Commercial Addition to Floating Slab Structure
A client is asking me to design an addition of a 60’x 30′ warehouse structure to an existing 40’x 30′ commercial structure. They drive forklifts inside, and need the slab of the new space to match up with the existing.
We are in Vermont with active frost/ground movement, and the existing structure is an old hodge podge of building styles. I’ve been told that the existing slab is floating, with a few sonotube piers added after the fact.
When people around here talk about additions they talk about making sure that the new and the old “agree.” A new floating slab will float with the old one like boats moored side by side, whereas a slab with a footing next to a floating slab will mean that they fight each other.
Have not done any excavation yet to see if what I’ve been told about the existing foundation is true, but want to give good info and advice to these clients before we go too far down the road on this.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
goodomens,
I'm not sure you have much choice from a code persecutive. The two compliant alternatives are to build down to below the frost level, or use a FPSF. Neither of those will move with whatever frost heave the existing foundation experiences.
And practically it doesn't make much sense to me to build an addition with the same problems the existing building has. Perhaps the best solution would be to stabilize the existing building by retrofitting a FPSF strategy.
I agree with Malcolm; while you could build another floating slab next to the existing and pin the slabs together with rebar so the joint remains aligned, they still may move independently of each other. It would be a bit of work but not terribly difficult to excavate around the existing structure and add foam insulation to create a frost protected shallow foundation (FPSF). https://www.greenbuildingadvisor.com/article/frost-protected-shallow-foundations
Thanks guys! That helps me get my head on right.