Foundation on Expansive Soil - PTI vs Safe

[NFExp]

I have been working on foundation design for a 4-story wood frame multi-family design on a very high plastic soil. The geotech has given the following parameters for the soil.

Bearing pressure = 2500 psf
PTI Parmeters
Center em = 8.3', ym = -2.2"
Edge em = 5.1', ym = 1.8"
ks = 125pci
Differential Settlement = 1:15

The exterior loading on the wall is fairly light (3500plf).

The contractor is a decent sized company with a lot of local projects. The geotech and the contractor claim that all the projects they work on are a 5" slab with some conventional reinforcement and "top steel".

WE have designed the slab with 2 methods. Both designs are much more robust than the contractors experience. The first method we used is the PTI method, which gave us the following design (post tension 4" slab with 18" x 31" ribs at 8' on center or a 10" equivalent slab).

The 2nd method, we modeled the building in SAFE. In the model, we removed the outer 5' of soil supports in an attempt to model the "Center lift" condition. Our model cantilevers the building 5'. As you can imagine, in order to get the model to work, the slab becomes very large. Even with the large slab, at the corners and jogs, the bearing pressure is higher than allowable. We tried flat slabs and ribbed design. All our designs were either not practical or bearing pressure was exceeded.

My questions are as follows:
1.) Can we use the PTI method even though the PTI method does not consider corner and building jogs?
2.) Are these soil parameters compatible with a shallow foundation or is a deep foundation required?
3.) Is our Safe model too conservative?

FYI, we asked the geotech to confirm his numbers and he stood by his original report.

Thanks for your feedback.

 

[JedClampett]

I don't know anything about prestressed slabs. But if your numbers come out different than the Geotech's and you're sealing the slab drawings, you need to stick to your guns. Or they can seal the drawings.

 

[KootK]

Even with the large slab, at the corners and jogs, the bearing pressure is higher than allowable.

I would expect allowable stresses to be exceeded, and redistributed, locally. Would that improve your results significantly? As for the specific questions:

1) I would say so. All buildings have corners and most have jogs.
2) The soil parameters don't bother me.
3) I suspect so.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bhiggins]

I've tried in the past to model the center lift condition in RISA foundation for a conventionally reinforced slab on plastic soils. The software was giving me #8 beam reinforcement so I suspect that this approach is highly conservative, especially at corners where you would have a double cantilever condition.

Here is a tutorial of the proper way to model a PT slab in adapt-SOG. The process is much more involved than simply modeling a lack of support at edges, you need to follow the PTI flowchart.

https://www.youtube.com/watch?v=ZQqdJUJO270

With your PTI beam spacing I suspect you have around 60 PI value. I wouldn't mess around with that and be influenced by the contractors.

 

[dcarr82775]

ADAPT has a module that will do post tension slabs on ground. I used it a couple times and it did a nice job on the center lift condition. Edge lift didn't work with a darn. When I did these types of foundations I ended up using the PTI values for the typical building, but at oddities (corners, or really large point loads at the perimeter) I would use that ADAPT module. I had to use deeper beams at the corners, matching frost depth instead of the shallower interior ribs. These deeper beams would extend a bay or two in from the exterior. That was the only way I could get the corner stresses down to calc out. Given the building designs I often had some very large point (30 kips service) loads along the front side. In those instances the deeper ribs extended into the building and often had 3-#7 in the top. In the end you do what you think is right, which is clearly not a 5 inch slab

 

[Huevo]

You could push back on the Geotechnical Engineer for more soil modification.

PTI DC10.5-12 R6.1.3 "Geotechnical approaches should reduce y[sub]m-center[/sub] to less than 2.0 in. and y[sub]m-edge[/sub] to less than 1.0 in."

 

[JoelTXCive]

I'm in Houston. We have highly expansive soils and your design seems very reasonable to me.

26" deep beams are a starting point in my book, so 31" is certainly reasonable given the soil conditions.

Maybe you could explore reducing the interior beam widths if you do not need the bearing capacity there?

If you want to triple check your design, you can download the WRI procedure for free here:

Link

http://wri.support/TF700-R-07.pdf

 

[steelnz2003]

If I got what I mean correct that you drop the edge the 5' this of course will not provide a correct result the soil profile follows a parabola so you need to match the soil profile in safe to get a realistic results

 

[NFExp]

All

Thanks for the feedback. We have decided to go with a 10” flat slab. Our slab meets the output from the PTI method. Our design did not satisfy our SAFE model but it seems that most engineers agree that he model was conservative.

The 10” slab did have the benefit of eliminating almost all the interior footings.

 

[steelnz2003]

Refer to AS2870 and Branz study report 120A (2008) that will provide you will good guidance
Refer to Mitchell method and design software slog
Subset national a deep perimeter footing will solve the issue