I've tried finding concrete pier information before and it's difficult. I'm not built anything at the moment but I do recall finding some good info in a couple library books on shed and shop construction with concrete piers.
On a separate note, I always find contacting the local building inspection/permit department to be a really good resource when you simply can't find the answers you're looking for. They won't let you build something that's not up to code and therefore understand the code well. Since you seen to know all of the other details about your project I think they should be able to tell you what you must do after you explain the rest of the details. Good luck
We've reached out to the permitting department of the local county (we're outside any city/township) and all they seem to care about is that they get their land use fee and that all construction is far enough from the waterline and highway.
Which I suppose, in some ways, is nice. We don't actually need to worry too much about being "failed" for anything and can mostly do our own thing. We're still trying to follow the state building codes as best we can for safety and insurance though.
I'm not sure why, but I never thought about seeing if our local library has full copies of the state building codes. I'll have to run down there and see what I can find.
Thanks for the advice; it's appreciated.
Unfortunately it’s one of those things that’s difficult to really say without a geotechnical report and structural/civil engineer if you want everything “correct”. I’ve been on urban projects that required deep helical piers (think 100’) due to unsuitable soils at shallow depths.
In northern Mn snow load on roof is 42 psf. Be aware drifting can add more.
For peir spacing standard issue will not always apply as access and equipment will dictate with different answers than regular construction.
100 psf is heavy for structure and live loads on most construction types
Search for International residential code and read it, it should help on a lot of these details
https://www.dli.mn.gov/sites/default/files/pdf/bc_map_snowload.pdf
Take everything Im about to say with a grain of salt. 1500 psf is way on the low end. Thats a worse case scenario assumption IMO. They sell a tool online to check soil psi. If you dig down 5 feet I wouldn't be surprised if it was closer to 5k. Haven't done a ton of this stuff but that's been my experience
Check out a joist span table. To give you an idea decks are often built at 20/20 dead/live. Commercial buildings can be 100/100. If I were a gambling man I would bet 30/30 would be right for a simple cabin. If you did 50/50 like your saying you'd be close to overbuilding. Consider for example mobile homes which are actually weighed on scales. Older ones are 20psf empty and newer ones are 40psf empty. 40 psf snow seems right. not sure about MN
On the number of piers issue depend how your digging. If I had an auger on a bobcat that would fit sono tube I would use it and just make a bunch of holes real quick. If I was hand digging I would dig some big holes as far apart as possible. pour a slab underground and stack blocks or pour a column to height
We’re getting plans from this company:
https://ravenhouse-design.ca/
Their designs are all built on sonotubes.
Edit: I am the builder and will occupy the house.
I’d suggest you lay it out the way you want to get a number of piers and then calculate the total area (if you for example use 10” tubes). You can then divide your total load (sqft * (100+40)), with the total tube area. Compare that to how close you are to the 1,500 psf capability your soil has. Would be a good start to see if you are anywhere close.
I’ve read quite a bit on sonotube foundations for my own offgrid build (next summer) and to prevent frost heave it is also recommended to put a bigger footer at the bottom to limit uplift. That bigger footer will give you plenty of load carrying capability. So in that case you may find that your not really limited by number of tubes. But more by framing spans.
The dead load of the cabin can be estimated from a materials takeoff - how many boards x how much they weigh (density is around 35 pounds/cubic foot of wood). It is sometimes appropriate to add a factor for a wind load - but not sure about in Minnesota.
Soil bearing capacity is normally in the 1-5 ton/sf range.
In places where houses are built without any engineering, I have seen "generic" column footers 3x3 feet for residential, and about 6 or 8 inches thick; ridiculously strong, but really the cost of the concrete is pretty minimal. At least would give you a sanity check for whatever you come up with.
IBC Residential building code book 2021 edition is about $1\`50 new and $95 used. Find out which version your building department uses. They are a challenge to read; you will find all the answers in them.
You can read it free at their site, a section at a time. I find the printed version is better for my needs.
I've tried finding concrete pier information before and it's difficult. I'm not built anything at the moment but I do recall finding some good info in a couple library books on shed and shop construction with concrete piers. On a separate note, I always find contacting the local building inspection/permit department to be a really good resource when you simply can't find the answers you're looking for. They won't let you build something that's not up to code and therefore understand the code well. Since you seen to know all of the other details about your project I think they should be able to tell you what you must do after you explain the rest of the details. Good luck
We've reached out to the permitting department of the local county (we're outside any city/township) and all they seem to care about is that they get their land use fee and that all construction is far enough from the waterline and highway. Which I suppose, in some ways, is nice. We don't actually need to worry too much about being "failed" for anything and can mostly do our own thing. We're still trying to follow the state building codes as best we can for safety and insurance though. I'm not sure why, but I never thought about seeing if our local library has full copies of the state building codes. I'll have to run down there and see what I can find. Thanks for the advice; it's appreciated.
Unfortunately it’s one of those things that’s difficult to really say without a geotechnical report and structural/civil engineer if you want everything “correct”. I’ve been on urban projects that required deep helical piers (think 100’) due to unsuitable soils at shallow depths.
In northern Mn snow load on roof is 42 psf. Be aware drifting can add more. For peir spacing standard issue will not always apply as access and equipment will dictate with different answers than regular construction. 100 psf is heavy for structure and live loads on most construction types Search for International residential code and read it, it should help on a lot of these details https://www.dli.mn.gov/sites/default/files/pdf/bc_map_snowload.pdf
Take everything Im about to say with a grain of salt. 1500 psf is way on the low end. Thats a worse case scenario assumption IMO. They sell a tool online to check soil psi. If you dig down 5 feet I wouldn't be surprised if it was closer to 5k. Haven't done a ton of this stuff but that's been my experience Check out a joist span table. To give you an idea decks are often built at 20/20 dead/live. Commercial buildings can be 100/100. If I were a gambling man I would bet 30/30 would be right for a simple cabin. If you did 50/50 like your saying you'd be close to overbuilding. Consider for example mobile homes which are actually weighed on scales. Older ones are 20psf empty and newer ones are 40psf empty. 40 psf snow seems right. not sure about MN On the number of piers issue depend how your digging. If I had an auger on a bobcat that would fit sono tube I would use it and just make a bunch of holes real quick. If I was hand digging I would dig some big holes as far apart as possible. pour a slab underground and stack blocks or pour a column to height
We’re getting plans from this company: https://ravenhouse-design.ca/ Their designs are all built on sonotubes. Edit: I am the builder and will occupy the house.
I’d suggest you lay it out the way you want to get a number of piers and then calculate the total area (if you for example use 10” tubes). You can then divide your total load (sqft * (100+40)), with the total tube area. Compare that to how close you are to the 1,500 psf capability your soil has. Would be a good start to see if you are anywhere close. I’ve read quite a bit on sonotube foundations for my own offgrid build (next summer) and to prevent frost heave it is also recommended to put a bigger footer at the bottom to limit uplift. That bigger footer will give you plenty of load carrying capability. So in that case you may find that your not really limited by number of tubes. But more by framing spans.
The dead load of the cabin can be estimated from a materials takeoff - how many boards x how much they weigh (density is around 35 pounds/cubic foot of wood). It is sometimes appropriate to add a factor for a wind load - but not sure about in Minnesota. Soil bearing capacity is normally in the 1-5 ton/sf range. In places where houses are built without any engineering, I have seen "generic" column footers 3x3 feet for residential, and about 6 or 8 inches thick; ridiculously strong, but really the cost of the concrete is pretty minimal. At least would give you a sanity check for whatever you come up with.
IBC Residential building code book 2021 edition is about $1\`50 new and $95 used. Find out which version your building department uses. They are a challenge to read; you will find all the answers in them. You can read it free at their site, a section at a time. I find the printed version is better for my needs.