Footing Question


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Old 08-03-07, 07:51 PM
S
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Question Footing Question

We are in the process of replacing our rear deck and want to address some water drainage issues as part of the project. In the past, large amounts of water have flowed by our deck during heavy rainstorms. To address this we plan to do the following:

• Improve the grading out from our rear foundation wall.

• Install a curb at the perimeter of the deck to divert water away from the deck and building foundation.

• Grade the ground outside of the curb so water flows towards a storm drain in the common area adjacent to our new deck.

Another function of the curb will be to support Simpson CBSQ66 post bases wet set in the curb that will support the outer deck beam. One of the requirements for the Simpson CBSQ66 post bases is that they have a minimum of 3” side cover in concrete. Since the post base is 6” wide, I have to add 3” inches of side cover to each side which gives me a 12” minimum for the curb width. According to what I have read about concrete footings the footing has to be 2x the curb width or 24” and the depth is the same as the curb width which is 12”.

Based on the above I end up with a 30’ long footing that measure 12” deep by 24” wide.

I have the following questions:

• Must the footing be 12” deep or would 8” be sufficient for this application? NOTE: My current drawing show 8”.

• Is drainage rock required under the footing?

Paste picture link for top view of deck.

http://www.fototime.com/727834931A2BA08/orig.jpg

Paste picture link for side view of deck.

http://www.fototime.com/8820347554BCC90/orig.jpg

Paste picture link for view of partially completed footing trench.

http://www.fototime.com/A45ACF481346A5F/orig.jpg

Thanks
 
  #2  
Old 08-03-07, 09:09 PM
H
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Can only give you our experience.

I believe on where you live, you want to be below the 100 year frost depth to prevent heaving). We are in central Florida so it isn't an issue, but we still were at 12" deep.

We installed a 130' footing.

Our concrete guy told us if we dug below "virgin" soil, you want to compact it, and if you've added more than 4" of dirt, he recommended 4" of crushed concrete below the footer. We were in virgin soil and used visqueen (6mil) in the footer before putting in the rebar.

Our structural engineer specified 12" X 24"w and we submitted and received a building permit (required for retaining walls over 18"h which we weren't but in our case, it attached to the house so the permit was required). Also specified was 2ea #5 rebar from 6-8" on both sides (2 bar chairs on each side, minimum 25" overlap on the bars).

We understood and had a slope less than 2 degrees that didn't require drainage but we did install french drain pipes (cotton sleeve added over pipes with holes) along the inward slope at the base of the footer and added 6" of lime rock horizonal and 6" vertical. We sloped the pipe 1/4" per foot, added 30 feet of (non hole) pipe till we got to the perimiter of the property line and trenched a slope the same 1/4" along another 40' of hole pipe w/sleeve in a bed of lime rock 12" deep.

For a roof base for a patio, I installed Simpson ABU66Z post base (6x6) and chose those because I can drill the 5/8" hole securing the ABU through the concrete with great percision rather than worrying about guys doing the pour whacking something along the way.

We used 3000 psi fibercrete. From standard 2500 psi, we specified 3000psi fibercrete which cost $1 for 500psi and $6 for fiber per yard. We just paid $87 per yard from Rinker.

Good luck with your project
 
  #3  
Old 08-05-07, 02:28 AM
S
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Thumbs up

Thank you for taking the time to read my post and reply.

I checked with our planning department for Santa Cruz County and there is no depth requirement for frost so 12” below grade should be fine.

I believe the bottom of our trench, which is currently 10” below grade, in virgin soil. I am no expert, but it appears to be a mixture of clay and sand. Why did you use visqueen (6mil) at the bottom of the footing?

We do plan to provide a method of drainage at the inside base of the footing and I really appreciate you providing the details of your installation.

I did consider the Simpson ABU66Z for the same reason you did, but decided to go with the Simpson CBSQ66 because I was concerned about lateral stability with a free standing deck with an enclosed fence extending 6’ above the deck surface. From what I have read in books the wet set type of post bases are inserted after the curb is filled and screeded. The Simpson CBSQ66 are huge and imbedded 7” into the curb. I suspect it will be a challenge inserting them correctly after the form is poured. I plan to use a square to make sure they are level with the form and mason line to make sure they are centered.

BTW, another contractor on a concrete forum suggested I eliminate the footing altogether and go with a 12’ x 18” curb. This would reduce the amount of concrete by 46%. Do you think this is acceptable for my application?

See picture link below to see a comparison of the two curb designs.

Paste picture link: http://www.fototime.com/8418519FEDCD73C/orig.jpg
 
 

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