Author Topic: Foundation plan help  (Read 523 times)

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Offline ben bb

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Foundation plan help
« on: June 04, 2020, 05:30:08 AM »
Hello Forum,
I've enjoyed lurking around this forum.  There's a lot of great information and help here.  I'm looking for some direction on my project.  My plan is to build a timber frame workshop/occasional residence(in loft) structure.  It will be roughly 20x30 or 24x24 ft.  The build site has a decent slope to it, between 3 and 4 ft drop depending on what dimensions I go with.  I want to build a wood pier foundation out of PT pine, then build the structure on top of that from oak that I'm getting milled on my property.  If I use 6x6 posts for the piers, can I connect 2x6 floor joists to that?  I've seen that done, but I feel like there should be runners going the opposite way to directly support each joist.  So my plan is to notch 2x10's into the posts, then run 2x6 joists across those.  Does that sound good?  If so, what is the correct spacing for the posts?  Would 3 rows of 2x10's be sufficient for a width of 24 ft?  That would make the 2x6's span just under 7ft (placing posts 2ft in from outside edge), is that too much?
Would 4x4 or 4x6 posts be a better option?  Do I need to do anything extra to account for the extra height of posts at one end?
I know I put a lot of questions in there.  Thanks in advance for help with any of them.
Ben B.
Memphis, TN     

Offline ben bb

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Re: Foundation plan help
« Reply #1 on: June 05, 2020, 04:37:59 AM »
Is there a similar pier foundation build thread on here?  I've been looking through a lot of builds discussed and haven't seen one.

Offline akwoodchuck

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Re: Foundation plan help
« Reply #2 on: June 05, 2020, 12:06:34 PM »
There's a reason for that....posts+slope usually = a downhill lean over time....often a severe one....




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Offline Don_P

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Re: Foundation plan help
« Reply #3 on: June 05, 2020, 05:28:07 PM »
You're thinking backwards, or rather you need to approach the design from the other end. A timberframe collects the loads on the building and they are carried by few, large, widely spaced members. A conventional stick framed house has many smaller load bearing members that uniformly distribute the load.

So there are a few things to think about. Your cantilevered floor built of 2x6's is going to have a post supporting tons landing on basically one of those overhanging joists. That'll break

If the piers do not align with the posts then the post load is carried from the joist onto the single 2x10 floor support girder. I'll about guarantee that is insufficient to support the floor much less what is above it. That girder is also supporting the loft floor, the roof, and all contents and snow. If the joist didn't break first this is next in line

If the wind blows or the mud flows, as akwoodchuck pointed out, the foundation posts stand a good chance of overturning.

It needs some rethinking.

Lose the cantilever, put the timberframe on a continuous perimeter masonry foundation. If its block be sure to reinforce and pour the cores under the TF posts.

Trace your loads looking down thru the building from top to bottom accumulating, quantifying and then resisting the loads along the way.

Offline ben bb

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Re: Foundation plan help
« Reply #4 on: June 06, 2020, 01:14:43 PM »
Thanks Don_P.  That's what I needed to hear.

Offline Don_P

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Re: Foundation plan help
« Reply #5 on: June 06, 2020, 05:30:52 PM »
Ben,
I have a sawmill and have timberframed in oak. If you would like more help post some sketches of what you are thinking about.

One thing that will save you some grief down the road is to get a sack of Solubor from a real farm supply. Mix 1lb of that per gallon of water and roll or spray that on your timbers very liberally right off the sawmill when the wood is green. It will keep powderpost beetles from getting a foothold in your tasty new timbers.

Offline ben bb

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Re: Foundation plan help
« Reply #6 on: June 07, 2020, 06:26:26 AM »
Don_P.  Thanks for the tip with the Solubor.  I'll definitely do that.

This is close to what I want: https://timberframehq.com/28x20-timber-frame-saltbox/
I think I'll go ahead and buy those plans before I start cutting.  I need to make sure there's enough beams in the trees I'm going to take down, or pick some more trees.

I've been doing woodworking for years, but I've only done a couple smaller sheds, which of course aren't nearly as complicated.  I want to make sure I do this right.  I wanted to do wood pier foundation, mostly because that's what I'm used to working with and am not experienced with concrete.
The build doesn't intimidate me near as much as the foundation, though I'm sure I'll learn plenty the hard way.

So with that design, would you recommend a full perimeter cinder block foundation?
Then would floor joist sit on that, or on girders first?  would vertical posts go through joists and sit directly on treated sill plate, with blocks underneath filled with concrete? 

For support of the center post, could I just plant a treated 6x6 deep in concrete underneath it, or should there be a cinder block tower there for support?
   

Offline Don_P

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Re: Foundation plan help
« Reply #7 on: June 08, 2020, 05:43:37 PM »
I would put a full perimeter foundation under just about any building, it is pointless to build a series of braced walls above and then put it on top of something that is unbraced. Foundation walls provide that bracing where piers do not. Inside of that set of bracing walls, it is fine to use piers. The wind's horizontal force is resisted by the exterior walls and then the walls of the foundation, that prevents overturning. Inside of the foundation if you need post or beam support piers are fine, that lateral load is not getting to them, it was resisted around the perimeter.

I'm not that particular on species of foundation walls. Block is probably the easiest and if you don't like the look of the one you just laid, pick it up, remove and replace the mud and try again. It is nothing to be intimidated by. I like stone, it is not that difficult but is slow relative to block, for me rocks are free, to the point of being a nuisance  :D. Poured reinforced concrete is probably the strongest of the 3 but requires forming to resist serious loads during and shortly after the pour, probably best contracted out.

I've poked around a little on that website. It seems kind of light/ optimistic in terms of engineering involved in design. I'd run any of those designs through an engineer myself, at the least double check design and assumptions against your local loads... what snow and wind loads did they design for as compared to yours, or is it simply a starting point for final design to begin working on.

I would frame the floor with pockets for the posts to drop through onto the treated sill, over solid foundation.
 I'd probably run a girder under the floor joists along that center post line, there would be at least 3 piers supporting the girder and a pilaster or beam pocket at each end wall.


Offline ben bb

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Re: Foundation plan help
« Reply #8 on: June 09, 2020, 06:31:10 AM »
Makes perfect sense.  Thank you.  I would love to have some rocks to use.  I've got a 6 acres and there's not a native rock on the place, so blocks it will be.  I'd like to come back and do a rock facade over the bricks outside.  That would take some serious scavenging in my part of the world to get enough though.

 

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