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Sr100pcfl
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# Posted: 10 Jul 2022 06:43am - Edited by: Sr100pcfl
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Hello, Friends. Here is my first post ever over the internet. I am in design stage but just a few weeks away from starting my 16x24 cabin. I will try to attach a picture file of what I am planning to do with cabin foundation, any advise will be greatly appreciated. 
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Cedar Fever
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# Posted: 10 Jul 2022 09:59am
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What size are the joists? If too small you will get bounce in the middle. I have a 16x24 and wish I had figured out how to put it on steel I beams.
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spencerin
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# Posted: 10 Jul 2022 10:40am - Edited by: spencerin
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You're probably pushing the limit on the span of the floor joists. The easiest way to fix that is to narrow their spacing to 12" OC.
You'll also be better off not encasing the wood posts IN concrete but attaching them to the top of what would be concrete piers.
What's your location? Doesn't look like you're going deep enough to get below the frost line on your foundation, unless you're in a warm climate.
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Aklogcabin
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# Posted: 10 Jul 2022 11:13am - Edited by: Aklogcabin
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Why not put support under the middle of the cabin ? Folks seem to think that putting 3 sows of beams under the floor is difficult or they will heave from frost.
If done correctly they will not heave. I built a 16x24 cabin. For the foundation I used cedar utility poles 6' on center. Then put 6"x12"x 12' beams shouldered in the posts. Fasten will 1/2" galvinized bolts. 12" tall manufactured floor joists spanning 16'.
This is a very solid floor that stays perfectly level. And being in ak has been through a 7.2 earthquake and 12' of yearly snow.
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ICC
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# Posted: 10 Jul 2022 02:58pm - Edited by: ICC
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First a comment re the floor joists... Species and grade makes a difference whether or not those floor joists will be sufficient. For some species that will not work as drawn.
Use the SpanCalc at AWC. You can enter and change the species, grade , sizes, spacing, live and dead loads... You can have it solve for a maximum span for a particular species, etc. which is the default mode, or have it supply the joist size (2x8, 2x10, etc) for a particular maximum span. Click the top right "Span Options" tab.
In case you are not aware, #2 grade is most common, deflection for a floor should be set at L/360, Live Load is 40psf by code for most rooms and it is best to strive for that IMO if you don't want joists to flex and bounce. If the joist size is close to being maxed out it is best to up the joist size of close the spacing. Be aware that closer spacing also makes some construction tasks a little more difficult because of reduced work space. Dead Load is the weight of the materials. Maybe up that a little if going 12" OC or using more dense woods.
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ICC
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# Posted: 10 Jul 2022 03:02pm
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Second, a question about location. As was mentioned is freezing an issue?
Is there a need for a building permit? Just because we might rather not have to get a building permit does not mean that the sometimes pesky officials might come asking for one at some time after construction has commenced.
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ICC
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# Posted: 10 Jul 2022 03:11pm
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I am NOT a fan of building a habitable building on the surface. If I interpret the supplied info and pictures correctly, it appears you have concrete sitting on gravel. Nothing is actually in the ground. I think that sort of thing is fine for a shed. Some folks disagree with that opinion. Attaching a wood post to the top of a concrete pier also has some issues; one being the wood to concrete joint can act like a hinge point. If the wind blows enough horizontal movement can occur, maybe followed by downwards movements of the cabin.
Need more information before more comments/advice.
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Sr100pcfl
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# Posted: 11 Jul 2022 08:04am
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Quoting: Cedar Fever What size are the joists?
Joists size pictured are 2x10 16"OC, but reading Posts, most likely, I will change it to 2x12.
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Sr100pcfl
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# Posted: 11 Jul 2022 08:10am
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Quoting: spencerin What's your location?
East Tennessee frost line less then 12". Should be OK to go 16"-20"down from ground level?
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Sr100pcfl
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# Posted: 11 Jul 2022 08:16am
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Quoting: Aklogcabin Why not put support under the middle of the cabin ?
Trying to make building process as easy as possible. Will up joist size to 2x12 16"OC. It should make floor more stable.
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Sr100pcfl
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# Posted: 11 Jul 2022 08:20am - Edited by: Sr100pcfl
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Quoting: ICC I am NOT a fan of building a habitable building on the surface.
As first picture shows building will seats on the slop, all piers will start at ground level and go down 16-20".
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Sr100pcfl
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# Posted: 11 Jul 2022 08:36am - Edited by: Sr100pcfl
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Quoting: ICC Attaching a wood post to the top of a concrete pier also has some issues; one being the wood to concrete joint can act like a hinge point.
One of the reason why I am going to encapsulate post in concrete. Should give me 15-20 years worry free time 
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gcrank1
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# Posted: 11 Jul 2022 09:02am
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Those years will go by far too fast.....
Better to do 'best practices' on anything at the bottom; it is way too hard and expensive to be re-working ANY of that when you are older.
Fwiw, our current cabin is the same 16x24 and was poorly done only 10yrs ago by the prev owner and builder. Dealing with what it really needs is more than a little problem!
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Aklogcabin
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# Posted: 11 Jul 2022 09:43am
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I don't believe that the floor will be solid without 3 support beams. Even with 12" material. Was a fellow from WI here that was just writing about how much floor bounce there is with only 2 beams. Also don't believe that you will find dimensional lumber that spans 16' and meets any codes if concerned about that.
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Brettny
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# Posted: 11 Jul 2022 10:49am
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Quoting: Sr100pcfl One of the reason why I am going to encapsulate post in concrete.
Dont do that. Just skip the post altogether and pour the sono tube upto the bottom of the main beams.
I would also echo 2x12 floor joists for that span.
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gcrank1
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# Posted: 11 Jul 2022 10:59am
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Before spinning your wheels too much, do you have to comply with any building codes there?
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travellerw
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# Posted: 11 Jul 2022 11:06am
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Quoting: Aklogcabin I don't believe that the floor will be solid without 3 support beams. Even with 12" material. Was a fellow from WI here that was just writing about how much floor bounce there is with only 2 beams. Also don't believe that you will find dimensional lumber that spans 16' and meets any codes if concerned about that.
I built pretty much the same cabin as him and yes there was "some" bounce.. but it really isn't that bad. No one in my family noticed it (except me). It completely went away when I added 2" in foam with a second subfloor screwed every 6". We both also used 2X10s not 12s! No 1 graded SPF does meet code.
Now with all that said, our cabins are "on grade". The OP is doing a pier cabin with piers below the frost line. If I had done that, I would have added a 3rd support beam.
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Sr100pcfl
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# Posted: 11 Jul 2022 11:18am
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Quoting: Brettny Just skip the post altogether and pour the sono tube upto the bottom of the main beams
I was thinking to go this route, but dont like an idea to deal with large amount of concrete (not young anymore), and does sonotubes required footing as well? (Trying to stay on budget).
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jsahara24
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# Posted: 11 Jul 2022 12:05pm
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Quoting: Sr100pcfl Quoting: Brettny Just skip the post altogether and pour the sono tube upto the bottom of the main beams I was thinking to go this route, but dont like an idea to deal with large amount of concrete (not young anymore), and does sonotubes required footing as well? (Trying to stay on budget).
I have a 13'6" joist span and have 2x10s on 16" centers. It is pretty solid but I wouldn't go any further personally.
We put piers down below frost line on footers, the sonotubes slide into the footer form so we did a monolithic pour. Whether you need footers or not depends on your soil, sf of bearing from your piers, and the weight of your building. We had the piers come up to the floor elevation (~2' above grade) and put the beams on the pier. I would recommend this route rather than having PT wood in the ground.
Will you have a concrete truck or using bags and a mixer? If you have concrete truck access its not that difficult to fill up sonotubes but does require some manual labor...
Good luck...
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spencerin
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# Posted: 11 Jul 2022 06:56pm
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If you end up encasing the wood posts/piers in concrete, then buy those sleeves online for this so the posts are better protected from moisture. They'll likely add decades to your posts. This way, you can keep to the original plan but have them last longer than 15-20 years.
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ICC
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# Posted: 11 Jul 2022 07:29pm
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Quoting: Aklogcabin Also don't believe that you will find dimensional lumber that spans 16' and meets any codes if concerned about that.
That may be what you want to believe but that is incorrect. IF you look, there are tables in the IRC that will show you what will pass code and there are many examples of 2x10's and 2x12's species and grades that are commonly available and do meet code for spans up to and over 16 feet.
The IRC is the code used in 49 states, the District of Columbia, Guam, Puerto Rico and the U.S. Virgin Islands. WI does their own but it closely follows the IRC. (each state government may omit or edit sections as they see fit.)
The 2021 IRC can be found online... https://codes.iccsafe.org/content/IRC2021P1/arrangement-and-format-of-the-2021-irc Earlier versions are also available online as not every state adopts the newest version as they are released.
Click on Chapter 5 in the left sidebar menu; many tables there are dedicated to floor joists. Not every species is listed but the IRC tables list all the commonly used species and grades. The AWC calculator on the other hand lists many species that are not in common use but that could be used. When referencing the IRC always read the footnotes; they can be important.
Now, whether or not a person wants to use a joist size and species/grade which is on or near the maximum span is another decision that a person can make. Code tables are all minimum sizes or maximum spans/loads and most people will agree that pushing the limits may not be the wisest thing. That goes for many pursuits not related to building. However, it can pass code inspections.
It never hurts to go up a size of material or reduce the joist spacing. Using thicker subflooring can also increase the rating, and make a floor stiffer. Thicker than 3/4" OSB and sheet products like Advantech are available, just not found at your usual big box store.
The AWC calculator that I often refer people to every so often, is not put out by some guy in a basement someplace. AWC is the American Wood Council which, IF you check, works closely with the folks who publish the IRC, the IBC, and several other codes concerned with building structures.
The AWC calculator is also available online... https://awc.org/calculators/span-options-calculator-for-wood-joists-and-rafters/ They have Android and iPhone version available too. I also find their Connection Calculator very handy to determine whether or not fasteners sizes and number are suitable for proposed loads.
THose tools are there for use by anyone and one does not need to be a professional to use them. That is the whole purpose behind those webpages. Correctly using the information from the tables or calculators can eliminate the need to hire an engineer.
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Brettny
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# Posted: 11 Jul 2022 08:44pm
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Quoting: Sr100pcfl I was thinking to go this route, but dont like an idea to deal with large amount of concrete (not young anymore), and does sonotubes required footing as well? (Trying to stay on budget).
The foundation is not the area to skimp on.
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