Hi Alexander,
I haven't forgotten about you, just really busy, and trying to finish this project while 3 more have already started...
I will try to address some of this about "frost heave."
Below quote is from
Stone wall collapse (frost) -- New England -- smarter solutions?
Jay C. White Cloud wrote:"Frost Heave" is often a "misnomer" in many locations and not...necessarily...all that is going on. That is why the many stone walls around the world...not just New England do not move. Many of these stone walls sit on...or near...bedrock, and/or a "mineral soil," not a "clay soil." The clay soil is part of the revelation...clays are an expansive soil when they get wet...even without ice to exaggerate this expansion. Get a "bentonite" clay in a soil and you have way more to worry about than frost as this is an extremely expansive form of clay...yet has many positive uses as well.
So the real issue is clay and/or water...if neither are present you have little to worry about. So the goal of any design is finding the clays, and removing the standing water which can freeze and expand.
No water...No heave...No clay...no heave....
Could not recommend the gravel trench more!! Best foundation system yet devised and still used today on just about every continent.
I think I covered frost heave also in the "Raised Earth Foundation" post...
Being in Vermont, "frost heave" can and is very much an issue in some soil types. Sharon Vermont, where this frame is, typically has very well drained soils in most of the area near the river.
For a project like yours, whether one uses large stone or a big chunk of concrete (artificial stone) the depth and installation of both is directed by the soil type as much as it is "averages in frost depth." Assurance of drainage is the primary issue to focus on whether a structure sits on made made stone or real stone.
In your case the other issue is a pre-existing structure you plan on tying into (i.e the house.) I typically will do not recommend tying into pre existing architecture just because of the risk of uneven settling This can be sometimes seen on some decks and porches added later to structures and/or added in new construction yet the "backfill" that the deck porch foundation is dug into settles more than the primary foundation of the house.
There is a great deal of info out there about "concrete foundations" and "frost." Not all of it is accurate, much of it is "incomplete," nor addresses traditional foundations. Empirically, whether here in New England, or anywhere that ground may freeze, all one has to do is look at historical homes and how they are built...They do not sit on concrete...They do sit on stone, and drainage is the primary focus along with depth excavation, and soil types. I would further add, that even with or huge building efforts...the majority of human architecture built in the world in history and into today...sit on wood and stone...not concrete.
As one more "food for thought" item...Let's consider the Brooklyn Bridge's foundation, one of the finest bridges of its kind. It is over 150 years old and still in great service, while the "concrete and steel" modern bridges all around it are either falling apart, need to be replaced and/or have been "down graded" in load capacity because of "issues.
What is the Brooklyn Bridge sit on?
Stone...
Scribing to plinth stones seems totally manageable, and there are several great posts on that, but it's all the other details -- how deep do you dig the footings, how exactly do they prevent frost heave, how do the posts attach to the stones, etc. -- that I can't seem to track down.
I am sorry...
...I feel like I may have left you hanging on those points and will try to make it clearer now. Please do ask more questions if you have them...
As for "how deep," that does depend on the soil type in your area and how far the frost specifically penetrates for your given area. To select a "safe" generic number...1.2 to 1.5 metres is safe for most of New England, and I add a bit more if not sure or don't know the exact soil types.
For example the "turnkey" project I just took on has a "sand/gravel" soil...It just doesn't freeze or heave from speaking with locals. We will be testing this out as we are building an all stone (dry laid) root cellar under the timber frame house I am designing for the client. The rest of this house (about 30 minutes North of Sharon in Bethel, Vermont) will sit on the same "stone plinth" foundation stone system as the Sharon Pavilion...Very much like most of the 300 to 600 year old homes do in Northern Japan.
As for how this prevents frost heave...again...no water...no heave...
Attaching the "post to stone" is by gravity in the traditional context and sometimes with a small "drift pin" or stone tenon in the base of the post. Just being scribe fit is typically enough for most simple structures with solid wall systems. Many of the old Minka farm houses of Japan (just like our old farm houses and barns here in New England) just "sit" on their foundations. There weight and gravity is all the "glue" that is needed.
Now to make all my "PE Friends" happy, on commercial/public buildings (and for extra insurance) we use "moment connection" (fancy word for mechanical fasteners in post foundations.) In the case of the Sharon Pavilion (if you look close at some photos) you can see that there is funny looking mortise in the side of the post near the base. This is where a 20 mm steel shaft penetrates up through the base of the wood post and is "fixed" with a large washer, locking washer and nut. I have had entire (small structures) like this lifted by crane...stone plinth and all...!! This is how strong the attachment is between the stone and the post in this configuration. The steel shaft (aka thread rod) is structurally adhered to the stone by an epoxy and is embeded 300 mm into the stone at minimum.
I would add again, I would not recommend using the house as part of the "structural support" for your "Canopy" area. I would make this a "free standing" and self supporting frame independent of the structure. Then the "main house" can move (or not move) and your frame can do the same. This does add more material and work, but I have always felt the gain in strength is worth it. Another reason to do this...
...if someone really likes it, they can buy it from you...
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Please do post more questions if you have them...I will do my best to stay on top of them...
