Pier Specs

[Cooter's]

Hello, I am building an 16' X 24' Accessory Structure (Cabin, but don't say Cabin) up North (Northern Michigan) on a Pier Foundation this summer. This building will be Canti-levered out over a rolling hill. So, grade is about 4' lower at the back of the Cabin than at the front. My plan was do 12" Concrete Piers for all of this (Sandy Soil), and I am looking for specs to do them. Rather than stopping all of the piers slightly above grade, and add posts to get up to the floor my plan was to pour them/form them to the finished height I need for the floor. At the back of the structure (16 foot wide wall) will be 3 of these piers, then 2 eight feet away, then 1 in the Center ten feet away (for ridge support), 2 more sixteen feet away, 3 at the front wall, and 3 more to support the porch for a total of 14 holes/piers. I attached a simple plan view sketch (sorry, it's a little light) to illustrate that last sentence.

Anyone have any insight or specs to accomplish this (rebar mesh, single rod, pattern of rods, which pier needs it, etc.)? Do you think I should connect any of them via footer? For the taller 3, I plan taper out the bottom of the hole a bit more, and of course pull the form up off the bottom some. Lastly, should I bias the piers to the inside of the footprint of the building to help prevent water from sitting on the tops of them or just center the brackets?

Cooter

 

[blkhonda1991]

one word...engineer, this is what is going to hold up your building and if you do it wrong the whole thing is going to fall

 

[kbs2244]

What a lot of people do not realize is that most of the support a pier post supplies is from the friction between the pier and the sides of the hole, not the bottom surface.

That is why driven posts work even with pointed bottom ends.
Most boat piers have driven posts and they work fine.

http://www.ehow.com/how_7173453_calculate-concrete-piers.html

http://www.permacolumn.com/pdfs/Rural_Builder_Concrete_Piers.pdf

http://www.countryplans.com/foundation/index.html

 

[pattenp]

This may be true for post for piers/docks and piles driven for beach homes out in sand, but that's not true for foundation pier construction on soil. Footings are required for support.

What a lot of people do not realize is that most of the support a pier post supplies is from the friction between the pier and the sides of the hole, not the bottom surface.

That is why driven posts work even with pointed bottom ends.
Most boat piers have driven posts and they work fine.

http://www.ehow.com/how_7173453_calculate-concrete-piers.html

http://www.permacolumn.com/pdfs/Rural_Builder_Concrete_Piers.pdf

http://www.countryplans.com/foundation/index.html

 

[KELLHAMMER]

A "pier footing" is poured in place and typically relies on the bottom spread footing on firm soil to support the load. As a simple determination, a 24" x24" footing on 2000 psf capable soil can support an 8 thousand pound load. A "piling" is driven into the soil. Piling depth, material and diameter is dependant upon numerous factors to achieve the desirable load capacity. Pilings do rely on the friction of the surrounding soil when determining the total load capacity. Pilings and pier footings are not equal. Pilings work best in loose soil such as sand or saturated soil. And are good in areas where flooding can scour away soil putting a tradition foundation at risk. Some piling work is done by filling a steel cassion with concrete and steel but that type is not something done in lightweight construction

 

[Trey T]

This is a post for geotechnical engineer and consider research the soil specs and slope of the land.

I would start by research for approved plan for nearby homes or builds at the city's office.

I would also google and see what others have come up with.

 

[K'ledgeBldr]

one word...engineer

This^^^^

 

[Cooter's]

I just contacted an Engineer. I was hoping for some examples of piers that worked and other suggestions/specs in the meantime. I can actually shift this building (to flat land if really necessary, but I like the view where I have it planned now) which would eliminate these questions, but I am trying to take in all I can to know I am in a good spot.

Thanks for the replies,
Cooter

 

[Trey T]

I pretty sure there are civil engineers on here that specialize in geotechnical engineering. Just wait and see.

 

[willymakeit]

Ask your engineer ,preferably in your area, about frost jacking. I hear it is common there. we dont experience a lot of it in the south.

 

[Cooter's]

Are there any other free opinions? My free engineering answers so far have only suggested upping to 16" Diameter Piers, but I planned to taper-out the bottoms of my 12s anyways, so I am still where I was essentially. Another thought that has come up is Locating the questionable piers (three on the tall end) with Cable, but I still think I will be fine.

Any other thoughts or opinions (other than hire an engineer) would be appreciated.

Cooter

 

[Cooter's]

I am going forth mostly as planned. I did double up the rim joists though, and I pouring the "tall piers" only to grade and using 6 X 6 posts for the above grade portion (with brackets and diagonals, of course). The weight of the building calculated out at 19,300 lbs, so the 13 Piers at 16 inches diameter should be fine. Even if the 10 of the main building were holding all the load, less than 2,000 lbs per pier should give me a safety factor of about 1.5.

I'm moving on!
Thanks for all the replies,
Cooter

 

[Cooter's]

It hasn't fallen down yet, but while I am still working on it does anyone have any more free advice? The one area I have in question now is the Open Area between the lofts. This area is now only 12 feet deep (still spanning 16). I will have two Collar Ties total in this area, but I wonder if it is enough with just a 2 X 12 No. 1 Ridge Board (acting as a beam really in this 12 feet).

Thanks,
Cooter

 

[Bojans]

Nice little place there. You in Northern Lower or the UP?

 

[Cooter's]

Northern Lower... Too many Panthers in the Upper (jokes). I would like to get a place in the Upper someday as there are some really good roads up there.

Cooter

 

[ilovevocs]

I have a place in Kalkaska. We really enjoy it.

 

[JDishong]

Engineer fee to design your slab - $250 (w/ peers) including specifications and certification.

Free opinions - $0

Congrats on saving $250 !

 

[TommyK]

It hasn't fallen down yet, but while I am still working on it does anyone have any more free advice? The one area I have in question now is the Open Area between the lofts. This area is now only 12 feet deep (still spanning 16). I will have two Collar Ties total in this area, but I wonder if it is enough with just a 2 X 12 No. 1 Ridge Board (acting as a beam really in this 12 feet).

Thanks,
Cooter

Eliminate the collar ties and make a structural ridge beam. At a minimum I would go with a triple 2 x 12 ridge and clip the rafters to it. Also clip the rafters to the wall. A properly sized LVL ridge beam would be better but for what you are building the triple 2 x 12 should be adequate for that short distance.

 

[ms fowler]

Make the piers wider at the bottom than the top. If they taper the other way ( wide at top to narrow at the bottom) frost will eventually squeeze them out over the years. As the upper ground freezes, it exerts upwards force due to the expansion of water. The taper permits that force to get a grip on the piers and push it out. Tapered to wider as it gets deeper, and the frost never gets a grip on the pier.
Make sure bottom depth exceeds the local frost depth.

 

[Cooter's]

Thanks for all the replies. Attached are a couple of updated pictures. I now have 4 Collar Ties (at half way up the rafters), and am planning to install a cable across between the walls within a rough log-beam and or install a beam (laying flat) on the underside of the eaves just in case.

The center section (12') could be considered a ridge beam (although it is just a single No. 1, 2 X 12) rather than ridge board, and I did split it this way (10', 18' in the center, and then 8') so the center section would hit three sets of joist-tied rafters above the one loft, and 2 sets of joist-rafters above the one main wall. I understand the aforementioned concerns of hiring an Engineer, but this building is going up complete for less than 20K and is just a shack. It is now named The Shabin (Shabby, Shack-Cabin) too which may or may not help with the concept of the thing.

Later

 

[Cooter's]

Roof On.

 

[Cooter's]

Snow Pic. Tails Cut.

 

[Cooter's]

I've made more progress and am moving on to walkways and landings. Has anyone here run into issues with the landing needing to be at the threshold vs. one step down? (I believe code reads it can be at a distance of one full step down.) I was told by the inspector that I only need three 3' x 3' Landings and stairs to get final. I am doing a walkway essentially, connecting the side entry door around to the back doorwall. The entry under the porch will just have a free standing landing with two perimemter steps.

Also, to stay true to this thread of questionable things (without a structural engineer), I am (only) doing 2 x 4 Joists 12" OC as the span is only 53.5 inches (5' wide Deck/Walkway on the back). This is supported by a Double 2X6 Beam with 4X4 posts 6' on center. I think it will be fine.

(An updated, summer picture is also attached.)

Thoughts? Suggestions?

 

[blkhonda1991]

I've made more progress and am moving on to walkways and landings. Has anyone here run into issues with the landing needing to be at the threshold vs. one step down? (I believe code reads it can be at a distance of one full step down.) I was told by the inspector that I only need three 3' x 3' Landings and stairs to get final. I am doing a walkway essentially, connecting the side entry door around to the back doorwall. The entry under the porch will just have a free standing landing with two perimemter steps.

Also, to stay true to this thread of questionable things (without a structural engineer), I am (only) doing 2 x 4 Joists 12" OC as the span is only 53.5 inches (5' wide Deck/Walkway on the back). This is supported by a Double 2X6 Beam with 4X4 posts 6' on center. I think it will be fine.

(An updated, summer picture is also attached.)

Thoughts? Suggestions?

one step down at doors is acceptable, where are the 3 landings you need? why not just use 2x6 at 16 inches oc, double 2x4s will cost more mo s t likely and 2x4,s arent even looked at in span tables i have as a joist

 

[Cooter's]

It turned out. 2 X 4 Portion isn't even bouncy... Note, I did 2 X 4's 12" OC (and each side of the posts), not doubles, and again the span is only 5'. I think I am going to extend the roof on the eave side over the raised portion of walkway. Then, I need to chop the posts to height. The two near posts in the picture I am going to leave long until I make up my mind if I am going to do anything with them. (Hammock? Clothesline? Drink Ledge? Small Canvas Shade (I like this idea)? Likely all of the above.)

Regards,
Cooter