Author Topic: Few Questions on Concrete Pier Foundation (Probably beating a dead horse)  (Read 1399 times)

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Offline goodasgone81

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I'm sure this has been discussed but I'm not coming up with the answer ... I'm looking to have three 6"x12"x20' beams supported by concrete piers, I'm looking to using Simpson EPB (elevated post base) to attached the beams to the piers.  That being said it looks like primarily the EPBs are used to support vertical post and not a horizontal beam, is there a better option than the EPBs or are they good to go?  I guess it's worth noting although probably obvious, I'm looking to build a "cabin" on top of the three 20' beams, so my concern is EPBs are designed to handle the load of a cabin.


Above is the main question but also I'll give a break down of the current foundation plan and if anyone has input on something I should reconsider feel free to let me know.

-Building a 20'x20' cabin
-Looking to have a total of nine concrete piers, three for each of the three 20' 6x12 beams
-Each pier will have a 24" base and 12" column
-Footprint of the piers will be 15' x 20' .... so on the 15' spread will have an overhang of 2.5' on each end of the 20' beam, with the other axis being flush with floor joist
- 6x12 beams will be three 2x12 secured together to form the beams

I'm by no means an expert but based on the research I've done I believe the piers and beams will be more than adequate to support the weight of the cabin.

Online Don_P

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Welcome to the forum  w*
Do some research in the building codes, whether enforced locally or not they contain good information on minimum structural practices.
Pier foundations require engineering. If the footing is 24" square and is only 1' tall I'm not too concerned with it overturning myself, whatever that is worth. However the shallowest footing in a frost free zone needs to be 1' underground so what I'm understanding is that you are in the deep south and that base is at grade on a level site.
 The connector does not restrain the beams from overturning, they need to be braced or blocked. The forces resisted by that connector were up, down and lateral but it has no moment capacity, it does not resist rolling. The overhang on the joists sounds like it is too long. Typically that is limited to the depth of the joist or less restrictively maybe, the cantilever table in the floor chapter of the code.

Things to research;
What is the frost depth where you are building, this is the footing depth.
What is the snow load
What is the soil bearing capacity
Use the design loads, both dead and live, from the building planning chapter of the code to determine the building's design weight.
Divide that by the proposed area of the footings in square feet and per pier. That weight should not overload the connector or the soil.

Offline goodasgone81

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Thanks for the input, does anyone have suggestions on other connection types to look at? .. I'm a little confused on the overturning part.  I'll be burying the piers 4', because I'm actually midwest :) Was looking to use something like this https://www.homedepot.com/p/SQUARE-FOOT-28-in-x-20-in-x-28-in-Plastic-Concrete-Footing-Form-SF28/300483908?cm_mmc=Shopping%7cTHD%7cG%7c0%7cG-BASE-PLA-AllProducts%7c&gclid=Cj0KCQjwy9LVBRDOARIsAGqoVnspeWFXinI1SI_Mm17ANx3qHqn3e0a3dX-bJaTejTEytSHiIC5rpLoaAs4SEALw_wcB&gclsrc=aw.ds&dclid=CM2ytK_9g9oCFQ59AQodFqUIYA (realize that's 28" and not 24" as I said but it's one I had saved) and then something like a 12" sonotube.

I had used this this equation (below) to determine how big the base should be and unless my math was wrong the 24" footers were adequate

Step 1
Divide the total weight that the piers must bear by the pounds per square foot that your soil will support. Compact gravel may support as much as 12,000 pounds per square foot, while soft clay may only support 2,000 pounds.

Step 2
Divide the result by the number of piers you plan to install, to calculate how many square feet the bottom of each pier must be.

Step 3
Take the square root of the result in Step 2 to calculate the actual dimensions of the piers in feet. For example, if each pier must cover 3 square feet, the square root of 3 is 1.73, which means each pier must be 1.73 feet by 1.73 feet. To convert to inches, multiply the result by 12, so 1.73 feet would equal approximately 20 3/4 inches. If you plan to install round piers, divide the result from Step 2 by 3.14, take the square root, then multiply by 2. The result will be the diameter of a round pier in feet.

Step 4
Add the depth of each pier in feet to the height above the ground that you want your building or deck to be. The piers should extend below the point where the ground freezes in winter. This will give you the total height of the pier.

Step 5
Multiply the length and width of each pier in feet by the height from Step 4 to calculate the cubic feet of concrete needed for each pier, if the piers are square. If the piers are round, divide the diameter from Step 3 by 2, square the result and multiply by 3.14, then multiply by the result from Step 4 to calculate the cubic feet for each pier. Multiply the cubic feet of each pier by the number of piers, to calculate the total cubic feet for the job. Divide by 27 to calculate the number of cubic yards of concrete necessary.

Source ... https://www.hunker.com/12375334/how-to-calculate-concrete-piers

Offline MountainDon

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I'm a little confused on the overturning part.  I'll be burying the piers 4'....

The overturning concern is for the 6x12 beams.  You don't indicate how the floor joists are mounted, however, the most common method is to place the floor joists at right angles across the beam tops, then build the structure on that. When the wind blows against the wall that is parallel to the beams, the forces generated try to move the top of the beam in the direction of the wind. The metal connection bracket is designed to restrain the beam from lifting or sliding sideways. The connector mentioned is also designed to hold the beam above the concrete so also has a specific downward load carrying capacity. All those values should be listed in the Simpson technical info.

So back to the wind. The wall and roof pitch that is facing the wind generates lateral force. The walls and roof are connected securely to the floor assembly (joists, subfloor... a rigid box). That lateral force is transmitted to the top of the 6x12 beam. The bottom of the beam is held in place by the metal brackets. The forces will be trying to overturn or rotate the beam along its axis. The metal brackets are not designed to resist that type of force.

The beams could be braced against that rotational force by dropping the floor joists between the beams using joist hangers. There are advantages and disadvantages to that.

Q:  Are the proposed 6x12 beams to be a built up beam of layered 2x12's or a sawn timber beam?

Rather than digging nine holes large enough for the 28" square plastic footing units  and still having a non engineered foundation have you thought of digging a trench for a conventional perimeter foundation?  Those are detailed in the prescriptive building codes and form a rigid box that supports the building on top?  One important difference between a pier foundation and a perimeter foundation is the piers act independently of each other as they are connected only at the upper ends. A perimeter foundation is a monolith with reinforcing steel for added strength. The loads on the foundation are spread out. A pier foundation can spread the load unevenly.  At least those plastic forms are large and will spread the load over a much larger area than what we usually see done with piers.

Are you in any of the midwest seismic zones?
« Last Edit: March 29, 2018, 09:56:56 AM by MountainDon »
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline goodasgone81

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Thanks for the input .... I'm following the side load due to wind, I am looking to build on top of the beams but could look at using joist hangers.  I'm looking to laminate three 2x12s to make the beams, I did think it would be odd to have a bracket that only support half of the beam but I'm not seeing other options to connect the beams to the piers, or at least not other options that address the issues.  I'm going to keep searching around, doesn't look to be much out there designed specifically for the DIY home builder, most everything I've seen is designed for decks.

This is being built in a clearing of pines and on a slight incline, with limited access to build site.... that said I think piers are my best option.  One thing that might help with the side load due to wind it is should be minimum due to have a forest around me, but I don't want to build sub standard. 

Offline goodasgone81

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Actually looks like I found a way better option but still might not be ideal .... at least it is designed for foundation beams

https://www.strongtie.com/columncapsforcmuandconcretepiers_masonryconnectors/ccqm-cctqm-ecclqm-cccqm-ecclqmd_productgroup_wcc/p/ccqm.cctqm.ecclqm.cccqm.ecclqmd

Offline MountainDon

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.... I'm looking to laminate three 2x12s to make the beams....

FYI, in case you were not aware, if there are splices, the splice points should be located over a pier, not between piers. The norm is to alternate or stagger the splices, but you do not have enough piers to do that. I'm not sure how that may affect the beam reaction to loads. If all three layers are spliced over the center post in effect there are two separate beams. If I recall correctly a load will then cause more deflection between the posts than if the beam was continuous or had staggered splices.   I'm not certain on that but something rings a bell.  Perhaps Don_P will have some input on that as he has worked with this sort of thing for decades.

As Don_P wondered, the end overhang / cantilever seems long at 2.5 feet. The floor chapter of the IRC does have info on cantilevering floor joists more than the depth of the joist, but I am not sure how that applies to a beam.
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline goodasgone81

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Thanks ... I'm only doing 20' beams so I'm looking to just be three 2"x12"x20' piece of lumber for each beam.  Shouldn't have any splices.  This has been helpful, given me a lot to think about.

Offline jsahara24

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-Building a 20'x20' cabin
-Looking to have a total of nine concrete piers, three for each of the three 20' 6x12 beams
-Each pier will have a 24" base and 12" column
-Footprint of the piers will be 15' x 20' .... so on the 15' spread will have an overhang of 2.5' on each end of the 20' beam, with the other axis being flush with floor joist
- 6x12 beams will be three 2x12 secured together to form the beams

As always most will recommend you not to use piers for your foundation, however I understand site conditions and personal constraints do come into play.  So please consider the following, for the record I installed concrete piers with the 28" bigfoot spread footers myself and I haven't had any issues (movement) with them over 3 years now.  With that being said I do wish I did a full perimeter foundation, its my biggest regret....My soils are also very well drained, mostly sand and gravel, which definitely helps keep a pier foundation from heaving.  If you have a lot of clay/silt you may have a different outcome...

If you are set on the pier foundation then your plan is the best that I am aware of, much better than 6x6 posts which will rot over time and require much more bracing.  I am not sure what your soil bearing capacity is so I am going to assume you have enough SF of footer to take the weight of the building.  I would only make 2 quick adjustments as follows.  You're saying you want to have a 2.5' overhand on either side of the piers, so you will have 15' of width with 3 rows of piers, resulting in 7.5' spacing between the rows of piers.  This seems excessive to me.  I would limit your overhang to the height of your joists.  You are only going to have a ~9.5' length to span using this approach with your floor joists which can be accomplished with a 2x6 I believe (check the tables), which doesn't give you the depth you really need for insulation anyway, unless you use spray foam which gets very expensive.   I would use at least 2x8 joists. 

I would also recommend you add a pier in each of your rows, right now you have a 10' span between piers for your beams which is longer than i'd recommend. This would reduce that to ~7'. 

Good luck and I look forward to pictures! 

Jason


Offline akwoodchuck

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The way I read it, the beams are cantilevered past the piers, not the joists past the beams.....if so, that gives a 7-ish foot beam span, which should be fine.....

Offline jsahara24

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The way I read it, the beams are cantilevered past the piers, not the joists past the beams.....if so, that gives a 7-ish foot beam span, which should be fine.....

Just read it again and you are correct, sorry I read that wrong!  I'm not used to people proposing to have the beam overhang, my biggest concern with that would be if you plan to have a cathedral ceiling with a ridge beam, and the posts come down to the beam that is overhanging the pier. 


Offline ChugiakTinkerer

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The cantilever of 30" seems really close to the limit, depending on snow load.  Out of the question if it must support a structural ridge, and it's cutting razor thin for a non-structural ridge or trusses.

Assuming trusses or non-structural, each side wall bears half the weight of the roof load.  If you have 2' eaves and rake then your roof footprint is 24x24, or 576 sq ft.  Minimum roof design load is (if I recall correctly) 30 pounds per sq ft (psf) and 10 psf dead load.  So you should design to support a roof load of 23,040 lbs.

Each side wall beam will also be supporting 1/4 the weight of the floor design load.  Live load of 40 psf for living space with 10 psf dead load and you get a floor load of 20,000 lbs.  Side wall beams each bear 5,000 lbs of that load.  Add in half the roof load and for a uniformly loaded beam you are looking at 826 lbs per lineal ft.

For the spans between piers, you are looking at a load of about 6,195 lbs at each 7.5' span.  Based on the IRC span tables for girders, Table R602.7(1), you could make do with three 2x10 to build your beam.  But the load on the cantilever ends is close to being too much for three 2x12, depending on wood used and actual snow load.  if you reduce your cantilever to 24" or less you might be okay.  It's close enough that you should either step up to four 2x12 or get someone with the right knowledge to assess the loads and wood strengths your specific project has.

Note that for the beams you'll probably be wanting to use incised, pressure-treated wood.  That treatment affects the material strengths of the lumber, which requires different design values.

Edit to add: The Simpson Strongtie anchors look to be a good fit, assuming you can use three 2x12.  If you need to go up to four deep, I don't see any that are 6" wide.  But pay attention to the footnotes on the load tables.  They assume a pier that is 16"x16" CMU (cinder block) filled with grout or concrete.  Also, the download capability is 7,000 lbs (footnote 9).  By my estimates the max load on a pier will be the center pier on each side wall.  Looks like 8,260 lbs based on my calculations.
« Last Edit: March 30, 2018, 01:15:26 PM by ChugiakTinkerer »
My cabin build thread: Alaskan remote 16x28 1.5 story

Offline goodasgone81

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Definitely a lot to think about, the only reason for the overhang was to try and have a smaller footprint as to not get too close to some near by pines I didn't clear..... that said I'm going to reduce the overhang to 1' on each side and possibly  just go the full 20'.

As for the "Column Caps for GFCMU and Concrete Piers" by Simpson, I am seeing now that my 12" tubes will not be adequate, saying a minimum of 14" square which would be 196 sq inches ..... a 16" diameter tube would equal out to a little over 200 sq inches so it looks like I'll need a minimum of 16" columns if I want to use those brackets  :-\

These concrete piers are starting to become a headache ... might reassess if I can do another type of foundation but just doesn't seem likely with the terrain 

 

Offline MountainDon

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These concrete piers are starting to become a headache ... might reassess if I can do another type of foundation but just doesn't seem likely with the terrain

Permanent wood foundation?   No concrete required at all. If you can get a back hoe in there to dig a perimeter trench, the machine does all the hard work, then you frame a foundation with a special pressure treated wood.

Info at southernpine      info at american wood council
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Offline Adam Roby

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I am so tempted to try those screw in type thingies.  They claim to support 5000 lbs and are adjustable somewhat.  Would avoid digging into the tree roots if that was your concern, but I don't think anyone has tried them on this site yet, and they are probably not designed for something that big.

Offline MountainDon

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My brother-in-law had to install some of those screw piles after the bldg dept caught him after building a non-permitted deck in his backyard that had only the so-called ground mount deck blocks under the piers. That was a very difficult install; lucky for him he could remove screws and lift deck boards to gain some access. He had to hand screw them in using pipe lengths as lever arms. Turn a little, move the pipe and turn a little more. Repeat ad nauseum.

Don't most of those screw piles that are meant for habitable buildings require an installer with power equipment to screw them into the earth? Especially if there are large roots to cut through. ???  I will admit I have only superficial knowledge of them.
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Offline Adam Roby

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Looks like they at least need some kind of hydraulic machinery, but looking on Google they still seem rather portable.

I was more looking at the deck type ones, 50" in total with maybe 42" under ground.  Using them like a skid foundation but with some support, so no posts to add a hinge.  Reading on some other sites that they can give a swaying motion inside the cabin, so not completely stable.  That sounds a bit annoying... 

I need to add some to a deck I want to reinforce this summer.  If that goes well then I might consider them for a roofed shelter for the tractor...  one step at a time.

Offline GaryT

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I will be building a camp in New Brunswick this year on helical piers.  The contractor I will be employing uses this outfit to drive the piers:



Gary

P.S. they are braceable to inhibit many of the problems inherent in the use of columns and piers.  The brand I will be using is GoliathTech.  The piers come in 7 foot lengths; I will be doubling that due to unstable fill that we have to get through.

Online Don_P

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Rather than letting them value engineer it as lightly as possible, step up a diameter or three on the piles. Lay it out precisely and then watch them like a hawk using a laser to keep them in line as they place the piles.

Offline GaryT

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I visited a site the same contractor had done.  The owner/builder was tickled to death with the job.  Perfect alignment, etc.   Our site will cost right around 10K (Canadian) for a 26 X 36 camp.  Good advice about the diameters.
Gary

Offline goodasgone81

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Annnnd I just closed out the tab on accident I was writing a replying to this thread on, I had been working on it for about ten minutes so lucky for anyone that reads this you'll get a shortened version :)

So now the long story short, I'm back to the drawing board for columns caps because the Simpson CCQM brackets I was going to use are over $220 a piece!  I'm now considering the Simpson PSB66 and trying to sort through if they are adequate

http://embed.widencdn.net/pdf/plus/ssttoolbox/2c1fhtnuv8/C-C-2017-p098-099.pdf

Specifically I'm trying to be understand "uplift" and the allowable loads relating to it, so for the PSB66 it's saying 2165 is the uplift allowable limit .... does this mean if I had a barbell (this is just a thought experiment to try and understand ha) attached to one of these brackets I'd have to push up with a force of over 2165 lbs for the bracket to break?  If this is the case and I also had say 60,000 lbs worth the building material on top of it then I'd actually have to push up 60,2165lbs for them to break?

Second question is more just a confirmation .... also with the allowable it's saying "download", I'm assuming this is just the downward force the bracket can handle, in this case it's saying 14,420.  So that specific bracket could handle 14,420 lbs on top of it before it "collapsed"?

Online Don_P

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Correct on both counts. Uplift that deep down the load path, in other words with all that dead load of the building above it is less of an issue than uplift say on a rafter tie. In an open structure like a pavilion where the wind can get under and lift the light roof against those brackets it would also be more of an issue. Download is not only the connector, think about the elements it connects to and the soil bearing capacity as well.

With this entire line of thought the real elephant in the room, the controlling failure, is probably going to be lateral, a pier or connection rotating. Most of these connectors specifically say that they have no resistance to that. Most people think of the ground surface as rigid in their minds thinking the pier rotates about that groundline, not so unless the piers are "constrained" by a surface slab. Stand rows of pencils in a pan of jello, stack a serious pile of books on top and push laterally on it, that is going to be closer to reality on that muddy day when the wind howls. Some form of bracing is required. Usually what you'll see in old threads is inadequate, when someone does brace well enough in all directions, takes care of girder spans and connections properly, etc it would have been cheaper and better to put in a conventional perimeter foundation... which is why it is conventional  :D.

Offline NathanS

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In our area I've never been in a pier structure that's more than a couple years old that doesn't have at least one pier that has significantly moved.

A mini excavator could dig out a perimeter footing in a day. If a concrete truck really can't get back there I would look at the permanent wood foundation route. Or buy a concrete mixer and mix/pour the footing yourself.

Even aside from all the problems with the piers themselves, then you have to enclose everything to try to protect your insulation, drain and water lines. I would rather mix bags of concrete for the footing and lay blocks than deal with all that every year or two.

I'd also clear white pines away from where they could snap onto the house. Beautiful trees, I love them, but not good next to a house in my opinion.

Offline Beavers

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I built my cabin on piers.  My reasons were cost and needing an elevated finished floor elevation due to flood plane requirements. 

I constantly kick myself in the ass for doing a pier foundation.  It seemed so much easier and cheaper at the time, but in the long run it isn't if you do it right.  Pretty much the rest of my cabin is built to or exceeding code...the one big weak link is the pier foundation.  I'm getting ready to start on an addition.  I'll be using a concrete block foundation and going back in and trying to tie in one of the original cabin beams into the new foundation.

The responses here have been pretty unanimous trying to steer you away from a pier foundation.  Don P tried too nudge me in the right direction back when I was building mine. I was too stubborn to listen though.  Hopefully you aren't as stubborn as I was.  d*

I think it would be pretty easy to find owner builders who would admit to wishing they hadn't used a pier foundation.  On the other hand I think you would be hard pressed to find someone saying what a waste of time and money their full perimeter foundation was, and how they wished they would of just used a pier foundation. 

Offline MountainDon

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I think it would be pretty easy to find owner builders who would admit to wishing they hadn't used a pier foundation.  On the other hand I think you would be hard pressed to find someone saying what a waste of time and money their full perimeter foundation was, and how they wished they would of just used a pier foundation.

Our cabin (16x30) was built on two rows of 6x6 piers. After several years of learning more I met an engineer here (online). He kept a very low profile but in the end we talked about piers and defeciencies that accompany them. I ended up building shear walls, sometimes called a braced wall, under the cabin, between the piers and the floor assembly and the ground. Basically a framed wall, studs, top & bottom plates and 3/4 plywood. All PT. All a big pita. I did it, not because any problems had popped up, but because I understood better, what could happen and how. Now I feel better about our cabin.

I used piers, in part because the transit mixers companies did not want to venture far off the county roads. If I was building now I would elect the perimeter  permanent wood foundation, or perhaps learn to lay block. Frost depth for us is 30 - 36 inches. I might even go deeper and have a full basement. There are things for which I would now find a basement very useful. Batteries, water cistern, ....
Just because something has been done and has not failed, doesn’t mean it is good design.

 

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