Can I change the Little House plans to 16x24?

Started by maggiethecat, January 08, 2022, 11:53:01 PM

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maggiethecat

Hi everyone, I was wondering if we could build the 14x24 little house as 16x24. Will this be alright? Thanks in advance!
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MountainDon

Floor joists, ceiling joists, and rafters may have to be increased in depth or spaced closer together, as well as being longer of course.
Just because something has been done and has not failed, doesn't mean it is good design.


maggiethecat

Quote from: MountainDon on January 09, 2022, 05:27:28 AM
Floor joists, ceiling joists, and rafters may have to be increased in depth or spaced closer together, as well as being longer of course.

Thank you Don, is there a formula for this?
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MountainDon

There are tables in the IRC and google can find others.
https://codes.iccsafe.org/content/IRC2021P1

The AWC Span Calculator is very handy
https://www.awc.org/codes-standards/calculators-software/spancalc
There is an Android and iPhone version available too. Links to them in the above page IIRC.

The AWC tutorial page explains use of the tables and the values used in the calculator.
https://www.awc.org/codes-standards/spantables/tutorial

Well worth becoming familiar with.
Just because something has been done and has not failed, doesn't mean it is good design.

maggiethecat

Thank you Don, I wonder if the number of piers and their positions would stay the same?
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Don_P

This is the foundation chapter of the code. Section 403.1 should answer that part of the design;
https://codes.iccsafe.org/content/VRC2018P1/chapter-4-foundations

maggiethecat

Hi Don, I couldn't find anything about it in that link, but I found this old thread: https://countryplans.com/smf/index.php?topic=14482.0

It had the following image, which I think is probably a good idea for the spacing:


I would need to build up the beams from three 2x6's. The difference is that in the little house 14x24 plans, the floor overlaps the posts by 2'2", probably so you can add skirting. These plans have the posts at the very corners.

I'm not sure where to go with these, I thank you for suggestions!
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Don_P

Prescriptively, that is, without an engineer or architect's seal, these are the minimums. If you wish to step outside of these provisions, be prepared to engage one of those registered design professionals.

For what you have pictured above the answer is ;
Quote
R404.1.9.3 Masonry piers supporting braced wall panels.
Masonry piers supporting braced wall panels shall be designed in accordance with accepted engineering practice.

In other words, engineer required.

So going back to the easy way,

This is section R 403.1
QuoteR403.1 General.
All exterior walls shall be supported on continuous solid or fully grouted masonry or concrete footings, crushed stone footings, wood foundations, or other approved structural systems that shall be of sufficient design to accommodate all loads according to Section R301 and to transmit the resulting loads to the soil within the limitations as determined from the character of the soil. Footings shall be supported on undisturbed natural soils or engineered fill. Concrete footing shall be designed and constructed in accordance with the provisions of Section R403 or in accordance with ACI 332.

Using the tables below that section, it looks like a 6x14 concrete strip footing would be about the minimum.

The foundation walls, refer to section 404;
Quote
R404.1.2 Design of masonry foundation walls.
Masonry foundation walls shall be designed and constructed in accordance with the provisions of this section or in accordance with the provisions of TMS 402. Where TMS 402 or the provisions of this section are used to design masonry foundation walls, project drawings, typical details and specifications are not required to bear the seal of the architect or engineer responsible for design, unless otherwise required by the state law of the jurisdiction having authority.

The easy way is 8" block, check the reinforcement schedule there.

For cantilever spans see table R 502.3.3(1)

For girder spans check table R602.7(1) and (2)
In the second table they address the doubled 2x8 girders across the floor supporting the 2x6 floor joists. There's room to interpolate and play but you are either a ply or a size too light.




Don_P

#8
This is non prescriptive but is using methods that I believe an engineer could seal if the building inspector required it. The posts rise from large piers (overturn resistant wide). The posts rise, unbroken from pier to top wall plate. The wall sheathing provides bracing for the posts. This is "post frame construction". At 16' wide you can make it with 2x12 joists and no mid girder or piers. That can be done either way, weigh foundation costs vs bigger framing costs.

Hmm, image isn't posting for me, this is where I've put the file;
http://timbertoolbox.com/cp/16x24postframe.jpg

Saw another pic while filing that. This is a 16x28 we built a couple of years ago. We sawed all the framing, floor and roof sheathing and siding for this job. It is on a full basement which put the washer/dryer, water heater, heater, etc down there.
http://timbertoolbox.com/cp/PatSE.jpg


Don_P

#9
It looks like I'm talking to myself. So to continue a conversation with the smartest man I know.
This is a perspective view from my first plan attempt.
timbertoolbox.com/Pat/PatPierFnd.jpg

This is non prescriptive but goes a long way to meeting the intent of the foundation requirements in the code. There is a braced "core" that anchors the building and through the floor diaphragm, the piers that support the rest of the structure. I have built this and it works. I wouldn't do it there there are high winds, seismic, etc. In this particular instance the building official chose to require an engineer if we went that route.

The quick calculus in my head was to think about whether it was smarter to hire the engineer to say I was right. Or, would that cost about the same as putting in a full basement.

Here's what we built;
timbertoolbox.com/Pat/patrick6.18.jpg

and a good bit of the rough framing exposed in this shot
timbertoolbox.com/Pat/patricksframe.jpg
The end rafters are not in yet. count from the right hand rafter going left, you have a good sightline on rafters 3 &4. trace them down to the wall, notice they "stack" directly over the wall studs. You cannot see the 16' floor joists on the main and loft floors but they are also in exactly the same plane as the rafters. Everything "stacks" wood on wood over wood whenever possible.

Ahh, my feet have finally returned, off to the shop.

MountainDon

#10
Hmmm.. The smileys have disappeared. ???  I have the keystrokes for that one memorized....  The icons for them are not displaying; that needs to be addressed.
"Thumbs Up" to Don_P

Just because something has been done and has not failed, doesn't mean it is good design.

MountainDon

Just because something has been done and has not failed, doesn't mean it is good design.

Don_P

#12
Smiley's, tags, its all gone here.

Knowing our driveway, you'll get a kick out of this. We had a pretty good snow a week ago. With Michelle still not 100% after knee and hip work, hiking in snow was out. I had already made backup plans. This is my email to my Sis.

Trying manual entering quote tags
Quote
We got Michelle down to the road and off to work today. I got plowed out and back up to the house, ran a load down to her car and came back up for her. ZZ Top was playing the "How how how" song as we loaded her on the armrest of the dozer, I spun it around and presented the sorta plowed 30% grade. "Have Mercy" left her lips, and away we went  :D

edit; Well, that worked at my end, trying img tags, I had no joy on this the past 2 days

Sawed these posts out last week for a shed, one is a corner post the other a midwall post.  Horizontal boards will stack in the grooves.

Hmm, still no joy there. The tags and img link are all here in the reply window but nothing in the viewed thread. That's what I keep getting.

maggiethecat

Wow I missed a few posts! Thanks guys!
But I think I figured this out.

I will keep the foundation plans just as they are EXCEPT the width will be 16feet and the floor joists will be 2x10 instead of 2x8.

In an old thread, John Raabe answered highplainsdrifter by saying he should increase the size of his floor joist from 8 inches to 10 inches to accommodate the added 2 feet.

https://countryplans.com/smf/index.php?topic=8702.msg112841#msg112841

So you get a 3'2" cantilever instead of 2'2" but I think that's going to be okay.

The other thing John suggested was: "If you are using the 14x24 Little House plans and will do an interior centerline beam and piers, I would set the outside beams to the outside of the platform. That would leave an 8' span for the joists. At 16" o/c you could use #2 Doug Fir 2x6 or go to 2x8 to get more depth for the floor insulation."

I will try and calculate which option is more cost effective considering our currently insane lumber prices and I hope this thread helps whoever else is searching for converting the 14x24 Little House plans to a 16x24 cabin.
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Don_P

The inspector will check spans and cantilevers against code. Don't find yourself in the position of saying "Billy Bob told me so", check the references above if there is an inspector involved. (The tables do come from basic physics, not someone trying to get in your wallet, they work reliably).

maggiethecat

No inspectors where we're building, so THAT is good!

One thing to keep in mind is that we will need skirting. That means we can't have the posts all the way to the edge. So we'll do a 6 inch cantilever on the edges.

So, three rows of posts, six posts each row, spaced as in the 14x24 plan, 2-4-4-4-4-4-2 in the long direction!
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Don_P

You've basically described a permanent wood foundation. Done correctly that may be easier and cheaper... and it provides bracing.

maggiethecat

Quote from: Don_P on April 05, 2022, 06:06:31 PM
You've basically described a permanent wood foundation. Done correctly that may be easier and cheaper... and it provides bracing.

Don! Do you have any opinions on placing treated posts directly into concrete as opposed to on a bracket on top of the concrete? Inspectors in Oregon, from what I hear, are okay with this and they're picky as hell. You dig a hole that is 20 inches diameter, stick a treated 6x6 post in there, 6" above the bottom of the hole, with some rebar and fill the hole with concrete.

I'm wondering how this will last thru the decades.
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Don_P

If you use foundation grade posts burial shouldn't be a problem. That said I prefer masonry in the ground.
The concrete under the post spreads out the punching load of the small post end over more ground, a good thing. The concrete around the post is an attempt to use the post as bracing. Likely more expensive and less strong than building a wall, of either treated or my preference of something that cannot rot or be eaten, rocks, concrete, bricks, etc.

Walls brace a building. Posts must be braced in some way to keep from overturning.

The permanent wood crawlspace I mentioned requires no concrete, it is simply short treated wood foundation walls on a gravel trench. Because those walls are sheathed, your desire for skirting, they are also braced. They don't need to be set inside the wall line above, although up to joist depth that is fine.

The section of foundation we are getting ready to do is rubble stone, rocks from the site, mortar and concrete. Scott and Helen Nearing popularized slip form construction back in the old Mother earth days. It's faster but doesn't rock my world. We've been under a house for 16 months now putting a foundation under it. It is a bit harder to fix a bad foundation decision after the fact than to get it right the first time.

Don_P

#19
This is another way;

I haven't tried to post a pic since things changed, let's see if this works... appears not, well here is a link to a pic I had;
http://timbertoolbox.com/cp/16x24postframe.jpg

Notice the posts run from the footing all the way up to the top of the walls. As long as the projecting posts are kept reasonably short, when the walls are sheathed the posts are braced. The weak hinges at the floor and footing are replaced by a solid post. The concrete pier footing Is how I do them here, in fact we poured 4 for a porch last Thursday. Because our frost depth is 2' and I know a 4 square foot (2'x2') footing can support at least 8,000 lbs on our soils... If I want to attach the wood to the concrete at grade, I pour a 2'x2'x2' cube slightly above grade. Then a post standoff base to protect the post bottom and secure the post to the 'crete and life is good.

If there is no objection to burying the post these are footings stepping up a slope but buried to frost depth.
http://timbertoolbox.com/sketches/1624barn.jpg
Same concept, brace the posts by extending them up into the walls.
Footing thickness... time out, into the weeds;
Remember above I was harping about keeping your joist cantilever to no more then joist depth? If the joist is 1' deep it can overhang the support by 1 foot. Hmmm, what angle does that form... 45 degrees. In building, and what is building but engineering, you'll see that thinking pop up a lot.
So we have a 6x6 post sitting on a 2'x2' concrete footing. If the footing is 2" thick and we have a few thousand pounds on the post, I think its obvious the post will punch through that potato chip. What does rational design say? Well we have a 6x6 CENTERED on a 2' square. That means it is 9" from edge of post to edge of footing. How thick is the footing... 9". The post load travels at up to a 45 degree angle through the footing, pure compression out to the edges of the footing, you won't, Ma Nature says you cannot, punch thru. The mode of footing failure goes from punching through a tater chip to now the post must crush the concrete to dust. In other words, I can worry about the next weak link, we got that one.

So why don't I use one of those flimsy cheap toilet paper tubes concrete forms on my piers? I'm 2' tall and 2' square. There's a 45 again. What is it going to take to overturn my pier. What is it going to take to overturn that flimsy cheap ash toilet paper tube sized pier. Not that I'm biased, I had to knock a few over to learn that lesson.