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hobo99
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# Posted: 27 Oct 2015 16:37
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Hello all,
Long time lurker here and now a new member with a few questions about cabin construction.
I'm working on a plan for a small 16x28' cabin/ham radio shack with a loft on some land I've owned for many years now and now have the time to work on this project. I hope to have it completed in the next 5 years.
The place will be just for me and my dog's and I don't plan on it lasting a 100 years and I doubt seriously if any of my inheritors will keep the land once I'm long gone. So construction will be sound but towards the cheap side of building.
The 25 acres is located in Zone 3 and the maps I've researched show the frost depth to be 5" to 10".
After reading way to many threads here about foundations I still have question begging me.
The local concrete company can do all the work and pour a 16x28' slab with 18" perimeter footing and 4" center deck with 3//8" re-bar for $3K.
Does the 18" depth sound right?
I'n also looking into just using 6x6 treated poles or telephone poles if their not too expensive. There's a place down the road that has a yard full of them.
My question with going with piers is the need to insulate the floor? I saw a video on youtube with a cabin using only a buble foil radiate barrier and the guy says it's all you need and he was building in up state NY. I plan to heat with a Jotul 602 and propane heater if needed.
Has anyone here in the lower zone 3 area used only a radiate barrier on top of their floor joist under the sub-floor and been happy with it through the winter?
Appreciate the guidance and glad to be a member of such a great resource. Sorry to be so windy for just two questions
Tom
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Don_P
Member
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# Posted: 27 Oct 2015 23:01
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Google radiant barriers vs the federal trade commission for the answer to one question.
The slab foundation sounds good and at a good price. A pier foundation is an excellent choice for a mailbox.
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bldginsp
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# Posted: 27 Oct 2015 23:20
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I think you will definitely want more insulation in a wood subfloor than a radiant barrier. A poorly insulated floor leaves your feet cold which sucks.
Pier foundations simply don't work for any kind of serious engineering calculations, so the best solution to that problem is to not do the engineering calculations. If you don't want to make a permanent building, piers will work, just get them below frost. But $3,000 for a slab foundation isn't bad. Your pier foundation and wood subfloor will cost you most of that and a lot more labor.
Don- piers are also good for chicken barns
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Don_P
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# Posted: 28 Oct 2015 00:06 - Edited by: Don_P
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Engineering calculations,
Stand up 4 pencils on a tabletop as piers, lay a book on top as the structure. Walk away and come back after a bit.
Model a slab, lay a piece of plywood on the tabletop as the slab, lay a book on the plywood...
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bldginsp
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# Posted: 28 Oct 2015 08:31
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Ok Don, here's what I did- I drilled four 3/8" holes into my butcher block table top, and put four pencils in them. Then, I drilled four holes in my copy of Roger's 'Baseball Statistics, Then and Now', and put that over the pencils. Then, I took a cutting board and put it on the other side of the table and put a copy of Martha Stewart's 'How to Invest in Wall Street' on it. Then, I slammed the table to the left about two feet. Baseball Statistics stood up, but Martha fell onto the floor! This conclusively proves the utility of baseball statistics, and the arrogance of Martha Stewart, IMHO.
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Don_P
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# Posted: 28 Oct 2015 09:06
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LOL The joys of modeling.
You've modeled a post frame. What you have been advocating up until now is 4 to 6 pinned connections, free to rotate, stacked over a hole full of mud. Now, how do you suppose you would actually create a cantilevered connection?
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bldginsp
Member
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# Posted: 28 Oct 2015 12:21
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Duct tape and baling wire
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Don_P
Member
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# Posted: 28 Oct 2015 14:12 - Edited by: Don_P
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Try this,
remember...
I drilled four holes in my copy of Roger's 'Baseball Statistics, Then and Now', and put that over the pencils
You cantilevered the posts out of the "structure". Piece of cake, run the pencil up into the structure enough to resist the racking stress on the post. In a post frame the post runs up through a sheathed wall all the way up to the top of the wall. The attached sketch shows one way.
You also cantilevered the posts out of the ground and made a very common assumption, that the ground is a rigid plate and the post you put in the hole is cantilevered from that hole. To get that type of cantilevering in soil, pilings are driven 25' deep or more. What you are actually creating is called a shallow pier. The engineering graphic for it showing the relative position of the point of rotation is below... much closer to the footing than most people appreciate, posts don't tip over at the ground level, the lever that you are trying to keep vertical is much longer than that.
A pier and beam really is a duct tape and baling wire solution. If you want to build on posts, a post frame building is a better way.
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bldginsp
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# Posted: 28 Oct 2015 15:11
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I'm glad we have your knowledge and experience available on the forum here Don, and I think newcomers contemplating their cabin design should understand the realities of building on simple piers with posts- it doesn't pass any muster for withstanding lateral loads like earthquake or wind. I don't have a problem with using piers for very small buildings because the loads are so small, but then the question is, at what size would I judge that it becomes an issue and piers should not be considered?
Hobo99 suggests building a 16 x28 cabin. I think this is too large for a building on piers anywhere where lateral loads are a potential issue. I suggest, Hobo, you do the slab, but that's up to you.
Now back to baling wire and duct tape- recent improvements in the tensile strength of the supporting fibers of common duct-sealing adhesive membrane have led to its implementation as part of several lateral load bearing structural assemblies currently being assessed for code compliance by several listing agencies, which though new on the American structural engineering scene, have gained rapid credibility with certain authorities due, perhaps, to allegations of substantial monetary compensation offered the political officials in the jurisdictions concerned; however, the agencies in question have responded to said allegations by stating that they are doing nothing more than providing the jurisdictions with necessary resources to do the analysis required in the situation, though the question asked is why such analysis has to be conducted in the gambling halls of the South of France with numerous ladies of questionable occupation in attendance.
Which proves conclusively that duct tape is a valid construction material.
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Steve_S
Member
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# Posted: 28 Oct 2015 18:51
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@Bldginsp: Love the comments about Martha Stewart... Ouch my stomach hurts from the laughs... Thanks I needed that.
PS Red Green is one of my Heroes ! King of Duct Tape. Geez, I gotta find those shows and download them...
@Don, your one heck of a great resource to have here... I'm always picking up valuable nuggets from you,
@ Hobo99 Good to see ya posting and starting the plans. The deal you've been offered for 3K on a concrete slab sounds pretty good, awfully good if it includes the site prep (removing the organics and putting in a packed solid base) and "Excellent" if that includes some insulation, which for you would be fairly minimal compared to my climate zone.
Having just had a somewhat unpleasant experience with concrete guy, I suggest asking around casually about their work... I should'a did more of that ! If it is an option to use Fly Ash in the concrete, it makes it lighter and somewhat stronger as well... Alternately adding fibre strands (fibreglass reinforcement) will help in crack reductions / prevention. Something else I should'a done... Danged Lessons Learned.
16x28 is a nice size, you haven't mentioned if your planning to use it year round or not... if you are, you may consider laying in 1/2" pex to a simple manifold. You likely wouldn't spend more than 200.00 on the materials but it's there if/when you decide that you want warmer floors and nicely heated space. Heck of a lot easier & better than any retrofit later.
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MtnDon
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# Posted: 28 Oct 2015 19:13 - Edited by: MtnDon
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Does the concrete quote include foam sheet insulation under the slab? Probably not, but would be nice, especially if you do radiant heating.
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Don_P
Member
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# Posted: 28 Oct 2015 22:08 - Edited by: Don_P
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That's funny, I was kicking around in the Red Green pile all day today converting old Lowes lumber racking and some sheets of salvaged galvanized sheet metal into a pickup truck reefer unit. Mr T could build a tank out of an old GMC in a 30 minute episode, awesome.
Thanks for mentioning the slab upgrades. To bring it back around, trying to insulate a radiant slab is where I learned about the radiant bubble wrap bogus claims.
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Steve_S
Member
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# Posted: 29 Oct 2015 08:19
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I got suckered into using that Radiant Bubble Wrap when renovation my old (built in 1886) house that I live in 10 years ago...
I also used a "Radiant Vapour Barrier" which was basically 6mil Poly with the shiny foil like face... It was a replacement for the standard 6mil poly vapour barrier. Seemed like a good idea at the time, as it performed the function of Vapour Barrier and reflector back into the living space... I can't say if it made a difference or not but it did work as a vapour barrier, so no harm there, just more $. (double the cost of reg. 6mil.) I haven't seen it for sale anywhere lately other than at the Habitat Re-Store, so I'm assuming it was a flop or not significant enough.
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hobo99
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# Posted: 29 Oct 2015 14:54
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Wow, thanks everyone for the responses, they are appreciated.
After reading threads here for over a year on foundations, I know there's not much love for pier type's used in larger structures. I can understand some of the logic behind this for certain areas.
My experience is a little different. Growing up, all of the houses we lived in were older 50's style farm houses built on piers and most used just red bricks and a few nothing other than flat Oklahoma rocks.
My adult years (last 35) I've lived in Dallas, TX and all were on grade slab style foundations and every single one of them had problems. In fact there are about as many foundation companies as there are roofing contractors here. Under $500K it's slab and over it's piers.
At one point i thought about just sitting it on blocks and using tie downs.
The land is next to my parents land where I lived the last years at home and sits in a valley of sorts. We have never hard high winds like I get in Dallas where it blows it seems 10 months out of the year.The area is surrounded with 40' oak and southern pine trees so lots of wind breaks helps a lot.
I know just sticking sticks in the ground doesn't give much
lateral support but if you keep a minimum above ground and below the frost line in my case 10" and they are braced both vertically and X-crossed between peers they can be a workable solution.
All of the houses I've lived in during my youth are still standing even the ones built on brick and rock for piers. Will they pass code, ha, there is NO code in rural OK. It's the land of the free and your god given right to kill yourself by building a crappy house if you want to.
Don_P, to your comment on radiant barriers, but there's this guy on youtube that says it works great and you know youtube is just like the internet so it must be true!
Your cantilevered solution might work for a smaller structure but I'm trying to keep the internal walls out of the picture with the exception of the bathroom so i could only have the outer walls tied structurally in this fashion, but it is a great idea for me to ponder.
I have for my pier plan 25 piers with 5 girders running the 28' length and then my 2x6 floor joust
on top spanning the girders. The slab plan is a lot easier but requires me doing more dozer work to prep the slab area before they can start.
bldginsp, If I have to do any kind of "serious engineering calculations" I might just as well hire someone to build it and that's no fun. I dream of laying in bed at night praying that the roof doesn't fall on my head during the night till I fall a sleep And NO real man would ever be with out "baling wire and duct tape", it's part of the manly-man code!
Steve_S, not much site work in the price. Low ball was $2700.00 and I figure they will hit me with a least one load of fill soil and have to packed this regardless so I would be happy with anything up to $3.5K. No insulation and thanks much for the comments on "Fly Ash " and "fibreglass reinforcement". I will live most of the time there and have no plans for a heated floor but it's something else to consider.
I asked for a quote on a basic slab with a perimeter footing and they came back with 18" for the perimeter footing and 4" slab deck and limited site prep.
Tom
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hobo99
Member
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# Posted: 29 Oct 2015 15:25
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My floor plan looks something like the attachment ( I hope it's showing).
With no internal walls I'll need both a ridge beam and beam to carry the loft.
Tom
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Don_P
Member
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# Posted: 29 Oct 2015 22:21 - Edited by: Don_P
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If you are in the same type of shrink/swell clays as Dallas, get an engineer, the slab will probably be post tensioned or the piers will include a specialized grade beam. In some soils it is a good idea to let his expertise or insurance work for you.
Your estimates don't seem to indicate that.
If the frost depth is 10" why is the slab going down 18"? Code minimum is 12" if that matters. Or he may be creating a stiffer (grade beam) edge. There's a little less digging and some mud you don't need to pour if it can be 12".
Another alternative is a strip footing and a few rows of block for a crawlspace.
The post frame (think about a pole barn) doesn't need internal posts, the goal is basically to create a braced box over a braced foundation. The outer sheathed walls perform this function. If the posts extending from the bottoms of the walls are rigidly connected to the frame AND if they are sized to resist all loads at their particular length (you can account for soil interaction now) then they would be considered "braced". For a 20" cantilevered post I think you can forgo a formal engineering calc on 6x6's.
Builders use piers inside the braced structure to support girders very frequently, the lateral is taken out by the braced perimeter box...if the floor is going sideways we have bigger problems than piers.
If you noticed somewhere on the post engineering graphic I posted earlier it notes that the post is unconstrained. It was showing how a post interacts with just soil, the typical pier and beam condition. Another way is to build it as a post frame, nail a 2x12 around the outside of the posts, backfill gravel to within 4" of the top of the 2x12 and pour a slab inside. The graphic then changes to one labeled "constrained post". The pencil (you remember the pencil), the pencil is in a neatly drilled hole in the tabletop. The fancy equations change to confirm it, it has to be true. In practice many post frame builders have gone to slab first with post brackets cast in and then posts above grade if there is going to be a slab. ...at which point forget the posts and stick frame it on the slab.
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hobo99
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# Posted: 30 Oct 2015 14:32
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Thanks Don_P, the soil is completely different, darker non cray and a little sand in spots across the properity. For the last 100 years it was a oak and pine forest.
The estimate is just the quote they gave me over the phone. The frost depth I googled and pulled from a map I found.
The "constrained post" pole barn you mentioned is the way we've built tractor barns in the past so I understand this method.
The center support I'm getting from having the width of 16' and seeing in the 2009 IBC tables around page 508 states a #2 pine 2x6 with a dead load to span 11'10". So I put in a center support girder to support this across the 16' width of the structure. I could have gone with 2x10's but I was trying to keep the height down. I'll redo my cost calculation versus the slab and see where I'm at.
Tom
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hobo99
Member
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# Posted: 8 Nov 2015 12:51
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Hello all,
I have a question concerning slab requirements for two loads that it will need to support at each end and possibly the middle for this 16'x24' structure.
Not sure if a 16'x24' with a full loft can be called a cabin or just a small house but here's my question.
This cabin/house/shack will have both a ridge beam and a beam carrying the weight of the loft and both loads will transfer the weight down via 4x4 or 6x6's to the slab at each end. And more than likely at the midway (14') point (unless I really start trusting the beam calculations I'm seeing).
Will the slab require any additional considerations (beefed up/deeper, etc.) to support the two loads and possibly the a center column (vertical 6x6) load?
Thanks for the guidance.
Tom
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toofewweekends
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# Posted: 8 Nov 2015 13:41
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Not an issue in SE Oklahoma, but one advantage to being a few feet off the ground via posts in the ground is that in winter when you get 4-6 feet of snow you don't have to tunnel to your doors and windows. The covered space is also a dry space for tools, handy for small amounts of firewood, project supplies, etc. My 16x20 is on 12 6x6 treated posts, and is standing straight.
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Don_P
Member
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# Posted: 9 Nov 2015 08:02 - Edited by: Don_P
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First, determine the load on each of those posts in pounds. Next determine the soil bearing capacity in allowable pounds per square foot.
Divide the load in pounds by the bearing capacity of the soil in pounds per square foot to yield the number of square feet of footing required at those locations. If it is a single monolithic pour these are thickened areas of the slab pour, you just neatly scoop out to virgin soil the size and depth needed.
Mull it over until you understand the logic driving the calculations, all of this is simply quantifying common sense... or it is a way of backchecking to see if one is using common sense. Oftentimes I perceive a problem, ask a few questions and it confirms what experience driven common sense has observed. We don't become enlightened when we buy the hammer, it takes awhile swinging it. When about done swinging it I looked around and try to save someone else a few blows by describing what I was thinking about as those pounds of nails passed by.
A beam of that size for a ridge is sized for free by the supplier. I like to check my work to make sure I'm using common sense. If you'd like I would be happy to take a crack at the ridge before you hand the problem to their engineer.
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hobo99
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# Posted: 9 Nov 2015 12:37
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Thanks Don_P, your response on the slab requirements part of my question was very helpful. I understand fully now how to approach the problem and solve. What information would you need from me to take a crack at the roof beam? My supported roof framing would be 16' and the clear opening would be 27.1' using a 50 PSF snow load with a 12/12 pitch metal roof.
On the Ridge Beam I'm using the "Southern Pine Size Selection Tables" from document 15-20L. Using Table 17, 50 PSF (better safe than sorry even for SE OK), it shows using No.2 Pine, I would need a 5-1/2x16 beam for a clear span of 24' which is a little short of my 28' structure. 24' is as long of a beam as this document goes. So I have some idea just seams I'm worrying about roof sagging 5 years down the road...
The loft beam is what's bugging me the most because of the live load the loft will carry. I planned to store a lot of my Vacuum Tube Radios and Books which are very heavy, but most of the weight will be along the walls and not in the center of the room. Just something else to waste brain cycles on...
Thank you for the help.
Tom
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Don_P
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# Posted: 9 Nov 2015 18:12 - Edited by: Don_P
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I dropped the total load to 30 psf, that should be plenty. The building is 16' wide... half of each rafter is bearing on its' eave wall, half of each rafter is supported by the ridge. The tributary area is 8' x 27.1'. The load on the beam is trib area X design load, I'm at 6500 lbs on a 325" span.
A double 18" LVL or a triple 16" will do it.
Now take your design to the local building supply and ask them to size it.. I figure it for prelim design and let them check it when I order.
I'm pretty sure that will raise your eyebrows here is what happened between 24 and 27'. The load increased by 720 lbs and the span increased by 3'. The maximum bending moment at the midspan of the beam went from17280 ft-lbs to 22032 ft-lbs a difference of about 4700 ft-lbs.
I've seen people use the logic that if a single stick can support some distance a doubled stick can span twice as far. The span doubles and the load doubles... the bending moment quadruples rather than just doubling. If you've ever tried to tighten a bolt to 200 ft'lbs the scale of the forces here should bring things into range. You know the guys, "I
jumped on the beam and its solid". Kind of like saying "a
fly landed in my truck, the suspension is good for a ton"
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hobo99
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# Posted: 18 Nov 2015 21:34
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Thanks again Don_P for responding. I've still got some reading to do to understand better the calculations I need to run for the live loads in the loft area for that beam. I'm reading through several of the tech specs documents for LVL's from GP, LP and Weyerhaeuser as time permits so I can be as knowledgeable on the subject as possible.
I've called two places so far trying to get a quote and both didn't understand what a ridge beam/board was and never returned my calls. I've just been to busy to go in and explain further either here in Dallas at a big box store or up in OK at the only lumber yard there.
I hope to go back at Christmas and have some time then to talk with someone local to the cabin site.
Tom
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hobo99
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# Posted: 19 Nov 2015 15:49
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A job I was planning to do today fell through so had time to go in to my local big orange store and ask about the two beams.
They called a place called Cedar Creek and I talked with their engineer who configured for the ridge beam a 3 ply 1-3/4 x 14" beam and for the loft a 2 ply 1- 3/4 x 24" with a 40# live load and 10# dead load beam.
Hoping the local guys in OK can order LVLs and site deliver, would hate to haul both all the way from Dallas.
Tom
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Don_P
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# Posted: 19 Nov 2015 20:42 - Edited by: Don_P
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The local guys can order it for you.
Looks like I was a size more conservative than he was, I was at a triple 16" and he called out a triple 14". I'm seeing it I think. I ran the deflection calc a little more conservative than required. I base it on the total load, it is correct to only use the live load.. but the sag is in response to the whole load. A 14" under full design load will deflect about 1-1/4", which is allowable. I kept it under an inch, actually about 13/16" under the same load, the double 18 deflects about 7/8". Hmm, if you have room, price the double 18 vs the triple 14. Structurally it will give a stiffer beam and it may cost less.
24" LVL's... I've had fun with those. check them for cupping before you accept them, if it's severe, apologize but insist that they try again. Then get them up that day and bolted together tight. Keep them dry, a roll of window flashing tape run along the top edge will keep water out of the middle. I'm telling you how hard it is to keep that flat. Allow yourself some room to shim between the beam and the finish wrap if possible, another opportunity to tune it up.
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hobo99
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# Posted: 20 Nov 2015 14:47
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Thanks for the tips!
The 24" is to support the internal loft floor to eliminate bounce from using 2x6 floor joist. 24" is killing my head room.
I'm now considering using 2x10's joist for the loft floor and possibly doubling up and just living with any bounce since most of the weight will be along the walls.
(Thinking out loud here)...
It will be 2x6 walls, so 8' main floor and a 4' knee wall for the loft. The loft floor joist will sit on the 4' knee wall plate and be nailed to each 4' knee wall vertical stud and plate. So I'm losing 11" of loft head room.
I could cut my main floor walls down to 7' keeping all the head room in the loft but losing it for the main floor.
Something else to work out...
Guess I could get rid of all my tall friends, problem solved...
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hobo99
Member
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# Posted: 4 Dec 2015 19:08
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Hoping there are some plumbers here that could help me with some Plumbing questions.
With a slab foundation does the main drain line run through the edge footing or under it?
Does each of the separate toilets, sinks and shower require a vent line or will the single one I have in the attached drawing work?
Thanks
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Just
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# Posted: 4 Dec 2015 19:58
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Anywhere within 5 ft. of the main vent a drain does not need its own vent . That's what my inspector told me and we passed inspection.
My main drain was under the cement floor but through the footing and it also passed inspection . BTW. I added a cleanout on the outside wall were the drain exited the cabin just in case of trouble, not required by inspector .
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hobo99
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# Posted: 5 Dec 2015 19:50
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Well that sure makes this easy, thanks Just for the help. The clean out is a good idea, you never know when trouble going to happen.
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