raft foundation design 3

The foundation is where soil-structure interaction happens; hence it is a difficult effort to get any proper "modern" modeling of a separate foundation, mat or whatever, that doesn't include the structure above itself.

At the present moment the industry seems be taking the stance that modeling such structure on soil springs is the way to go. This should be bettered to grasp the nonlinear responses expected to occur at limit states, where linear springs wouldn't give an accurate model. So you need essentially a FEM program with sound plates or solid brick/tetrahedral elements and the ability to properly model support. Most surely the big FEM programs such ANSYS and ABAQUS are well suited to do that, but are a bit off-the-mark of what most structural designers do. So a program like SAP 2000 may be better suited to the generality of the intent, having built-in combinations and plenty of codes within.

Particular programs may excel in some aspects of this kind of design. For example, CYPECAD has great look output (reinforcement itself) and easy input (you model the whole building. You have also a number of codes within. The inbuilt analysis capacities, even if safe and enough for the ordinary practice I wouldn't call "state of the art". For example, I understand that till recently plates were modeled by an irregular grill of interconnected line elements.

At the present moment the industry seems be taking the stance that modeling such structure on soil springs is the way to go.

Click to expand...

The geotechnical engineering community does not agree (well at least my voice). Structural engineers like to look at the spring constant as the solution to deformations as it's worked for slab on grade design. However, a soil raft is managing areal loads that result in stress increases to depths. Soil modulus values vary with confining stress (hyperbolic moduli) or you may have soil layers that are compressible (i.e., influenced by the soil's ability to move water). The spring analogy doesn't effectivelly work on these sorts of problems, irrespective of draft ACI committee recommendations to use some magical "long-term k" value.

Do a proper geotechnical engineering study, look at the appropriate soil behavoir and run the numbers. A spreadsheet may help, but first and foremost, understand how to do this on paper.

(rant over)

f-d

¡papá gordo ain’t no madre flaca!

Try out RISAFoundation. It's pretty good for the design of structural slabs on grade.

Fattdad has some good points related to the geotechnical aspects of the site. But, I'm not sure those are structural issues. To me that is something where you get the recomendations from the geotech about what sort of foundation you're allowed to use and what sort of differential settlement you can expect.

Then you design the structural aspects of your slab within those guidelines.

Lastly, I should point out there is a decent ACI document (335.2R-88) which talks about how to do an FEM analysis on a mat foundation which partially accounts for the cupping or dishing effects of the soils. It's not exactly light summer reading. But, if you get really into Mat design, then it has a number of good suggestions.

The problem with the geotechnical question, "what sort of foundation should you use?" is a geotechnical engineer may conosider 3,000 psf for a 300 kip column and you'd come up with a 10x10 footing. Now the owner comes up with the notion that they need a distributed floor load of 1,000 psf for their product inventory or such (big racking system). You, the structural engineer, feel as if the "bearing capacity" is 3,000 psf, this shouldn't be a problem. But it is. . .

A 10x10 ft footing at 3,000 psf may result stresses exceeding 900 psf within the upper 5 to 10 ft of the bearing grade (I didn't do the calculation right now). However, a 200x200 ft area loaded to 1,000 psf will result in stresses exceeding 900 psf to depths of maybe like 30 or 40 ft (conceptually). This is where the intersect lies between the structrural and geotechnical engineer. You may then integrate the column into the mat and say, voilla, my FEM model show no place where contact pressures exceed 3,000 psf, so I'm good. Well you may not be good.

I think that ACI needs to better address this and so far the "solution" of "long-term" k doesn't do it.

f-d

¡papá gordo ain’t no madre flaca!

i am interested in RISA foundation ? who knows about this program? who has used it ? if someone has a book or any digital filea about raft foundation (even if it looks simple and basic) please send it to me this will be helpful. thankssss

A demo version of the RISAFoundation program can be downloaded (after filling out some basic sales contact information) from the following web site:


The program is geared towards the design of structural slabs on grade. But, it's also works for the design for combined footings, or single spread footings.

That same spot where you dowload the demo, you should be able to download a simple tutorial that goes over the interface and the basic features of the program. Tutorial was originally written for an older version of the program and needs to be updated. Still, it still shows you most of the basics and will help a new user get started.

Keep in mind, of course, that this program is geared towards structural engineering assumptions for foundations. That means the true geotechnical properties are generally simplified to the type of input that a structural engineer would typically use for the design of their foundations.

I entirely agree with fattdad in that just spring models do not work properly particularly for dishing action, edge or surrounding effects (not to add actual nonlinear or irregular behaviour). I was mostly saying (and implicitly subjecting to critic) the fact that, in effect, a modulus of subgrade reaction is (here at least) being proposed (even by geotecnicals) as what you are expected to be doing ... not even a Young modulus is given, many times.

I also entirely agree that considering (in my view, too much shortsightedly) each footing separatedly os entirely inadequate; I had in Mathcad the opportunity to integrate the effects of foundations for the same set of flexible loads on (floating) piles, grill, footing and mat on elastic halfspace and overall settlement didn't differ but about maybe 10%. Hence, what matters for settlement issued where controlled by an elastic behaviour is Magnitude AND Position of the loads. The only modifier to that is stiffness added by the foundation and superstructure in its interaction with the ground. So if you want any degree of accurate evaluation of settlement you need to go for the whole set of footings or applied loads, never a footing by itself.

Contrarily, this also must expose other blatant misconception on these kind of works, that is: that, except for very good soils, you need to extend the depth of the tests far less for footing works than for mats. Doing so you forfeit the required info for proper evaluation of the settlements, and you will be getting unconservative appraisal of the settlements on dishing action and edge effects. Remember, the pressures exerted by individual footings and piles integrate in the terrain and subject it to one overall average "same" field of new behaviour, hence you getting akin settlements at the interface.

All what above subject and referred to the particularities of the foundation systems themselves, of course.

errata


... is entirely inadequate...

... what matters for settlement issues ...

let's just say the geotechnical engineer can have a real bad "interaction" with the structural engineer by giving out a spring constant as elucidated in this valuable thread. geotechnical engineers need to be involved in the design which is an iterative process and to follow up with a settlement and global stability analysis of the final designed structure.

Geomatrix will send you this paper (google search) - primary author was Jim French

Clarifying the Application of Subgrade Modulus in Structural Analysis and Design

It's very helpful in translating between structural and geotechnical engineers. I've had to send it to several structural engineers who wanted to use the modulus of soil reaction for a CL right out of the FAA chart.

Regarding Ishvaag's and Fatdad's comments about dishing and cupping effects.

There are ways to handle the node coupling effects that produce the dishing and cupping that you are referring to. The ACI report 336.2R titled Suggested Analysis and Design Procedures for Combined Footings and Mats has some good discussion and recommendations about this.

One method the report mentions (the one that is easiest to accomodate within a typical FEM application) is to adjust the modulus of subgrade reactions based on different "zones" in the foundation and based on the type of soil that you have.