Question - Existing slab on grade analysis

[PE_JRM]

I am renovating an old cabin (DIY for myself) and having some trouble determining if the existing slab is adequate to support what I want to do. The existing slab was not well implimented. The existing slab is 4" thick, old enough (1965) to be only 3000 psi, probably unreinforced, no thickened perimiter, sort of level and flat and no sub-base. 34x20 feet dimensions. Poured directly on stiff clay with about 1500 to 2000 psf bearing). The only thing the slab has going for it is that it is uncracked after 60 years although it is lightly loaded.

For various reasons I want raise the floor a few feet by building a wood framed structure above the slab, essentially making the slab the floor of a crawl space.

Framing this conventionally with long beams and joists with edge supports exceeds the strength of the slab. This could be dealt with by placing a square footing under each beam support but this is expensive and a lot of work if I do it myself. This approach is probably the simplest and best solution but I'm looking for a way to transfer the load to the slab in a distributed fashion.

I've been using excel spreadsheet GRDSLAB and BOEF as well as LISA FEM (free limited node license) to compare various approaches. Due to the limited nodes and uncertainty in the soil parameters I don't really trust the FEM analysis for much more that a gross understanding of where the load is going. I've been using it as a gauge of the practicality of various ideas.

The idea I'm currently exploring is a grid...beams supported every few feet (maybe 4'?) and perpendicular joists 16" OC. This would give me a 4'x4' group of point loads on the interior region of the slab. The slab edge load can be controlled by the edge spacing around the grid. I've been referring to 1) Slab thickness design for industrial floors on grade and 2) ACI 360R-06. The issue I'm having with analysis is that I'm off the nomographs for these references, probably because industrial slabs are typically thicker than 4". I've also referred to the OLD Army manual TM 5-809-1/AFM 88-3, Chap. 15.

I'm a retired structural engineer (PE), although concrete was never my specialty. Any ideas you might have that I could use to show my existing slab good with a bunch of point loads on a grid would be appreciated. I'd like to keep the solution simple and something I can do with a calculator, pad and pencil since my buget for the project is small. This solution is kind of fiddly but its worth considering.

Another idea I've considered is to pour some load distributing reinforced pads (2'x2') on top of the existing slab and just let the existing slab crack (could saw cut around pads) if it wants to. These pads would be for the beam supports. I don't really like this solution but it has its merits as being the easiest to DIY impliment and probably the cheapest.

 

[SWComposites]

Why not just pour new footings along the 34' edges and span joists across the 20' length? Sitting a bunch of wood beams directly on the slab sounds like a good way to get them to rot.

 

[dik]

If it's uncracked for 60 years, can you just leave it?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[PE_JRM]

Replying to both responses so far. SWcomposites: Its on land leased from the federal government at a Corps of Engineers lake and I'm not allowed to increase the footprint. I have three pages of federal restrictions that I have to comply with. You may have misread my post. I'm talking about supporting beams on posts (like a deck) on top of the existing slab but a few feet above it. Agree that beams resting directly on the slab is a recipe for rot and mold.

dik: The slab is at grade or a little below it in places and I'm not allowed to change it since its on leased federal property. I am also not allowed to mess around with the grade.

I'm working with what I am allowed to do and my post and request for thoughts is based on that. Basically , as long as I stay within the existing footprint AND don't tear down the building or mess with the grade I can do what I want.

Essentially, this post is for the creative out of the box thinkers. Standard solutions aren't going to work here. The property is zoned agricultural so code compliance and standard practice is much less of an issue.

 

[lexpatrie]

If the concern is punching shear from point loads I've seen something in Structure magazine some time ago regarding load limits.

 

[Tomfh]

Build it like you would a house.

Perimeter masonry walls with engaged piers supporting the ends of the beams.

Internal masonry piers to support the beams internally, or simply build walls to support your joints.

If doing piers then 4’ should be fine.

You may struggle to get those industrial floor guidelines to work, as they are designed for genuine point loads. Forklifts, warehouse racking eg.

Just keep the bearing stress in your piers and walls below allowable soil bearing stress and it will be fine. You can spread the load out a little into the slab.

 

[XR250]

If you have a stiff clay subgrade, I would not worry too much about a distributed loading. I normally design pony walls every 8 ft. to support the joists. The load only ends up being like 400 PLF max. By the time it spreads out thru the 4" slab, it is about 400 PSF soil loading - which is minimal.

 

[TRAK.Structural]

Agree with others here about pony walls or piers to help distribute the load.

If you are against those options for whatever reason I don't see anything wrong with conc pads or continuous curbs placed on top of the existing slab to accomplish the same load distribution. Just use something reasonable for thickness and reinf to justify that the new curbs can indeed spread the load.

 

[PE_JRM]

lexpatrie, I'm not concerned about punching shear. Tomfh, XR250 and TRAK pony walls and piers or a combination is what I would like to do. However, CMU walls are pretty heavy. I've already considered that option. The first cut at my loads has me at about 350 plf maximum on one interior wall and about 250 plf around the perimeter.

GRDSLAB which is pretty conservative and old school tells me that the allowable wall load at the edge of the slab is 256 plf and 330 plf away from the edges. A 2 foot high CMU pony wall puts me over the allowable around the perimeter and I'm already over it for atleast one interior wall.. The allowable wall load figures are based on a factor of safety of 5.6 (recommended by westergard *I think) a long long time ago), 3000 psi concrete, 4" slab and a soil modulus of 50 pci. I don't know the soil modulus and 50 pci is the bottom of the range for clay. The slab isn't well constructed. Its an obvious DIY and 1/3 third of it is a porch that is spalling. Here in Atlanta where we don't get much freeze thaw action. This tells me it was placed with too much water in the mix ... typical DIY nistake. So the concrete may not be sound meet the 3000 psi I've assumed. Based on my observations that portion of the slab isn't to be trusted structurally.

I've never designed anything with a factor of safety of over 5. But with the uncertainties I don't think its that outlandish.

In writing this post I've collected my thoughts and think that my best option is pouring reinforced pads right on top the existing slab. These pads would be coupled with CMU piers. Around the perimeter I can fill between the piers with an apron of some sort. This solution would be cheap, easy to do myself and if I saw kerf the existing slab around the pads I'm not relying on it to do anything structural. The only downside I can think of is that I might get some differential settlement from concentrating the loads on piers. That said, although I'm designing the floors for 50 psf and the roof for 20 psf its unlikely the building will ever get loaded that heavily.

 

[jhnblgr]

Doesn’t really seem to be an issue here, simplest thing would be to just frame up a wood wall to hold wood floor joists.
Seems like you are over thinking the issue.

 

[XR250]

Another idea I've considered is to pour some load distributing reinforced pads (2'x2') on top of the existing slab and just let the existing slab crack (could saw cut around pads) if it wants to. These pads would be for the beam supports. I don't really like this solution but it has its merits as being the easiest to DIY impliment and probably the cheapest.

Can't imagine this being more economical and effective than wood framed pony walls as I and others have suggested.

 

[Denial]

If you have been using Beam-On-Elastic-Foundation type analyses as part of your investigations, you might be interested in a pair of spreadsheets I have developed for Slab-On-Elastic-Foundation analyses.[ ] One of these analyses an infinite slab with loadings on its interior.[ ] The other analyses a semi-infinite slab with loadings on or close to its edge.[ ] These are freely available to be downloaded from my website

https://rmniall.com

[sub][ ]—————————————————————————————————[/sub]
[sup]Engineering mathematician / analyst.[ ] See my profile for more details.[/sup]

 

[PE_JRM]

jhnblgr and xr250. Yes wood framed pony walls are a cheap and easy solution if the load on them didn't overstress the slab. I thought about wood framed pony walls months ago but there are issues with that. The crude analysis techniques I have access to in retirement don't really cover my worst case issue of a 350 plf wall intersecting a 250 plf perimeter wall. I have no way to check that intersection. However, the crude tool that I have tells me that the perimeter wall and the interior wall are both right at the allowable load. I'm also concerned about the corners. BTW, I'm using ASD analysis because that what the spreadsheet uses.

A few things I didn't include in my origional post. 1) The edges of the slab are undermined a few inches all the way around the perimeter. 2) The slab is not well consolidated - the edges are full of voids. 3) Part of the slab was once a porch and it is in poor shape. So, based on 1) 2) and 3) I'm a little iffy on relying on the strength of the slab which is stressed right up to the "allowable" based on GRDSLAB. 4) Two of the 4 perimeter walls are far enought below grade that they flood (not a lot but enough to never dry out) when it rains. The sill plate on both of those walls is completely rotted. So a good portion of the pony walls will be wet since I'm not allowed to regrade. Even if they are PT wood thats a recipe for mold. True I can put the pony walls on a curb but I'm sure you're aware that you cant keep water out. Once it gets past the curb its only way out is to evaporate. So I'm not convinced that pony walls are a great solution for many reasons although I agree that they would be cheap and easy.

It seems that both of you have experience with construction of this type. My construction experience is with water and oil storage, pressure vessls, smoke stacks, offshore vessels, marine structures and other steel stuff(Chicago Bridge & Iron Co.). I have no gut feel for house foundations. All the same priciples apply but I never did dirt. I am perhaps overthinking it as you say because I am the end customer and if I screw up the foundation I have no recourse other than spending more money to fix it.

 

[PE_JRM]

Denial: Thanks for the link. I'll take a look at it. I don't really have good soil data so I've only been using beam on elastic foundation as a sanity check.

 

[SWComposites]

JRM - is your plan to (somehow) jack up the existing cabin and put new walls/blocks/piers under it?
Or are you allowed to tear the cabin down to the slab and rebuild?
And is this in a remote area where no one will actually inspect it?
Can you shoot grout under the existing slab to fill in the undermined void areas?

 

[PE_JRM]

swcomposites: yes, I plan to jack the cabin up a few feet and drop it back down onto the raised platform. The cabin is on Federal property that is leased from the Army Corps of Engineers. I am not allowed to tear it down and anything I do needs to be within the existing footprint. The property is currently zoned agricultural but the Corps is considering a propsal for a land release allowing me to buy the land. If that happens the zoning may change so I want to hurry up and do whatever I'm going to do. Right now I don't need a permit but if the land release goes through I would like anything I do to meet code. The foundation is a problem since it doesn't meet code. The voids are in the concrete itself, not underneath the slab. Interesting that you are in aerospace. After about 10 years working in heavy construction I went back to school and specialized in aerospace structures. Airplanes and offshore platforms have a good bit in common but they aren't the same. Thus, my comments a few hours ago about having no gut feel for foundations.

 

[SWComposites]

Here’s another suggestion - assuming the existing cabin is one story and not going to add another - build some formwork around the existing slab, 4-6” higher than the slab. Put some wire mesh on chairs on the existing slab. Pour 4-6 inch thick new slab. Then use filled CMU to build piers at appropriate spacing. Set cabin down on new piers.

(there’s a saying in the aircraft Stress community, “when in doubt, make it stout”)

 

[PE_JRM]

swcomposites: I already explored the slab topper idea and I haven't ruled it out but it has some disadvantages. The raised floor allows me to run plumbing, hvac and electrical anywhere I want to place it. I'm changing the floor plan dramatically so that is a big advantage. Also, I'm trying to DIY most of this to keep the cost down. I can't DIY a 20x34 slab. The other disadvantage is that the existing slab doesn't meet code. Local code (Atlanta. GA) requires that the edge of the slab is thickened and reinforced. I've just got a flat 4" unreinforced slab without a thickened perimeter.

 

[jhnblgr]

I would say you an underestimating the capacity of the slab. The perimeter if there are voids due to washout I would fill them with grout. Run multiple walls and cantilever the perimeter to reduce edge loads. Use naturally rot resistant wood species. Ventilation of the area below is important.

 

[PE_JRM]

jhnblgr: I agree that I am underestimating the capacity of the slab. Ideas for an analysis tool that I can use was the reason/motivation for this post. So far I haven't gotten any any input on that. I've gotten a lot of suggestions and ideas I've already thought of. Modern analysis tools are extremely expensive....way out of my budget. Old school tools are almost always very conservative.

Do you know of a means by which I can estimate the slab strength without spending a lot on something I'll never use again?

Remember, too, that there is no gravel between the slab and soil and all I know about the soil is that it is a red clay typical of the Atlanta area. Also, among all the other quality issues I've mentioned there is no vapor barrier. That's not really a strength issue but it further adds to the arguement that this slab wasn't done professionally.