Building post house on surface pads

[alex trent]

I am building an elevated stilt type house on a slight grade in a no frost area. 30 X 30 feet square. Posts at 10 foot intervals so 16 total. Posts will be 3' above grade to deck on high side and 5 feet on low part of slope...so not a 'tall' post structure. One story.

Can I place the posts on stone pads (24 x 18x 6 inches) that are resting on the level undisturbed soil. The grade is not steep so I can easily level where the pads go.  i would place one pad  below ground level all around and the other on top. Not having columns into the soil is desirable as hauling in concrete and mix and rebar is tough and also want to minimize environmental impact. The posts can be anchored to the stone with small concrete pads poured on top of the stone. Or do they need to be anchored to the pads at all.

I see nothing about this one line, so maybe i am on the wrong track. Any references/websites??

Thanks.

[Don_P]

The building code calls for a minimum 1' deep footing or below local frost depth, whichever is deeper. Without engineering is also calls for a continuous perimeter foundation, ie crawlspace or basement. This is to collect lateral (wind or seismic) loads from the building above and safely distribute them to the soil without tipping, sliding or overturning. If you do use piers of some type they need to be well sized to resist vertical loads and well braced to resist those horizontal loads mentioned above. This is best handled by an engineer.

[alex trent]

OK, so I will be one foot deep with my two 6 inch stone pads.  As to the perimeter foundation...how is it that stilt houses do not have them.  Is it because the posts are buried?  i assume then that the main potential problem is not vertical but horizontal wind loads?

[alex trent]

Here is where i got part of the idea...what am I missing about this and what I want to do?

http://coyotecottage.com/cabin/cabinconstruction/foundation.htm

[Don_P]

Stilt houses are an engineered foundation taking into account the loads involved both vertical and horizontal. If the soil and post are not up to resisting the horizontal load then additional bracing is required to keep the posts vertical. This is an engineer's territory. A shallow pier typically cannot count on the soil to brace it adequately. "Just because something has been done and hasn't failed does not mean it is good design"

When I first joined this forum we saw the results of an inadequately braced pier and beam foundation, it leaned and before the gentleman stopped posting he said he was going to have to take the house down and try again. We sure don't want to see that happen again.

[Squirl]

I remember that post.  I have been searching for it for a while.  It is a good example of calculating and planning what you want to do.  People post the builds but not the problems later.  And just because it worked for one person, doesn't mean it will work for everyone or even most people.

Areas with no frost depth tend to be southern, which has a regular occurrence of things called hurricanes and tropical storms.  That is very different from the area this was built in. Also the girders are attached directly to the piers in this as opposed the 3 ft post you proposed.  The taller the posts the more leverage it can put on the base. The calculations of which can be exponential and more complicated physics.  Also that house is not 30x30.  Loads multiply with size.

There is another reason to dig down.  The first few inches of soil are the organic layer.  It is great for growing food, but bad to put a house on.  The vegetation is going to die no matter what. (lack of sun).  Then this decomposes and gives way.  I have been struggling with this in my current build.  I scraped off the top layer.  I was not as carful with the trenches and piled it in with the rest of the dirt as I dug around.  Now during backfill, I have to be more carful not to place this back in the crawl space area.  As it is rotting it smells like a mix of a cow patty/swamp.  I have heard the environmental impact debate many times.  From experience I can tell you the best thing is to scrape off the entire layer of good top soil where the house is going to be and use it in the gardens, your house doesn't need it.

You are on the right path.  You found a method and asked the question, why isn't everything built like this?  It only costs $100 and a day's labor.  Is everyone else just a bunch of suckers?  It is because it has a limited design capability/ applicability and is less safe and sure than other methods.  

I did the same thing on a very small shed. The foundation shifted.  It will serve its intended purpose of a tool shed until the full house is built.  
http://countryplans.com/smf/index.php?topic=7305.0

[Squirl]

A PWF with a crawl space and balanced backfill can be especially good for a mildly slopped terrain.  Just be careful you are not in the danger zone of termites.  No concrete required.

I also reread your first post.  I noticed that you want to space the post 10 ft apart.  This is the sizing up issue.  Notice the posts in the example you posted are only around 4 ft apart.  10 ft is usually the danger zone for most buildings.  I would strongly caution against this.  You are resting the house on far less soil and putting a large strain on the beams. They will have to be much larger than most people assume.  

Since you are looking for more information, here is how to read a girder sizing chart.
http://countryplans.com/smf/index.php?topic=10511.0

It is also a link to the IRC.  You can click around the different chapters to read how to design almost every aspect of a building.

[alex trent]

Thanks.

Lots of food for thought in  your and other on here.

This is in Nicaragua on the side of a volcano in tropical forest.  Hurricanes are rare and wind is surprisingly light... I measured as had hopes for some wind power. The loaction is decently sheltered.  nevertheless, I know it only takes one big blow and zappo!

My changes..

1. Downsize to 14x24 and 14 foot side is most exposed to wind.

2. Shorter posts...pretty much 3 feet all around.

3. More posts/pads...every 6 feet both ways.

4. Need to get soil tested but it appears to be the same from 6 inches below surface to 15 feet. so a bit of scraping might work as planned.

This next one is not a normal thing, but let me ask for opinions anyway as down here we need a lot of workarounds.

If I am OK on vertical bearing (and I will ensure that), the challenge is to keep it fro shifting horizontall and to prevent uplift.

What about if I drilled three 3/4 inch holes through the pads (two stone pads at 6 inches deep each) and drove rebar down 10 feet. Capped the top with a fastner.

This do anything or just make me feel better...opinions welcome.

I restate the problem on digging...causes me lot of problems with the environmental folks as this borders a protected area and i am a gringo.

[UK4X4]

I would dig down the posts as much as possible 4ft is a pretty usual depth using a auger or spade

with then a concrete pad filling the hole- the post then on top of that

backfill compressed around the post - or concreted in

The depth helps the sideways movement - the wider -pad at the bottom the uplift and punch through

For looking at your loading I use the "Bigfoot" technical uplift sheet to guestimate footings size V's soil

really for a foundation you need to start looking at the loads from the roof down to the foundations as the numbers quickly mutiply up.

Side of a volcano in lush vegatation I guess- so you'll probably have very deep loamy topsoil made of decaying vegitation.

ie not the best soil type for a foundation, but thats just a guess- soil conditions are a major part in sizing the support structure

Volcano = active area + earthquake potential

so not only the wind will have to be concidered when you do your angled supports on the posts.

How long do you plan the building to last ?

Most ethnic stilt like structures have a short lifespan- the people accept crooked floors and curtains for doors
as the mostly shift during use- it all depends on what you expect to accomplish

[MountainDon]

I remember that post.  I have been searching for it for a while.

http://countryplans.com/smf/index.php?topic=4952.msg82920#msg82920

[Squirl]

Thank you.  It is one of the few members that shared when something went wrong.  I believe it happens more than we hear about in many situations.  Human emotions, pride, denial, come into play.  He hasn't posted since this.  I hope he didn't get discouraged.

[alex trent]

WOW. Bad scene.  Seems like he did it pretty much 'lightweight" from the start.  Posts three feet in the ground and seemingly not in concrete.  No braces. i guess he should have come on here first and asked questions.  I think mainly did not know his site and soil.  A real shame given the amount of work it looks like he had done.

Of course. I am the guy who wanted to build on pads.  The info i have gotten has pretty much convinced me that is a bad idea.  I have looked up a lot of stuff about this. All the work arounds and still with risk are not worth it.  I will dig and put in concrete piers and top with wood posts attached to the piers.

For the big version of this, which is 24 x 40 I will put in 24 posts on 8 foot spacing. Question...if I bury the piers 4 feet, do I need cross bracing on the wood posts at 4 feet from grade to deck?  looking at the pics in the link give me the shivers and from what I know, they do a lot to prevent this.

My main concern now (always another one, huh) is the bearing capacity of the soil.  i am up to the property now and will do a jar test...sandy clay loam I think. But as mentioned likely lots of organic matter.  If I figure the house weighs 30 lbs./sq ft (it is a simple one story house), I have about 30,000 lbs. So, 24 piers at 1 sq ft each means each will need to support about 1,300 lbs.  I see that in most charts the lowest soil bearing weight is 2,000 lbs/sg ft.  But, i also see some down to 1,000.  That makes me nervous as i cannot really test subsidence.  You think a decent loam but with lots of organic could  end up at 1,000 (or less)?  I will try to broaden the bottom of the pier hole, but only can do so much without major excavation, which i must avoid.

[Squirl]

The average calculation for floor loads is 40 lbs per square foot of live load.  I usually see calculations adding 10-15 lbs to that for the material weight of the floor. Don't forget your roof and walls have weight too, including wind loads. Also note in your calculations that roof loads aren't usually distributed evenly between all piers.  In most designs all of the weight of the roof bears on only two of the outside walls. 

Buildings are designed from the roof down.  The style of roof and how you distribute its weight, dictates how you distribute the weight and frame the walls, which is how you design the floor, then the foundation to how much soil you have to bear on. A good read on the subject.
http://countryplans.com/smf/index.php?topic=10675.0


Does Nicaragua have the same dimensional lumber as we do?

I hope you post pictures of your build.

[MountainDon]

I had written something and then all of a sudden when I clicked "post" it poofed instead.

Questions:
How permanent is this structure to be? Short or long term use?
What materials are readily available for use there? PT lumber?
What is the typical type of construction there for this type of use?
How do those stand up to the local conditions?

Note that, IMO, 4 feet of post above grade, with 4 feet in the ground will result in a building with some movement even if the posts are firmly locked to the soil. Diagonal bracing can help a lot, but can only do so much; issues may arise at the points where the fasteners secure one member to another before the materials themselves have problems.

The larger in diameter the posts or the concrete that fully encases the posts, the better the resistance to that post moving laterally in the soil. Like 20" concrete is much better than 8 inch.

[alex trent]

Keep them thoughts coming.

I will post photos of the build. Should be interesting.  I am currently looking for wood and some folks who know about building with wood.  This is concrete block country.  There are some  very good interior wood craftsmen, but many house builders.

The points you guys are making make me very glad I am on this...hopefully i will be able to pay some back sometime.

I likely shorted the house weight a bit 40-50 is better.  But inside walls are minimal and I am a minimalist in furniture too. No insulation and exposed ceilings, etc. The roof/wall distribution on the two side walls is a great point...especially since I am a bit worried about the bearing capacity.  Likely will tighten up the spacing on those pilings, or try to excavate bigger pilings.  I will recalculate everything once I get a better feel for the soil. turns out that organic matter is likely not high and not too deep, so I will be well under it.

I might get by with 3 feet above grade and could go down 5 feet.  Likely will.  to minimize the dig, I want to do 8 inch pilings and can get a 14 inch base pad...all with rebar and a rebar net at the bottom.

To answer the specific questions...

1. long term

2. Got good wood...just got to figure it out. Names don't alway match up with what it is.  been a bit of fun running it down.  Framing will be a wood halfway between SYP and white oak in bending, etc. So plenty strong. Siding is another wood, related to mahogany...really pretty...goes red on drying and sun. Hard as hell, will have to drill.  will also use for the floor and outside deck.

No PT, but I have Bora-Care that I treat with and none of this is in soil contact.  I will air dry for two months...dry season we are in the 90's and no rain at all! I will sticker and stack where there is good air circ.

3. Not any building up where I am on the slope...down a bit on level ground are the typical concrete block houses. Tile or tin roof. they do well. some owned by the less well off are old and patched and need maintenence.  But liveable.

Yep, I am different than a lot...but good reasons and some just don't want to live in a concrete block house.  My house in the town is Adobe...i redid it 3 years ago, nice but not for the mountain.  I want this to blend in to the woods.

[UK4X4]

5 deep and 3 high in that soil would be good

rather than dig individual holes - whats the chance of digging a 5ft deep by building long trench

Putting in a footing the whole length and posts up from that

ie you get the intrusive look above with just posts with a large and stable foundation below

Which way arround is the ridge - will you have posts down from the ridge line ?

[Don_P]

The wood handbook from the US Forest Products Laboratories has some non US woods listed and their ultimate mechanical properties... deriving allowable design values from those is a bit of a trick but better than nothing.

Block corners, say 4' wide out of each side of the corner, would go a long way towards bracing the foundation.

[alex trent]

Here is a site that has a lot of good info on non-USA wood.

Matches what i have found on some other site about a particular wood...so seems OK

This page is for the framing lumber i plan to use.  if I am not mistaken it has about 30% more strength than SYP.

http://www.thewoodexplorer.com/maindata/we231.html

As far as the trench and footing around the perimeter, not possible. Way to much excavation (all by hand).  Then I still have to deal with horizontal movement and filling in with the dug out stuff is not good.  Gravel is out of the question as is disposal of the excavation.  And, as far as bearing weight, I can handle that part with piles and big bases. If need be I can do 2 sq. ft. each.  At 30 piles that is 60,000 lbs. @ 1,000 lbs bearing wgt per sq foot, which is pretty poor soil. Not to mention that last 1000 feet is by hand or in wheelbarrow. Gravel and concrete for that is more than just an extra chore.  

I am to talk to a man who has a big house on piles...but on flat land and that will give me an idea.

[Squirl]

More posts = more soil to bear on and smaller beams.

UK makes a great point about seismic activity near a volcano.  Most construction in tropical humid areas is out of stone and concrete.  It doesn't rot and doesn't get eaten by termites.  But it also doesn't move in an earthquake. Wood is the best material for survival in that situation.  That being said, I believe you house will be heavier than most because of the hydroscopic nature of wood and the humidity of a tropical environment.

[Arky217]

Here's what I did; it may give you some ideas.

My frost depth is just a few inches and my house is on a very rocky ridge.
The house is 24'x 48' and sits on (4) rows of (4) 6"x8"x12' SYP beams which in turn sit on (36) 28" to 34" high, 6"x6" red-heart cedar posts spaced 6' apart.

The posts sit on 3" high, 10" dia. piers that go 18" deep and are tapered larger to about 16" dia. at the bottom.

The piers have 5/8" rebar anchors with a 2"x4" flat bar welded on with a 1/2"
hole. All (piers, posts, beams)  are tied together with 2"x6" SYP braces using 1/2" bolts, angle brackets, etc.
Braced and X-braced every way but Sunday.
I know, I know, overkill, but it's not going anywhere.

I cut all my lumber on a chain saw mill, but you could use 6"x6" treated for posts,   
2"x6" treated for braces and build up 2x for the beams.

Arky

[Don_P]

Hygroscopic... water loving  
The local humidity won't make a large enough difference in the equilibrium moisture content to change the weight of the structure significantly, the dead load of andiroba framing might add a bit.
This doesn't have Nicaragua but does have several SA locations... My wife's family is Dutch, look at the Netherlands for a comparison.
http://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf
I know the wood but not well enough to say much. There is a somewhat weaker relative we may be talking about judging by the latin name carapa nicaraguensis as opposed to the commonly known C. guianensis It carries a lower specific gravity of ~.45 which puts it closer to southern pine than C. guianensis. My "Wood Handbook" doesn't have the major mechanical properties of carapa nicaraguensis.
This is the more common species of andiroba;
http://www.fpl.fs.fed.us/documnts/TechSheets/Chudnoff/TropAmerican/html_files/carapa1new.html

Their comment on preservative absorption is disheartening although I doubt they were testing a borate solution. If you have a choice borate diffuses in much better if the wood is green, right off the sawmill. I've built several wooden dip tanks and soak the wood when possible.

[alex trent]

Arky..

Thanks loads.  Always nice to hear about a real life example.

I will be higher above grade...like 3 feet, so plan to go deeper into soil and I need more piers as my soild is not rock.

But you have a big house on this!!

Any pics?

[alex trent]

Don..thanks

This is the about same wood as the one you looked up, lots of variations to the name and this likely a different variety. Common name in NICA is Cedro macho...although not a cedar. likely is a bit weaker as the other is called Cedro real and not available as all cut.

Used a lot in building and has a generally good reputation for covered uses. Mine will be all covered and posts somewhat exposed but not in ground.

Most is used untreated. I will treat dead green with Bora-Care and the glycol is supposed to get sucked in the wood.  All the date i see say this will be good stuff for me.

Strength looks OK to me...you?  i make it to be a bit stronger than SYP.

There is another possibility...

Hymenaea courbaril ...here called Guapinol .

Looks to be about the same as Cedro macho in properties and i may use for the outside walls as takes on a nice rich red color.

I am looking at other wood possibilities as well.

Nispero is one...very hard and strong , and insect proof.  May use for the posts.

[alex trent]

I went back and looked at the house in KY that was falling over. Seems like a pretty big house for the posts.  But even more important NO BRACES.  Is not that a bit odd.  I have never done a house, but on sheds the braces are mandatory. You think there might have been a different outcome with decent braces?

I plan to brace mine real good.  So, am I fooling myself with this as well....or will they make a big difference.  Got a guy here who says with the right bracing I can go just 1 foot deep with a 2 X 2 concrete pad and attach posts to that. He is showing me some big houses he did that with. But on level ground. Tempting, but after being on here I am leery.

The new plan is back to the bigger house..24 X 38. Max 3 foot above grade for the posts (moved the site a bit) and 5 feet deep pilings.  Going to pour 10 X 10 inch squares with base of 16 x 16 inches.

Four rows of 6 foot spacing (24 posts/piers). Soil looks OK to hold it...60/40 silt/clay,,,about no sand.  So at 2,000 lb/sq feet that gives me about 90,000 lbs. Good drainage (or so it appears).

[Don_P]

I've been typing slow and haven't had a chance to look up your other wood choices.

The glycol is really a wetting agent. Glycols dry slowly and borates travel on the wet, they diffuse from areas of high borate concentration to areas of low concentration but only when the cells are above fiber saturation point, above about 25% mc. The glycol in bora-care is ethylene glycol... anti freeze. It keeps the wood wet longer to allow the borate to diffuse deeper. I use Solubor or Beau-Ron, ag soil ammendments and cheaper than bora care or timbor, all of these are DOT disodium octaborate tetrahydrate. If the wood is already dry I'll add a glycol. If you look up various mixes' MSDS's you'll also find propylene glycol (RV antifreeze) and polyethylene glycol.

If you know nothing else about a species, specific gravity is the most telling. Density is generally an indication of species strength, BUT, and especially with tropicals, extractives can raise s.g. without contributing to strength. On an individual board grading is the most important, look for normal, straight grain for the highest strength spanning pieces, headers, beams, joists and rafters. Lower grade sticks are used for vertical pieces like posts or studs where compressive strength is the main stress. Use the lowest quality for blocking, redundant short cripples, etc.

More than a few of the tropicals can cause respiratory distress or contact dermititis, local knowledge is worth gold here.

You can build on a parking lot if the structure is properly braced, there's the rub, getting the bracing right. A few nails or small lags aren't going to be adequate. Folks are generally overly optimistic about the bracing capabilities of soil on a shallow pier. If you look at the bracing on trestles, bridges, etc you'll see heavy bolts on sturdy X bracing and generally unseen shear developers like split rings or spike plates embedded in the joint. A house is tall and catches more wind that is levered down through the piers. Timberframing uses braces that are fitted into mortises in the posts. One engineering prof in a deck building seminar told us that most nailed on braces are simply something else to hit you on the head when the deck tips over, brace well. And yes the failed house had no bracing despite being told a couple of times early in the thread to get his bracing on. In the rush for the roof he went beyond the soil's ability to hold the piers upright. IMO go as deep as possible but brace well, the soil is an unknown "extra" rather than a primary line of bracing. Healthy sized welded steel angles would go a long way if you have access.

If you look in the wood handbook or your online sources you can usually find some familiar comparable species with similar ultimate values and then be conservative using the comparable's allowable design values. It is tempting to be optimistic but is better to err conservative.