Rebar spacing question

[Jimmyspeach]

Guys I'm in the process of building a 600 square foot garage that will be attached to my house

My question is about the rebar spacing

My slab has a frost protected trech footing the footing is 8 inches wide and 48 inches deep

The slab will be a mono pour

I know the inspector said use #4 rebar in the trench

my question is

I cant figure out the spacing of the rebar

I would think for a 48 inch footing i would have 2 rows of rebar

The 1st one would be 1 foot from the bottom of the trench

The 2nd would be 2 feet above the 1st row

This is my guess

Also im sure i could call the inspector monday but I have some free time this and would like to wrap it up asap so we can pour the slab this week

This forum has been a great resource for me and helped me alot thanks guys

 

[mcbane]

Obviously calling inspector is the safest route. But if you want to gamble, here are my thoughts:

If it is just a frost wall with no computed forces the only reason for rebar is to control cracking. ACI allows plain (unreinforced) concrete for non-structural foundation elements like a frost wall. UBC has prescriptive rebar requirements for "no design" inverted T footings but doesnt sound like your footing is a conventional load bearing inverted T.

I'm guessing your inspector is wanting to see minimum shrinkage steel in the frost wall that looks something like what would be in a T footing stem wall. Those prescriptive requirements work out to something like 0.002 minimum steel ratio.

A #4 bar is .19 square inch area and 48*8*0.002= .77 square inches. So 0.002 ratio would be met with 4 bars, equally spaced with no bar closer than 3" to the dirt. Put first bar 3" off the bottom, and space bars about 14" apart. You would also want a vertical bar every 14" because the steel ratio applies in both directions.

 

[ConCretin]

In your case, rebar isn't really structural from a load bearing standpoint. While horizontal rebar would serve to hold thermal or shrinkage cracks together, vertical rebar wouldn't provide much benefit. It's not strictly necessary and unless it's a code requirement, I'm not sure I'd bother. It's difficult to install and position rebar in an narrow trench unless you have the proper accessories.

If you want rebar, I'd install a #4 bar at the bottom and top including corner bars. You could use a few vertical bars to help hold them in place. If code requires it, you'll obviously need to know the specifics. The challenge will be keeping the bars centered in the trench when the crew starts dumping concrete. As previously mentioned. You want to maintain a min of 3' of concrete cover around the bar.

One additional thought. Since you are placing the perimeter trench with the slab, the slab will be restrained from contracting with drying shrinkage making cracks more likely. You have a relatively small slab but I'd install a couple control joints to hide any shrinkage cracks.

 

[brownbagg]

its two number 5 in bottom of footing 3 inches off bottom and two inch off wall, but your footer need to be 18 inches wide at minimum

for vertical, number 5 hook bar every 48 inches and number 4 every foot hortzontal continous, with 30 inch corner bars

 

[joes169]

Are you talking about pouring the wall inside of a dirt excavation, without any forms or a wider footing at the base? Not sure where you are located, but this must be something your local code official is letting slide, because it's not allowed in the UDC of WI. Seems they're more common in other areas, but typically, grade beams or footing/stem wall construction is required in WI.

 

[mcbane]

Are you talking about pouring the wall inside of a dirt excavation, without any forms or a wider footing at the base? Not sure where you are located, but this must be something your local code official is letting slide, because it's not allowed in the UDC of WI. Seems they're more common in other areas, but typically, grade beams or footing/stem wall construction is required in WI.

I don’t live in WI but just wondering: does WI prohibit concrete cast against earth or just for specific foundation elements?



Sent from my iPhone using The Garage Journal mobile app

 

[ConCretin]

I don’t live in WI but just wondering: does WI prohibit concrete cast against earth or just for specific foundation elements?

Sent from my iPhone using The Garage Journal mobile app

I can't speak for Wisconsin but earth formed foundations are usually limited to shallow foundations in regions that aren't susceptible to frost. Several reasons for this;

Firstly it's rare to find soils that will 'stand up' in a deep, narrow trench long enough to get concrete placed. Another shortcoming is the difficulty of installing rigid insulation on the face of the foundation to keep frost from penetrating into the soils under the slab. Finally, they aren't as resistant to frost movement. Instead of a vertical faced wall with a footing to key the structure into the ground, you typically end up with a wedge shaped section that is susceptible to lifting.

I'm not sure what the frost depth is in Wisconsin but the fact that the OP is going 48" down suggests there is some need for frost protection.

 

[GMCGarage]

its two number 5 in bottom of footing 3 inches off bottom and two inch off wall, but your footer need to be 18 inches wide at minimum

for vertical, number 5 hook bar every 48 inches and number 4 every foot hortzontal continous, with 30 inch corner bars

What code is that from??

 

[Jimmyspeach]

This is 9 inch trench only formed at the top with 1 inch rigid foam on the exterior

The earth will serve as the trench walls

I actually wanted to do a frost protected shallow foundation but i was told by the inspector this wasnt allowed

The codes i found in the WI statues allowed for a frost protected shallow foundation but i didnt want to challenge the inspectors word

He told me a frost protected trench pour would be allowed make sure the depth is 48 inches below grade

The trench is already dug and the trench walls are holding the form for the past 3 days

I live in southern WI

 

[matt_i]

I think if you want to do a frost-protected shallow foundation, you need to download the HUD "cookbook" (google search frost protected shallow foundation HUD to come up with a .pdf) for your area of the country and submit all of the "engineering" which I use in quotes because the formulas are all right there and build the resulting details (the measurements of depth, gravel, foam wings and thicknesses) into the cross-section for your foundation as well as attach the way you came up with the numbers.

My opinion is that has to be done at "plan review" or when you submit your initial set of plans to the building authority.....And not in the field to an inspector who is intent on spending less than 10 minutes out of their car looking at your plat before you break ground to verify the setbacks are proper.

In other words, asking any inspector to make decisions for you is a poor direction. Getting approval for your plans and then asking an inspector to verify that your as-built matches the approved drawings is a much higher pathway to success.