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Author Topic: Choosing foundation for small timber frame picnic shelter  (Read 1357 times)

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Offline DOCatRU

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Choosing foundation for small timber frame picnic shelter
« on: March 26, 2019, 02:39:57 PM »
I am looking to build a small timber frame picnic shelter in the backyard of my 1773/1810 farmhouse in Vermont.  It'll be 12 x 14 using 6x6 hemlock bents using a plan from TimberFrame HQ.

I can dig 5 foot holes for concrete piers, but I started to wonder if something this small could just be built on grade.  After all, the two timber frame barns on the property (1770's and 1890's) are just resting on rubble stone foundations and have stood OK for a long time.

All advice is appreciated

 

 

Offline Don P

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #1 on: March 27, 2019, 07:16:34 AM »
If it heaves or settles there aren't doors and windows to bind, it is a less important issue. One thought with that plan is uplift, make sure you have it tied down well. RU... Radford?
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Offline DOCatRU

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #2 on: March 27, 2019, 01:08:58 PM »
Yes, retired music professor from Radford University (42 years of teaching...30 of them at Radford).  Living the dream with a tractor, brush hog, wood shop, lathe, organic garden, a stocked trout stream in the backyard, 3 enthusiastic dogs and a loving wife.

Thanks for the feedback.


Offline doc henderson

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #3 on: March 28, 2019, 05:29:30 PM »
what did you want for the floor.  could do a slab and fasten to it.  That far north might be good to go beneath frost line.  here it is only 24 to 30".  with a roof and open sides might blow over in a storm.  great property.
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Offline Don P

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #4 on: March 28, 2019, 06:55:50 PM »
That far north... umm, we don't actually use that word in polite company around here doc :D. Typically we go 24" on footings, where DOCatRU is you can probably get away with 18, which is a valid point. Never go less than 12 even if no frost just to be sure you're on undisturbed mineral soil, not much more to frost depth and it allows more weight down below.
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Offline doc henderson

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #5 on: March 28, 2019, 07:31:25 PM »
yes we do mass footings for walls at 8 - 12 inches deep but 2 feet wide. with wall on top.  our frost depth is about 20 inches so they want things 24 to 30 inches deep.   i would think if you dug a little footing around the perimeter with some rebar, it could all be poured together, slab and footing.  that is if you are planning a slab.  if gravel/dirt then get a 12 " post hole digger an make a piller/footing for each post.
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Offline Don P

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #6 on: March 28, 2019, 08:38:48 PM »
Maybe a good opportunity to delve into some foundation stuff.
If the soil is good but we don't want to call in a geotech we assume it has a bearing capacity of 2000 pounds per square foot, psf.

A 12" diameter footing =.79 square feet, so 2000x.79=1580 lbs allowable on that footing.

If we assume a 1' overhang all around, the roof is 14x16, 224 square feet.

Snow load is 25 psf, dead load is probably 10-15 psf, go conservative and call the structure 15psf so a total of 40 psf x 240= 9600 lbs.

Divide by 4 posts =2400 lbs per post. The allowable was 1580.

Go carefully calling out footing sizes, we don't have enough info.

I tend towards the other direction for spot footings, once you've dug a hole it isn't much harder to make it 2' square, now I have a snowshoe down, 8,000 lbs capacity. it is not likely to tip from eccentric loading, you know, when you don't put the jack dead in the center of that little board in the mud and start up and the jack decides to tip over, usually after you got the tire off and so it sits down on the drum and nobody's happy. If I attach the building to that snowshoe and tamp dirt back up to grade, well do the math for up, wind. We've probably gotten up to 20 psf up in a few of our really big winds, now to be conservative I'd use 10psf for the dead weight so 10 psf up... see where I'm going? There is a long ton of uplift potential. That bigger footing helps in that direction as well.

Footing thickness; load is assumed to travel through the thickness at a 45 angle. If I put a 6x6 in the middle of an 8" thick 2'x2' footing there will be 9" to the edge, I run a slight risk of punch thru, rebar in a # configuration 3" up from the bottom on the tension side would certainly be a good idea but I also usually pour about a foot thick. With our 2' footings and if I'm doing a pier I'll just pour a 2x2x2 cube to about grade and start laying stone up. The general rule is make the footing thickness at least equal to the projection. Thicker is good.

Open shelters still take a lot of lateral from the wind but they lack bracing walls. We didn't pick up anything there yet, make sure there is good bracing in both directions above. A lot of people assume soil bracing of buried posts and there are ways to check that. A couple of thoughts based on observations over time. Most folks don't tamp the posts, most have never tamped a fencepost. I can walk up to most fenceposts and rack them at 4' tall with the less than 80 lbs of horizontal a human can produce. I don't consider that a good idea, pick up the lateral bracing with the structure. Welp, went longwinded again ::) :D
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Offline Dana Stanley

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #7 on: March 28, 2019, 09:44:07 PM »
That far north... umm, we don't actually use that word in polite company around here doc :D. Typically we go 24" on footings, where DOCatRU is you can probably get away with 18, which is a valid point. Never go less than 12 even if no frost just to be sure you're on undisturbed mineral soil, not much more to frost depth and it allows more weight down below.
Just curious, why would you say 18" for Vermont? Here in southern Mass the frost line is 48" I would think Vermont would be closer to 5'? I assumed that's why he said 5' holes. Here if your local inspector required a permit, then because it has a roof, they would likely want big foot footings. Equivalent to a 2'x2'x2' footing with a 10 or 12" tube, the bottom of the footing at 4'
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Offline Don P

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #8 on: March 28, 2019, 09:52:41 PM »
Whoa, my bad, I had him right down the road still near Radford University in VA, he is in the far north. My snow load numbers are gonna be way light as well. What I wrote was not meant as design specifics, just how I think about the problem. For most places this would be an exempt accessory structure, but every jurisdiction interprets that their own way.
That's funny, looking at the pic I said to myself, he's over near Elliston somewhere :D
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Re: Choosing foundation for small timber frame picnic shelter
« Reply #9 on: March 29, 2019, 08:06:58 AM »
You might consider a completely floating situation.  We once built a large deck in a cedar swamp.  We poured dry concrete mix into (approx) 4-ft.-deep footings holes that were full of water.  These all solidified just fine and the deck went up.  But after 2 winters, that thing was frost-heaving like crazy.  Long story short, we had to go back and cut all the legs shorter and put them up on those concrete footing blocks.  Zero problems since.

Some here have cautioned about wind picking whole thing up.  I suppose that could happen.  Is the location sheltered or more on the exposed side?  If more sheltered, I wouldn't worry about that.

tom

Offline doc henderson

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #10 on: March 29, 2019, 08:44:21 AM »
I think a floating foundation could work, but also think it needs to be well secured to ground.  I am from Ks and don't think it will go airborn, but may get ruined with open sides and either lay over, or could roll into other buildings. be safe.
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Offline Don P

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #11 on: March 29, 2019, 08:54:58 AM »
When thinking through a problem don't dismiss without consideration load from any direction, down, up, lateral, then make sure you provide adequate resistance. Up can be from self weight but not always. A friend lost a structure in those 50+ mph winds a week or so ago.

The poured in place footings in wet soil heaved because they were in saturated soil and they were rough, poured in place rather than in a smooth form. The ice grabbed the sides and lifted. Frost heave requires moisture, in well drained soils it really isn't an issue, we build for worst case there. The deck blocks are actually prohibited in most codes, doesn't stop sales though :D.
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Offline curved-wood

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #12 on: March 29, 2019, 11:02:00 AM »
If I have to dig for foundation with all the mess, mind as well do it deep, here it is 4 feet. Why stop at 2 feet if the standard around your place is 4 feet with all the trouble of digging, putting a form and pouring ? Also consider if the snow will be remove or compressed like beside a laneway...might have to go deeper. Here it is heavy clay so I have no choice. In sandy soil well drain , floating foundation might be an option providing good anchorage. But I would go either floating or digging deep but not in between. 

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #13 on: March 29, 2019, 01:04:36 PM »
Our floating deck sits on pure muck soil. And yes, our failure to A) use sono tubes or B) place smooth heavy-mil plastic on our footers, was a contributing factor in our deck heaving so badly. And the saturated soil, which I had already mentioned.  We learned.

There is simply no problem with using those floaters though.  I wouldn't build an important structure on those, of course, but for decks, etc, or for your gazebo, it will work.  Most buildings never get blown away, down, sideways, or over in their entire lifetimes.

tom

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #14 on: March 30, 2019, 06:11:42 AM »
It might be better rather than comparing that line of thought to "most buildings" to thinking about it in terms of unanchored floating buildings. Older trailers, carports, chicken coops, etc do indeed seem to roll across the landscape fairly frequently.

This is the wording of the exception in my state code, it is pretty much the same as most although the area specified by the exemption varies quite a bit, the 256sf is larger than many so do check your code, notice anchorage is required, this can be the screw in anchors which from memory are good for about 1800 lbs each. Also notice the limitation in exception 5;

Quote
SECTIONR403
FOOTINGS
R403.1General.
All exterior walls shall be supported on continuous solid or fully grouted masonry or concrete 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.
Exceptions:
  • 1.One-story detached accessory structures used as tool and storage sheds, playhouses and similar uses, not exceeding 256 square feet (23.7824 m2) of building area, provided all of the following conditions are met:
    • 1.1.The building eave height is 10 feet or less.
    • 1.2.The maximum height from the finished floor level to grade does not exceed 18 inches.
    • 1.3.The supporting structural elements in direct contact with the ground shall be placed level on firm soil and when such elements are wood they shall be approved pressure preservative treated suitable for ground contact use.
    • 1.4.The structure is anchored to withstand wind loads as required by this code.
    • 1.5.The structure shall be of light-frame construction whose vertical and horizontal structural elements are primarily formed by a system of repetitive wood or light gauge steel framing members, with walls and roof of light weight material, not slate, tile, brick or masonry.
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Offline DOCatRU

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #15 on: March 30, 2019, 01:56:57 PM »
Thanks for all of the insightful replies folks!  I knew that people with your experience would be the best to talk with about this.

I'm going to go with precast concrete piers from E-Z crete dug down 5 feet.  They are rated for beams up to 6x6.  The soil is loam (old dairy farm). 

We do get a fair amount of wind (particularly in the winter) so being anchored to the concrete piers will keep the structure from going "Mary Poppins" on me in a big storm.



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Re: Choosing foundation for small timber frame picnic shelter
« Reply #16 on: March 30, 2019, 02:13:00 PM »
I think more is better and meeting code as well may help if you someday want to sell and get full value.  If a home inspector decides there is a problem you will be asked to fix or deduct from the value.  Or the buyers may not get financing.  not to mention being safe.
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Re: Choosing foundation for small timber frame picnic shelter
« Reply #17 on: March 31, 2019, 03:42:29 PM »
Friday, I attended a session at the JLC Live trade show held by Simpson Strong tie about deck post anchor systems, as we're all interested in these connections.
they had two very nice samples. One is wet set, that is placed in wet concrete as it is poured:



 
This bracket show a 1" stand off above the concrete. And he suggested that the bracket be hung from the forms and not set in by hand. Because sometimes it sinks in too deep and you don't get the 1" stand off.
Another one that they showed was this one:


 
This one also has a 1" stand off above the concrete and is secured to the concrete with either a wet set "J" bolt or in an epoxied drill hole. That is a threaded rod set into the drill hole and the bracket is then held down with a nut under the stand off.
I found these two brackets very interesting.

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Re: Choosing foundation for small timber frame picnic shelter
« Reply #18 on: March 31, 2019, 04:29:51 PM »
I found these two brackets very interesting.

I used both on my cabin foundation.  The wet set was used for the 8x10 posts:






And the bolt down type for the ends of my main floor beam:


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Re: Choosing foundation for small timber frame picnic shelter
« Reply #19 on: March 31, 2019, 07:39:26 PM »
One thing an inspector told me... uhh busted me for,  and it does make sense. They want any anchors that are set in concrete to be hung in place prior to the pour. When you "wet stab" a J bolt or any other anchor into place after the pour what happens is you push the aggregate out of the way and then you puddle it a bit and all that moves back over the hook is cream. You don't really get the withdrawal strength that you think you have.  So I just make holders from scrap and screw them to the forms prior to placing the mud. That also assures that the connector is in the right place and depth, it really saves time and aggravation in the end.
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