Raft slab foundation & foam compressive resistance.

[Mark Wilkerson]

I'd like to learn what other people have experienced with raft slab foundations, specifically reinforcement and foam type.

We are designing a raft slab foundation for a passive house and the structural engineer has specified geofoam rated 40psi (5,760 psf) @ 10% deformation and 2.4 lb/cu.ft.  This is more dense, and more expensive, than the material I've seen used on other passive houses (type 9, rated 25psi (3,600 psf) @10% deformation and 2.0 lb/cu.ft.).  The soil bearing capacity is just 1500 psf.  

I also see on the spec sheet that at 1% deformation, this 40lb foam has a compressive resistance of 15 psi (2160 psf).  

We are also using two layers of rebar @ 12" O.C., top and bottom.  

Is this way out of line with what others are seeing?  

Thanks, 

Mark

[Adam Cohen]

Mark i will tell you that all our rafts are built on structural geo foam - 2.8#, 6400 PSF and termite treated.  

This is just good practice and as a design builder, good insurance.  

i listened to this Harry Belafonte song when I was young, and this taught me a lot.  Click here to learn what i listened to.

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Adam Cohen
Certified Passivhaus Consultant - North America and Europe

Registered Architect MD, VT, NH & CO, LEED AP, NAHB Green Professional

Design/Builder of the First US Passivhaus Public School Building

2012 VSBN Green Designer of the Year

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[Dylan Lamar]

Adam, we gotta get this soundtrack playing in all the houses at the next International Passive House Days Tour! 

It's a sure sell!

[Adam Cohen]

Would be awesome

Sent from mobile. Please excuse mis-types.

[Albert Rooks]

Well geez. How could you not like that! I'm picturing musical client meetings

Sent from my iPhone

[Mark Wilkerson]

Thanks for the entertainment Adam - that's hilarious.  Nice visual aide.  

Your strategy a strong counterpoint to something I've also heard which is, "The foam doesn't need to be stronger than the soil it sits on."  Surely your soil bearing pressure is not rated at 6000 psf.  What benefit does foam with a higher compressive strength than the soil add to your structure?  As if the Tower of Pisa wouldn't be leaning if they had used a more dense foundation.  

(My visual aide)

My suspicion is there may a difference in the standards used when rating soil bearing pressure and foam compressive strength.  Our geotech rated our soil pressure at 1500psf, with possible settlement of up to 1 inch.  The 12" thick 40psi (5,760psf) foam is expected to compress 10%, or 1.2 inches, under the full rated load.  

Does anyone have a good link to share with more detailed information on this topic? 

​Thanks,​

Mark

mobile         206.963.4039

4826  45th  avenue  south

seattle, washington 98118

www.wilkersonlab.com

On Thu, Oct 23, 2014 at 2:22 PM, Adam Cohen <adam.c...@gmail.com> wrote:

[Rob Harrison AIA]

Adam, why would the foam need to have a higher compressive strength than the soil it sits on? 

Not sayin' I disagree with the notion of "good insurance" but frugality is good too, where it makes sense.

Rob

Rob Harrison AIA CPHC

HARRISON architects

1402 3rd Avenue, Suite 515

Seattle, WA  98101-2120

O: (206) 956-0883 iP: (206) 794-2738

http://harrisonarchitects.com

http://twitter.com/#!/robharrisonAIA

lyrical sustainable design  ::  passive house plus

On Thu, Oct 23, 2014 at 2:22 PM, Adam Cohen <adam.c...@gmail.com> wrote:

[Mark Wilkerson]

Thanks for putting it more succinctly Rob.  

Mark

mobile         206.963.4039

4826  45th  avenue  south

seattle, washington 98118

www.wilkersonlab.com

[Adam Cohen]

Rob, 

It is very simply my nature.  As a design and builder there are things I do not compromise on, my motto has always been "I will  not  build for you what I would not have".  For example:

• I engineer all my homes wooden floors for 80# live load, (code is 30# and 40#), becasue I am a large guy and i do not like to feel my homes floors move or hear the dishes in the dining cabinet rattle.
  
• In VA the code says I can put one coat of tar and 6 mil poly and call a basement waterproofed.  I use a spray on membrane, drainage/protection board and then run both a footing drain and a french drain to daylight.
• All my slabs (basement, garage, etc.) are reinforced with rebar 24" oc for long term movement placed with the correct plastic chairs, have fibermesh for curing shrinkage and install control joints no wwf in any of my slabs!

My goal is cost effective PH, which I can obtain without compromising my overarching belief in building at a level well beyond code (similar to PH energy savings, well beyond code right now)

SO, why would I put the foam @ 3# instead of 1.5#.  I did a bunch of research and that is what I decided I would want on my home.  Then I designed a perimeter bearing system to minimized the use of the costly foam and optimized the design of the system so that is very low labor and cost neutral to a standard slab.

I hope that explains it, I am not saying it is right, just said that was the way I did it.

~ Cheers!

Code is the least you an do and still be legal.

[Hayden Robinson]

Mark,

 

For a Passive House I designed, with conventional spread footings on 3,000 psf soil, the engineer specified  the same 2.8 lb foam under the footings. This strikes me as reasonable, given the soil strength and the foam's compressive resistance of 2,160 psf at 1% deformation.  In your case, specifying foam with 1% deformation at 2,160 psf over 1,500 psf soil doesn't strike me as necessarily out of line. Hopefully the foundations are designed to impose a load of less than 1,500 spf, and the stiffer foam will limit movement. The remaining questions are how much is the load and how much deformation is acceptable? At least that's my possibly oversimplified, non-engineer reasoning. It would be nice to hear what your engineer says - it might educate us all. And, of course, it's always possible that it wasn't a carefully considered decision on their part.

 

-Hayden

 

Hayden Robinson Zertifizierte Passivhausdesigner

hayden robinson architect

206.691.3445