Footing

Started by busted knuckles, April 02, 2016, 09:43:35 AM

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busted knuckles

Hi everyone, finally starting my build. Getting ready to pour footing. Couple questions, the trench is on a bit of a slope. The bottom of the trench is deeper at the bottom of the slope than the top. So if I pour as is, footing will be 19 inches thick at bottom of slope and 14 inches at top of slope.  Will this be an issue?


I am then pouring piers on top of the footing. For a 16x24 house with 1.5 stories. Would 2 rows of 5/8 rebar running the perimeter of the footing be enough? Footing is 18 inches wide..
you know that mugshot of Nick Nolte? I wish I looked that good.

Redoverfarm

Quote from: busted knuckles on April 02, 2016, 09:43:35 AM
Hi everyone, finally starting my build. Getting ready to pour footing. Couple questions, the trench is on a bit of a slope. The bottom of the trench is deeper at the bottom of the slope than the top. So if I pour as is, footing will be 19 inches thick at bottom of slope and 14 inches at top of slope.  Will this be an issue?

Is not an issue other than the expense of the extra concrete.


I am then pouring piers on top of the footing. For a 16x24 house with 1.5 stories. Would 2 rows of 5/8 rebar running the perimeter of the footing be enough? Footing is 18 inches wide..

Just make sure that the rebar is situated off the edge and toward the center of your depth.  In your situation I would located it 6" from either side and 6-8" from the bottom of the trench.  Nothing wrong with elevating the rebar tied onto shorter sections driven into the ground.  On the deeper end you could probably elevate the rebar pair a little. If you can estimate the location of your piers you can fashion a ladder design there using shorter pieces tied to the parallel pair.

Any particular reason for pouring a perimeter footing when you are just pouring piers?  Just wondering from your description that is what you are doing unless you plan to build a stem wall from the footing up to enclose the crawlspace.  Most building on piers would just locate the pier locations, pour the footing suitable to hold the piers and build off those.



busted knuckles

Thank you very much, Redoverfarm!
you know that mugshot of Nick Nolte? I wish I looked that good.

Don_P

Pouring a continuous footing helps keep the piers from settling differently, it also makes it very easy to make corners out of block that run ~4' each way out of each corner, bracing the building... not as well as a full perimeter wall but much better than an unbraced pier.

Start at 8" thick at the top end... plenty thick for a footing under this and then run thicker to hold level if desired, not strictly necessary but easier to work from level. The rebar should be in the bottom 1/3 of footing depth but never closer than 3" to soil, the steel does its work on the tension side of things.

busted knuckles

Don,  you think 8" at the top is thick enough? I was concerned, having read that the corners usually take more weight than the others.
you know that mugshot of Nick Nolte? I wish I looked that good.


Don_P

Unless the soil is unusual 8" is plenty thick that is the normal callout for residential construction including brick, stone and log homes.  Actually the next piers in takes twice the load of the corner, there is the load from halfway to the next pier on either side. Use that windfall to make those wide corners I described and fill them with rebar and pea gravel concrete.

rick91351

Quote from: Don_P on April 05, 2016, 05:53:38 AM
Unless the soil is unusual 8" is plenty thick that is the normal callout for residential construction including brick, stone and log homes.  Actually the next piers in takes twice the load of the corner, there is the load from halfway to the next pier on either side. Use that windfall to make those wide corners I described and fill them with rebar and pea gravel concrete.

Rebar is a real key component for a tough footing and wall.  Footing brakes - wall will crack.  Another key is good solid soil - if your soil or matrix or native material is going to settle then your footing is going to settle.  When it settles your footing is going to crack unless it settles just a small amount then the rebar does its job.  I am not addressing small hair line cracks but the huge structural cracks. Steel is a real key.   :D   
Proverbs 24:3-5 Through wisdom is an house builded; an by understanding it is established.  4 And by knowledge shall the chambers be filled with all precious and pleasant riches.  5 A wise man is strong; yea, a man of knowledge increaseth strength.

NathanS

I don't understand all the structural engineering behind footers and foundation walls, but the HUD Residential Design Guide was helpful to me.

https://www.huduser.gov/portal/publications/residential.pdf

It is less concerned with 'What's the Code' and focuses more on the numbers.

"For the vast majority of residential footing designs, it quickly becomes evident that conventional residential footing requirements found in the residential building codes are adequate, if not conservative"

"For most residential foundations, the primary resistance against differential settlement is provided by the deep beam action of  foundation wall; footing reinforcement may provide limited benefit"

Also it's interesting when you look at a lot of the math, they already include lots of fudge factors in their calculations. Once something is in the IRC, there is likely already lots of safety included in those numbers, so when we beef things up we are probably going double overkill.

All that said, I'm still planning to put rebar in my footing because a 20' length of #4 rebar is only $7.   :D

Don_P

That puts steel on the tension side of that deep beam, where it does the most good for that direction of bending.  "If its worth the concrete, its worth the steel".


rorybagleys

Quote from: Don_P on April 05, 2016, 11:00:43 PM
That puts steel on the tension side of that deep beam, where it does the most good for that direction of bending.  "If its worth the concrete, its worth the steel".

I like that: "If its worth the concrete, its worth the steel".
A good motto.