Monolithic Slab Flootings Residential Eccentricity

[southard2]

I'm in Florida and I've an issue that arises every now and then. Typically builders like to use monolithich turned down slab foundations to support 2x4 or ICF walls for residential construction. I have always told architects and builders to switch to spread footings when the soil pressure is exceeded due to the eccentricy of the loading (the load bearing was is placed at edge of slab). However I've often noted that this threshold can be very low at times. The width I usually use for my monolithic footings is 16". Sometimes when I get the soil pressures failing I've often noted (particulary with ICF or CMU) that if I REDUCE the footing width all of the sudden it works. This occurs of course because the eccentricy of the load is reduced.

It does not make sense to me that 16" footing would not work while the 12" footing would. I suppose our equations are assuming the soils will be infinately stiff in our analysis. As the 16" footing is loaded wouldn't the outside edge soften a bit allowing a more uniform load on the soils. Like a form of load redistribution. I know this can't be quantified but it would make sense that if the 12" wide footing works than adding four more inches to the inside isn't going to make the footing less capable of supporting load.

In the case of ICF walls and CMU walls I was thinking of applying a rule of thumb, that if the soil pressure is good for a footing only as wide as the wall itself (zero eccentricity) than any size monolithic footing could be used provided it was wider than the wall. Is my thinking right here or should I stay with my conservative ways?

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[BAretired]

If you know the subgrade modulus, you could analyze it as a slab on an elastic foundation, but I think you would be safe using your last suggestion.

BA

 

[hokie66]

Agree with BA. Common sense is your best guide in this instance.

 

[Ron]

Agree with BA and hokie...and I'm in Florida.

Don't try to "fine tune" geotechnical parameters to fit structural conditions...they can get to be mutually exclusive in theory, yet work just fine in application.

 

[southard2]

Well the three of you seem to be in agreement. But I'm not exactly sure if you are saying my rule of thumb would be OK or if I should just stay conservative..... I'm trying to be extra careful not to misunderstand anyone here. I think what your saying is that the rule of thumb should work based on common sense of the logic.

And I certainly agree that fudge factoring or fine tuning the soil assumptions can be a very bad idea.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[hokie66]

We are agreeing that if the bearing is satisfactory with a footing the width of the wall, making the footing wider on one side does not make the situation worse.

 

[southard2]

Thank you so much Hokie. You guys are always so helpful on these boards. For the last 10 years I've pretty much been a solo engineering firm so this is a really nice place to be able to bounce ideas off of each other and to ask questions. Often I search and find my answers or needed references just by reading through these boards.

Thanks again!

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[BAretired]

You are right southard2. I wish that I had been able to benefit from this forum over the last forty some years since I first hung out my shingle. Too late now. Pity.

BA

 

[dhengr]

Southard2:
The three guys above certainly are helpful and full of wisdom. I agree with BA in wishing that I had had access to something like E-Tips over the last forty years or so. I spent an awful lot of time at Uni libraries, and collected quite a bunch of reference and text books over the years.

Maybe one of the reasons some of the older fellows are so good at giving advice, guidance, and mentoring in general is that without an easy/quick resource like E-Tips they had to dig a little harder and deeper, on their own, for their knowledge and understanding of any given problem, and that extra effort caused the learnin to stick with them a little better. Obviously more years of experience has something to do with it too. Also, without instant access to the high powered software almost everyone seems to have on their computers these days, they had to resolve problems in a little more straight forward, common sense approach, which could be practically analyzed in less than forty pages of printout.

 

[apsix]

If I understand it correctly the monolithic down-turn footing can't rotate freely due to restraint from the slab; which is an other reason why the theoretical peak bearing stress won't actually occur.

 

[csd72]

What eccentricity?

The load is applied off centre but there is also a slab load applied on the opposite side and a force couple between the slab at the top and the passive resistance in the base that resists the overturning.

I do not design for eccentricity on monolithic footings as long as the slab is reinforced and the depth is significantly greater than the width.

 

[ron9876]

In my opinion the soil will compress and relieve the high stress. However when it does this it rotates and this can cause cracks in the walls. My first house in Florida had exactly this condition and there was a crack in the stucco at the top of the footing.

If you design a portion of the slab on grade to cantilever off the footing you will find that the numbers work out much better. I often assume about 3'-4' of slab effective and I reinforce it to reduce the moment. Something like #3@12"c/c top.

Another thought is that if you are using lightweight wall systems you need to be sure that your footing is heavy enough to resist overturning when the walls act as shearwalls.

 

[southard2]

Apsix I believe your are right its just hard to quantify.

Ron9876 - I've done that before as well and assumed the rotation would be restrained because of the reinforcement extending into the slab. Similar to was Csd72 is suggesting. I've gone back and forth a bit over the years. I've never thought about using the weight of the slab as a counter balance but that is a good idea to increase the range of when a monolithic foundation can be used. For real heavy loads when everyone was insisting I've also extended the mono pour outward farther and notched it like you would for a brick ledge.

Ron I suspect that in residential it happens a lot especially when poorly compacted. Houses with siding probably have this problem concealed. I too have seen the crack your talking about. And the one thing I've learned over the years is that a building has to have a certain amount of mass one way or the other. It can be in the walls or the footings but one way or another you need weight when dealing with hurricanes.

csd72 - I actually agree with you also. Especially if welded wire fabric is in the slab and lapped properly. I do think there is a limit to how far this should be pushed. I'd say probably OK for most residential applications. Again its just hard to quantify unless you do very detailed calculations. You don't necessarily want to risk the slab cracking.

Thanks again everyone for you thoughts.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[rowingengineer]

agree with others while the slab helps often there are footing there to help with the torsion making for a very strong in torision footing.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”