Shallow frost-protected or passive house foundations 1

[EmilC]

Hi folks,

I'm a structural engineer, specializing in timber and used to designing pretty simple frost-wall foundations up in the tundra of New England. I'm looking to design more haunched-slab or raft-slab foundations at residential scale, both for warmer climates and for shallow-frost-protected and passive-house type foundations. My company does custom houses, so we get into some exciting loads on the foundation from timberframes, Simpson Strong-Walls, steel moment frames, etc.. With frost wall foundations, there's a lot of dead load, member section, and room for rebar to sort that stuff out, but I need to up my game for thin rafts or haunched slabs, especially when they have a thick layer of insulation beneath. I'm looking for key concepts that I should understand, and/or textbooks or references that I should dig into.

For a specific example, say I have a haunched-edge slab on top of say 6" of foam insulation, with soil below, and somewhere along the length of that haunched edge is a big point load, like a column or Simpson Strong-Wall moment reaction. I can look at the haunch as a beam on elastic foundation, but how would I envelope a reasonable subgrade modulus for the foam + soil combination?

Any advice is appreciated!

 

[oldestguy]

Where in this world south of New England (warmer climates) do you intend to do this where 6 inches of foam insulation is needed under the slab foundation for frost protection ? We sometime might do that much insulation under ice skating rinks for summer protection of the ice skating floor or under a refrigerated storage zone. More commonly buried sheet 4 ft. wide is laid just under the exterior surface and its topsoil. Then 2 inches is plenty usually. That way frost is kept out from under the foundation.

 

[EmilC]

It's the superinsulated passive house foundations that are going for the thickest foam layers. 6" is light for some of those jobs- they're going for "thermal isolation" more than "thermal insulation". Agreed that it's similar to a cold storage facility... what would you do for that sort of design?

I haven't done the thermal modeling, but my suspicion for frost protection is that a heated building with a superinsulated slab is closer to the unheated design case for frost protected shallow foundations.

 

[CarlB]

With 6" insulation below the floor, it is approaching the unheated building case, but still benefits from some heat loss to slow down frost encroachment.
The wall insulation will need to be thickened, per Table A.4 in ASCE 32. The table has a column for floor resistance =28; 6" is close to that.
Wing insulation should be same as for a heated building.