Mat / floating slab foundation for three-story multifamily
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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