspection before pouring concrete, I wouldn't get to much steel in there, the inspector can call for more if needed.
     
  13. Apr 16, 2019 #13

    JHopkins

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    I see the big footings, but I do not see where the rebar is placed unless that is shown by the arrows in the concrete. Here is a good picture of what I am trying to accomplish. What do you think of this design - only replace the wire mesh with #3 rebar.

    [​IMG]
     
  14. Apr 16, 2019 #14

    Snoonyb

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    Your original design, less the rebar cage is sufficient, and you can "J" hook over the inside bars and avoid interference with your foundation bolts.

    If your concern with this design is the rebar moving during the pour, it will anyway and the pump volume can be reduced ti mitigate that.

    In a monolithic pour the footings are first and the slab or flat work last, so once the footing are full the rebar isn't going to move.
     
  15. Apr 16, 2019 #15

    nealtw

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    We call this a pan slab, like an upside down frying pan, we only see it when the soil is real questionable, like building on a marsh. Because of the poor soil, the engineers go nuts with the amount of rebar. So I don't know what would be normal on good soil. Usually the bottoms are about 18" wide and the inside is angled up to the level under the slab.
     
  16. Apr 17, 2019 #16

    JHopkins

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    Rebar_design.jpg jeffs pics 294.jpg
    Not sure what You mean by "J" hook over the Inside bars, so I changed my drawing. Please let me know what you think. I also attached a picture of my worksite so you can see my current progress - no rebar yet. The drain lines for the bathroom are hopefully going in tomorrow if the plumber comes. It is obviously not graded yet, and the inside boards are just keeping the excess from falling in the trench.

    Thanks again
     
    Last edited by a moderator: Apr 17, 2019
  17. Apr 17, 2019 #17

    hornetd

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    I am concerned that you will miss the installation of the National Electric Code (NEC) required concrete encased electrode; which is often erroneously referred to as a "Ufer Ground". Most Electrical "Authorities Having Jurisdiction" (i.e. the local or state government Electrical Inspectors) will not except any other electrode in lieu of the required Concrete Encased Electrode. By that I don't mean to imply that they will wave the other electrodes which the NEC requires to be used but rather that they will insist that the Concrete Encased Electrode be used in new construction or additions were a concrete foundation is being built. viz.

    "250.50 Grounding Electrode System. All grounding electrodes as described in 250.52(A)(1) through (A)(7) that are present at each building or structure served shall be bonded together to form the grounding electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes specified in 250.52(A)(4) through (A)(8) shall be installed and used.

    Exception: Concrete-encased electrodes of existing buildings or structures shall not be required to be part of the grounding electrode system where the steel reinforcing bars or rods are not accessible for use without disturbing the concrete."

    Your addition is not going to be treated as "existing buildings or structures." It is new work so it must have a concrete encased electrode.

    Most Electrical Inspectors require inspection of that electrode before the encasing concrete is poured. When the installation of the Concrete Encased Electrode was not done before the concrete was poured I have seen Inspectors require that the work be done to expose the rebar and make the connection to it with a connector which is suitable for use in concrete. In a case were the only rebar was in the footer I have seen inspectors require that a trench be cut into the concrete pad so that a Concrete Encased Electrode could be installed when it was missed during the foundation's construction.

    Section 252(A) (3) of the NEC defines a Concrete-Encased Electrode as:

    250.52 Grounding Electrodes.
    (A) Electrodes Permitted for Grounding.
    "(3) Concrete-Encased Electrode. A concrete-encased electrode shall consist of at least 6.0 m (20 ft) of either (1) or (2):
    (1) One or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (1⁄2 in.) in diameter, installed in one continuous 6.0 m (20 ft) length, or if in multiple pieces connected together by the usual steel tie wires, exothermic welding, welding, or other effective means to create a 6.0 m (20 ft) or greater length; or
    (2) Bare copper conductor not smaller than 4 AWG

    Metallic components shall be encased by at least 50 mm (2 in.) of concrete and shall be located horizontally within that portion of a concrete foundation or footing that is in direct contact with the earth or within vertical foundations or structural components or members that are in direct contact with the earth. If multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system.

    Informational Note: Concrete installed with insulation, vapor barriers, films or similar items separating the concrete from the earth is not considered to be in “direct contact” with the earth."

    Be sure that you stub up a piece of rebar out of the forms only slightly inside the basement or first floor wall at an accessible location as close as practical to the Service Disconnecting Means which is usually but not always in the main panel of the home. You must connect the Concrete-Encased Electrode to the Neutral Bus Bar of the Service Disconnecting Means enclosure with a copper conductor which is not smaller than #4 American Wire Gage (AWG). If you used a #4 Copper wire as the Concrete-Encased Electrode then use a Schedule 80 Rigid Nonmetallic Conduit 90 and stub to bring it up out of the floor. The tail of the 90 should be long enough to allow it to be tie wired in place. Using Metallic conduit would require you to obtain and install a bonding fitting to bond the Concrete Encased Electrode to the conduit at both ends. Using Schedule 80 Rigid Nonmetallic Conduit avoids that expense and extra work.

    Please note that a Concrete-Encased Electrode is not actually a "Ufer" ground. Ufer was the electrical engineer that originally developed the use of Concrete-Encased Electrodes. Since he was dealing with lightning initiated explosions in ammunition bunkers located in the desert South West he had all of the rebar double tied so that it all connected together into one conductive grid. Since concrete is quite conductive because of it's stubbornly stable moisture content it electrically connects the rebar to the undisturbed earth beneath it. Read the informational note carefully. If you use insulating panels beneath the slab or plastic sheeting; either as a vapor barrier or to limit the infiltration of concrete into porous soils; the reinforcing bar in the slab is not suitable for use as a Grounding Electrode. To be effective the concrete encasing the electrode must be in DIRECT contact with the earth.

    In addition to the Concrete Encased Electrode a partial Ground Ring can be installed if the trench for the footer is 30 or more inches deep. Research done by the National Institute for Standards and Technology (NIST) shows that a #2 Copper conductor that is 20 or more feet long and buried at least 30 inches deep is a very effective grounding electrode. When it is installed in the footer trench it is sometimes referred to as a partial Ground Ring. That is because the minimum length of a Ground Ring is 20 feet. A well house or a spring house might well have a total length of outside wall of less than 20 feet. When that is the case you have to bring the Ground Ring out from the too short pathway until it will end up 20 feet long without overlapping.

    Bring the #2 Ground Ring itself up out of the ground through a protected Rigid Nonmetallic Conduit stub. Rigid NMC does offer some protection to the Ground Ring conductor but landscaping equipment can still damage or destroy it. The protection could consist of a larger piece of RNMC around the one used to stub up the Ground Ring very close to the building wall. If the outer piece of RNMC is large enough you can fill the space between it and the stub with waste concrete. The Grounding Electrode Conductor which makes up any portion of the pathway between the Ground Ring itself and the neutral bus bar of the Service Disconnecting Means need only be #4 AWG but some electricians just use a longer length of the #2 AWG Ground Ring conductor to connect to the Service Disconnecting Means as a matter of convenience in installation. To markedly increase the effectiveness of the #2 Copper Ground Ring you can install 2 driven rod electrodes at least twice their length apart through the bottom of the trench and connect them to the Ground Ring conductor. A Harger 305 ground rod clamp is a very convenient way to do that because they are easily installed mid line without bending the wire or threading the wire through the clamp before installing it.

    --
    Tom Horne
     
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  18. Apr 17, 2019 #18

    Snoonyb

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    The drawing actually shows 2 "J" hooks.

    I'm assuming you have, in your forms, established a finished grade level for the plumber.
     
  19. Apr 17, 2019 #19

    Snoonyb

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    " One or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (1⁄2 in.) in diameter, installed in one continuous 6.0 m (20 ft) length, or if in multiple pieces connected together by the usual steel tie wires, exothermic welding, welding, or other effective means to create a 6.0 m (20 ft) or greater length; or
    (2) Bare copper conductor not smaller than 4 AWG.
    "

    Tom, although I've legally built 100's of additions that employed the existing elec. service, I was never required to provide a UFER grnd., unless the service was being relocated, and I've always found the 20' encasement requirement, ambiguous at best, because rebar is stock length at 20' and in other than slab foundations, sometimes up to 2' is stubbed up to account for the joisting.
     
  20. Apr 17, 2019 #20

    JHopkins

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    Tom,

    I will need to read this a few more times to make sure that I understand, but thank you and I will be sure to talk to the building inspector about this before I pour the foundation.
     

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