Retro fitting a post and pier foundation

[bn031648]

I purchased a lake cabin in December, cool right?  Well.... not exactly.  The cabin that was placed on the property is on a slope and is supported by cinder blocks.  The cinder block supports started to shift and it really became a concern, if it's not addressed I will have a house boat soon.  I decided to retrofit a foundation for it.  Since I couldn't get under the cabin to drill the holes, I got as close as I could.  The cabin is 12x36ft and the supports are 6ft on center down the length of the cabin.


The iffy cinder block supports


The supports I poured as close to the cabin as I could get with the skid steer (the other supports to the left are for a deck/walkway)


I have 2 major issues.  One is beam size/support placement and the other is racking.  So here goes...

I had originally planned to use 3 ply 2x10 for the beams (7 beams total), however as the cinder block supports started to shift and they sunk into the ground enough to where the 2x10s will no longer fit underneath at the back.  I would have to raise the entire cabin just over 3/4 of an inch to get them to fit.  I'm working here by myself and I deem it too dangerous.  So I've decided to try and use 3 ply 2x8s.  I have enough space (as of now) to fit them between the supports and the cabin.  So using using the calculator supplied on CountryPlans I came up with the following.

sq ft: 432,
span of beam 84"
sq ft divided into 6 parts 72 sq ft.
total floor load 55 psf.
so 72 sq ft x 55 psf is  3960 lbs on each beam

These are the number I input into the calculator using SYP, the calculator says everythign passed but the way I read it, it didn't pass by much.  Is this correct?




One oddity about this cabin is that the joists will not be supported directly by the beams, the joists are on 'runners' and the 4x4 runners are supported by the beams if that makes any difference.  This next image is what it would look like if I ran a support down the middle of the cabin.  The supports in red are the supports that has already been poured. 



So if I'm near the max of the 3 ply 2x8s, would  2 supports under interior of the cabin be best?




As far as racking goes, the back of the cabin is connected directly to the concrete supports so I think that's good (isn't it?), the front of the cabin is about 5ft high, I'm highly concerned about the wind blowing it back and forth and the 6x6 posts that I'll use will end up racking and everything falls down and slides into the lake.  My plan was to cross brace the front 2 rows of supports with a torsion type of wall.  I have made torsion boxes before and they are really stiff.  Obviously this isn't the same type of application but do you think it would work?


both sides of the studs would be skinned with 3/4" PT ply, I removed the near ply for clarity.

If you go this far thanks for reading. 

[MountainDon]

First, Hi there. Also note I am not a professional engineer nor a licensed contractor.

Before getting into the "nuts and bolts" where did the 55 PSF figure come from?

Even in areas with no snow it is good practice to use a roof load (LL= live load) of 20 PSF as a minimum. That helps take wind, rain, and so on into account. DL (dead load) for the actual roof materials is normally counted at a minimum of 10 PSF and could easily be 15 or 20 PSF depending on variables regarding the roof structure.

A floor load for normal residential living spaces is 40 PSF, with the IRC allowing a reduction to 30 PSF for sleeping areas. Those are design loads and cabins without the large refrigerators, etc that are found in a typical residence may not come near the 40 or 30 PSF. Then there are the DL for the main floor and walls. The materials for a wood framed floor, subfloor, carpet or vinyl can be 10 PSF. Wood finish flooring may add a couple PSF to that, tile can add 3 to 5 PSF.

Is there an attic floor? Is that upper space used as living or sleeping or storage space, and have those loads been counted?

Exterior wall framing, sheathing, exterior siding, windows can  8 to 10 PSF.

So, before going any further I am wondering how accurate the 55 PSF figure is.

*********************

You are absolutely right with the concerns about lateral forces from wind having the potential to cause trouble. The construction of double faced shear walls across the width is probably a good idea. Probably also a good idea to have some to deal with forces down the length of the structure.

One of the problems with a foundation that has concrete piers with some length of pier/post between the concrete and the floor box (beams, joists, subfloor unit) is that there is a connection at the concrete pier top and another at the point where the post is attached to the floor box. Each of those connections can be a hinge point.  To picture what I mean take a short dowel, pencil or straw. Place one end down on the table and place your finger on top. You can easily move your finger in any direction. Yes nails or bolts at each connection make it less shaky. But nails bend, wood can split.....

Where are you located? Many areas in the US are in seismic zones. Some seismic info and maps may be found here 

[MountainDon]

One other thing... where did the values used in the timbertoolbox beam calculator originate... and/or what grade of southern pine and were the correct, post June 2013 values used? The Modulus of Elasticity (E) and Fb  appear to be a little high, maybe... 

I may not be understanding the 84" beam span...  Is that the span used across the 12 foot width with a pier in the center?   Is that center pier going to have the same bearing capacity as the new outboard piers?  And what about the logistics or placing those central piers... how are they going to be dug, what size footing, etc....

How deep are the new outboard piers.

Sorry, lots of questions I know and yet no answers. 

PS... when using the timbertoolbox calculator, I'm pretty certain the value for the field "Modulus of Elasticity (million PSI)" would be entered as 1.4, if the value was 1,400,000 PSI.  I don't understand how the table performs the math but the results fields all provide the correct figures except for the "Deflection (inches)" field, when the "E" value is entered as 1600000 instead of 1.6      Notice that the deflection is shown to be 1.350116855959654e-7    The  "e-7" at the end is an error or something... maybe denoting not enough or too many decimal points.  Anyhow, the calculator seems to ignore that except for that field, but it does come up with a correct PASS or FAIL in the "Deflection Pass/Fail" field.  If the "E" value is entered as 1.6 note the actual deflection is then listed as 0.13501168559596538" which is correct.      The short response to what I'm getting at is that because the table asks for the value of "E" in millions, weshould enter 1.6  for the value of 1,600,000      I hope that is understandable.

[bn031648]

Thank you MountianDon for reading my post.  I got the load values and SYP values from one of your old posts from May of 2012.
https://countryplans.com/smf/index.php?topic=12159.0

******************************************
Let's see what I can do...

Let's call that a 32 x 42 building; that = 1344 sq ft. 
Divide that into three parts = 448 sq ft per section
Let's call that 450 sq ft.
If we call the LL to be 40 PSF, let's call the DL out at 15 PSF.
That's a floor load total of 55 PSF
450 x 55 = 24750 on each floor segment.
Half of that will rest on the center beam and a quarter on each side wall.
24750 / 2 = 12375 on the center beam for each segment.

Using Don_P's simple beam calculator and the following data for SYP...
Modulus of Elasticity (E)   1600000 psi
Bending Strength (Fb)   1120
Shear Strength (Fv)   175 psi
(the calc is fixed to use a value of L/360 on floors)
But let's see what happens when we use...
a beam length of 13'4"
and 12500 lbs as the beam load
************************************************

After reading through that thread again, I think I inferred incorrectly.  Using the values you gave me for beam load

roof live load   20
roof materials   10
floor load   40
dead load main floor   10
exterior wall materials   10
   
total    90

That obviously changes thing a lot.  Using 90 psf, everything fails using 2x8 (3 ply) at 84".  84" is the distance from the already poured outer piers to the proposed center pier support.  If I used 2 rows of pier supports under the interior of the cabin that distance is 51".

I also looked up the updated values for Southern Pine ((number 2) (at least what I think are the values, I'm not familiar with all this elasticity and shear talk))


   southernpine.com (2013)   NDS 2018
bending   925                           1100
elasiticity   1.4                           1.4
shear   175                           175

As best as I can tell southernpine.com and NDS 2018 give different values for bending.  That difference in bending value is also the difference between a 'pass' and a 'fail' on the fiberstress in bending portion of the calculator when the distance is 51".


fail


pass

I'm located in Texas, I did not see my area on the seismic link you provided thankfully. 

The upper area if for sleeping loft.

The outboard piers are 48" deep.  The interior piers will be dug by hand, I'm thinking 12-18" deep with.  The supports in the back the concrete will meet the beam (attached by Simpson Strongtie connector), the supports in the front I'll run a 6x6" PT post up to the beam (also using Simpson connector on top and bottom of the post).

If the number I input into the calculator are correct, then it would seem I'll have to use 2 rows of interior supports with 3 ply 2x8s.  Also build the double faced sheer wall with cross bracing down the length probably using 2x6s or 2x8s for anti-racking.

[bn031648]

Shoot.... I know the pictures I posted show I used 1,400,000 for modulus of elasticity but I also used 1.4 and got the same results.  I just posted the wrong screen shots.

[MountainDon]

In 2013 the values for SP were derated as it was recognized that modern plantation grown SP was not as good as older slower growth SP.   That and the fact that in 2012 I had not learned as much as I have today      I wasn't an engineer back then either.  I didn't read that entire old thread but it does appear whatever we were doing we were only counting LL and DL for the floor and "never mind about the roof". 

I'll look this over later today and comment.   I must admit my first thought when seeing the images was that this was a scary build.

[bn031648]

Thank you for your reply MountianDon,

..."my first thought when seeing the images was that this was a scary build."  Well that's not good.

Oh look, an emoji that depicts my lake cabin dreams...

[MountainDon]

Since SP was re-evaluated for June 2013, there are more grades than for most other woods; they have dense and non-dense in addition to the numbered grades. Where I am we don't see much SP unless it is PT. SP is a very good wood to use for PT as it accepts the chemicals better than some woods. So, I am not familiar with what would be most likely to be found in the local lumber store.

One way to easily find the values needed is to run the size in question through the AWC joist calculator. Find it here. They have a smartphone app too which is handy.  Use the drop-down menus, select sizes and grades, run the calculator and the results table gives the E and Fb numbers as well as what the joist could span. Those E and Fb numbers can then be loaded into the timbertoolbox beam and column calculator. I find that easier than looking up the NDS info.

Yes, a twin row of inside piers would solve the beam load carrying capacity. I suppose if the hillside does not erode to effectively make the concrete piers less deep in the earth that may work. Nature does try her best to wear down the high spots, wash dirt and stone down slopes and fill in the lower spots.

Is there any reason you can't use 2x8 only where necessary because of the limited vertical space, but use 2x10 or 2x12 further down the slope?

[Dave Sparks]

Oh look, an emoji that depicts my lake cabin dreams...      

   NICE !

[GaryT]

helical piers (aka screw piles) can be used to correct a foundation problem:  https://www.goliathtechpiles.com/applications-en/underpinning-2/

Gary

[Bladesmith]

I'm not sure, but are the piers buckling in all directions  or are they leaning, all in one direction..?