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Mat / floating slab foundation for three-story multifamily

cody_fischer | Posted in General Questions on

I am in the midst of value engineering a three story six plex and wanted a gut reaction to my structural engineer’s default assertion on sub-slab insulation. 

The building has a rectangle footprint of ~1,500 sf.  The geo-tech engineer said that soil conditions were good enough that the only excavating required would be removing 2ft of topsoil.  (I can provide more detail on soil if needed)

The engineer says the Frost Protected Shallow Footing design found in places like GBA/Ecohome won’t work with the loads of a 3 story building.  The only option in his view is a floating or mat slab.

He hasn’t done the engineering yet, but estimates a minimum slab thickness of 18″ and minimum PSI of under slab insulation of 40, possibly 50.

1) Is he right about a FPSF not working for 3 stories?
2) Is he over shooting on mat slab thickness?
3) Is he WAY over shooting on insulation PSI? 

Number 3 matters to me because it forces me to use some of the nastier foam I am trying to avoid to reduce the carbon of this project.

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  1. Expert Member
    Michael Maines | | #1

    Cmfischer, I don't know all of the parameters of your design or location, so I can't give you definitive advice. I can say that many engineers are leery of building on foam and other non-standard techniques, while others are more creative or open-minded. You might want to talk with another licensed engineer about your project.

    I can say that the IRC building code does not specify a limit on height or load when it comes to FPSFs, and they do allow 3-story buildings to be built on crushed stone footings, so I don't see why a well-designed FPSF would not work for 3 stories. I have done raft slabs with 8" of concrete that were engineered to support 18,000 lb point loads, so I don't know why the same wouldn't work for a high but well distributed load. 40psi foam is probably a good idea; I have had engineers require it under point loads for 2-story buildings. You can get 40psi EPS.

  2. cody_fischer | | #2

    Thanks Michael - the examples you give are a helpful reference point.

    Do you happen to know any structural engineers in Minnesota :)?

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