Midspan support?

[PlicketyCat]

Quick recap/refresher on my project: I'm building this 16x24 post-framed cabin on permafrost, which means that I can't build a foundation wall, nor sink piles; so the floor frame will be supported on piers and pads that are on grade. This is the recommended foundation for building on permafrost. But it also means that we will be putting timber jacks at the top of the 10"d concrete piers, supporting our 6x12 beams, so that we can relevel the house seasonally.

Since we know we'll need to relevel, we're making the sill a full box to keep the structure from flexing too much, running 2 internal girders roughly 8' oc perpendicular to the ridge line, and our joists 2' oc parallel to the ridge line (yes, I know this isn't typical orientation). Almost all of the load of the building is resting on the beams and 8 posts/piers on the side walls  (with the truss roof there is no big ridge beam that needs end or midspan support). The 6x6 wall posts more than pass load, even when I went crazy and calced for double the snow load on the roof.

Here's the floor frame (the loft floor/header frame is identical except the joists are 2x6 instead of 2x12):





The 4 sill beams forming the box and the 2 internal girders are built-up 6x12 (rough), and the joists are 2x12.  Yes, I know building up beams is not pure timber framing, but I can't get solid sawn and the heavy machinery to lift them onto my remote lot :(

The 16' beams & girders should need to support 3200 lbs (40 live and 10 dead)... if I did the math right (half the span (8') x the spacing (8') x load (50 lbs)). According to the calculator, the 6x12 beams and girders (rough cut #2 doug fir or better, if I can afford it) can more than handle this load and I should not have to put any posts on the midspan... but I'm paranoid that I didn't calc my load correctly :D  If I should have calculated the full span, not just half of it, then 6400 lbs fails.





If I did my math right, then I only need to posts where the red Xs are, but if I screwed up then I should add ones where the green Xs are.  The concern I have with putting piers in the midspan is that this is one more point where the house can get out of level and bend, etc when we're adjusting the jacks (not to mention the cost of the extra concrete and jacks!).  Jacking the house would be easier and safer if I only had the long beams to worry about. Theorectically, if the math is right, I *could* get away with only 3 post/piers on the long wall, but 8' is a nice dimension to work with.

So, what do you all think? Did I screw up my math and need those posts at the midspan? Or did I do my math right and have a happily over-engineered floor?

[fuzzybear]

  well I never was good at math : 
  We have the same permafrost problem you have. You should have supports at the green X also.  I wouldn't bother with piers, just a good white channel base and pad with blocking.  With the frost heaves and spring run off it is a little extra support.  Also check with code.....unless your like me and have none to follow.
  If everything stays true after building you will only have to add or subtract shims as you need.   This is the way 90% of the buildings in Dawson are supported.
  You also have to remember that Alaska is an earth quake zone and even a little rumbler3.0 can break the permafrost, and there are lots of little rumbles each year throughout Alaska/Yukon, so extra support is like insurance.
  Good luck and remember to have fun and not stress over the whole thing.
FB

[PlicketyCat]

So you don't think a 6x12 can handle the 16' span unsupported, huh?  I see so many cabins up here the same size with just the two beams (not even girders!) and no center supports under the 16' joists. Floor's a little bouncy without the girder(s), but I can't see too much bending happening... that's what really got me wondering.

No codes out here either, just trying to follow the recommendations at the co-op extension and the Cold Climate Housing Research Center, as best I can.

Only reason we're going on piers is that we're about 36" above grade. We had to be up at least 18" with good floor insulation so as not to melt the permafrost beneath the house, and 36" is just easier to crawl around under   Plus, it's not unheard of for us to get 3+ feet of snow and I didn't feel like digging out my front door all winter - hahaha  We'll be spreading a couple inches of gravel base, then a square concrete pad (36x36) under the piers... which are just 10" quickrete tubes. Quakes are a bit of a problem, so we'll be shooting in earth anchors (nifty driver works with 10ga shotgun shells - hopefully I can acutally get those anchors up here!).

[Hilltop366]

It's not what you asked but I was thinking it would be a lot less work to frame both floors with 2"x 10"s x 16'  running on the 16' at 16"oc with 2 rows of blocking or X bracing (if you want less bounce 12"oc) with a rim joist running down the 24' that way you would have clear span across the entire floor,  set the floor on top of the built up 24' beams or flatten the top of a couple of good logs and then you just have to flatten the bottom of the logs where they set on the post. If you don't want the floor on top of the beams you could still use the built up beam on the 24' built in to the floor but I think the beam or log under it is stronger.

[PlicketyCat]

We need 12" for the insulation in the floor so that's why the beams and joists are 12". 

Hmmm - just checked AWC. 2x12x16' joists @ 16"oc makes the span fine. Just have to figure out if that saves me lumber and how I'd get that length out here.  If I spliced/scarfed/scabbed together 2-8s, then I'd probably want to add midspan supports under the joint and I'm trying to get rid of the midspan supports if possible. If changing my joists means I could eliminate all 4 of the short beams/girders and just have the two long beams, that would be much easier to frame and probably save us lumber as well. Well, if that didn't weaken the structure's ability to handle getting jacked up and down.

Glad I trusted my gut and didn't place my lumber order last Friday :D

[PlicketyCat]

Ran the joist plan through the budget (thank God for excel spreadsheets) -- all total, if we can also get rid of those 4 center posts, it'll save us $775 in lumber, concrete and miscellaneous hardware.  Certainly nothing to sniff at, and may actually cover the cost of having the lumber yard deliver our order to the edge of the highway (thus eliminating how to get the 16' pieces down the Elliott, and we'd only have to get them 1 mile down the trail where I don't care if they hang out of the truck bed funny).

Just might be doable :D  Thanks for suggesting it.

Now... just need to come to a conclusion on whether we can jettison those midspan piers!

[PlicketyCat]

And for those just joining us:  according to the AWC joist calculator, #2 doug fir joists that are 2x12 and spaced 16" oc can have an unsupported span of 15'4" with a combined floor load of 70 lbs (heavy), or 18'1" with a combined floor load of 50 lbs (avg).  Since the span between the sill beams is only 15' - looks like we can safely eliminate the midspan piers (the green X's) and still have a plenty sturdy and structurally sound floor.

The only thing I might do is put a pier/blocking right underneath the wood stove for a little extra insurance since that sucker is pretty heavy.

Am I being overly optimistic?

[Hilltop366]

A few rows of blocking or x bracing will really stiffen things up and is usually required also if you put the beams under the floor joist you can set them in from the edge a bit which will reduce your floor joist span to around 14' and keep most of the beam out  of the weather. I'm not sure how heavy your stove is or it's shape but it would have to be quite heavy to cause a trouble, one option is to double the floor joist where the legs set and/or add extra floor sheathing in that area to spread he load out. Some other things to watch out for are if there are any loads transferred from the second floor or roof to the first floor they have to be supported and any opening for a stair well if going through the floor should have the joist doubled on all sides.

I know it's not a appealing as a high vaulted ceiling but if you look at the extra cost involved and space taken up by the stairs in a small building like this it's got me wondering how much longer you could make your building for the same cost if it was only one floor, there are some other advantages of a single floor like being able to use smaller roof rafters and ceiling joist and easier to get lots of insulation in the ceiling also easier to get tight, lower ceiling will be much faster to warm up, easier to build and maintain. Just some thoughts as I was typing.

ps: I have a vaulted ceiling (19' high) and loft in my house and there are days I wish I didn't. It looks like free space when your drawing it out but it does cost more and is more work.

Cheers

[PlicketyCat]

We'll be adding stiffeners to all the joists to reduce bounce, they're just not in the drawing. We have two somewhet heavy loads upstairs - 100 gallon water tank and our battery bank - both of those are located right next to a wall post and have additional support from the downstairs pantry and bathroom walls. Our staircase is a fairly tight L-winder in back corner, we've doubled the joists on the floor knockout, and they also rest on the pantry and bathroom walls (the bathroom is partially tucked under the stairs) for additional support.

Our woodstove is about 650 lbs if you include the firebrick & tile pad it sits on. It's pretty much in the dead center of the cabin since we use it for heat and cooking, so I'll beef up the floor right there and add some blocking underneath just in case

Making the cabin longer/wider to get the same sq footage in a one story just isn't cost effective for us. Increasing the footprint means we'd have to get more gravel for the pad, more piers, more roofing material and even longer/fatter beams. The loft floor goes all the across (except the stair knockout), and we have a heat reclaimer on the flue upstairs and grates in the floor to help with heat issue. Our ceiling upstairs is only 7.5' to the rafter tie, so we aren't losing much heat to an "open" cathedral ceiling. Since the loft is only the bedroom, a desk area, and a place to put the water tank and battery bank, we didn't really need a whole lot of space.  Since our roof had to be steep for snow, seemed a waste not to use the space under the rafters as somewhat livable space... really, how much room do you need to sleep     (this is where having lived on a boat with bunks and limited storage/head room makes for good practice - hahaha)

[Hilltop366]

100 gallon water tank and our battery bank - both of those are located right next to a wall post and have additional support from the downstairs pantry and bathroom walls.

Water & lead that's some heavy stuff, I'm thinking if there are load bearing walls or post holding that kind of weight you might want to continue the load through the first floor to the ground?

Since our roof had to be steep for snow, seemed a waste not to use the space under the rafters as somewhat livable space... really, how much room do you need to sleep     (this is where having lived on a boat with bunks and limited storage/head room makes for good practice - hahaha)

Living on a boat probably IS good pratice, I didn't want to de-rail your plans and I sure you weighed all the options anyway from having drawn and redrawn my own plans a few years ago before building,       and redrawn and redrawn and.......

[PlicketyCat]

95% of the water and lead weight is carried by the exterior wall posts & beams right to ground, since it's right up against the wall and vertical rather than horizontal across the floor. The other 5% wouldn't really add that much to the two internal walls, I don't think

Draw, redraw, redraw, redraw, redraw... yes, I know that drill :D   I think this is the 17th iteration of the "final" design, and I scrapped about 5 other designs along the way as well. Things really start to get complicated when you factor in the constraints of building in the bush, on a lot without any inkling of easy access, on permafrost, in a wind/snow/seismic zone, with only two moderately competent workers, virtually no heavy equipment, and a budget that would fit on a gnat's back --- hahaha