Foundation Design Approach of Past, Present and Future
Foundation Design Approach of Past, Present and Future
(OP)
Colleagues:
Ran into an interesting paper the other day - on the net but protected. Please refer to Geo-Strata Magazine, November/December 2008 issue. The paper of the Subject title was written by Dr. Fellenius.
He puts forth the idea that many of us have - that capacity, except in few instances of very soft to soft clays is not the relevant design characteristic to consider - but that design should be based on serviceability issues, i.e., permissible settlements, rotations, etc. Many of us have basically said the same thing in many of the threads. This leads one to wondering why design codes put so much emphasis on capacity determinations and so little on serviceability - i.e., AASHTO LRFD says to use Hough's equations for settlement (yet in some 15 text references I have with me in Indonesia, none of them mention Hough (I know, OldestGuy, you like Hough - and perhaps rightly so)) So few codes, that I have seen have given any more than a scant sentence to interaction of adjacent footings - concentrating only on single footings unencumbered by anything nearby.
Let us know your thoughts on Dr. Fellenius' article!
Ran into an interesting paper the other day - on the net but protected. Please refer to Geo-Strata Magazine, November/December 2008 issue. The paper of the Subject title was written by Dr. Fellenius.
He puts forth the idea that many of us have - that capacity, except in few instances of very soft to soft clays is not the relevant design characteristic to consider - but that design should be based on serviceability issues, i.e., permissible settlements, rotations, etc. Many of us have basically said the same thing in many of the threads. This leads one to wondering why design codes put so much emphasis on capacity determinations and so little on serviceability - i.e., AASHTO LRFD says to use Hough's equations for settlement (yet in some 15 text references I have with me in Indonesia, none of them mention Hough (I know, OldestGuy, you like Hough - and perhaps rightly so)) So few codes, that I have seen have given any more than a scant sentence to interaction of adjacent footings - concentrating only on single footings unencumbered by anything nearby.
Let us know your thoughts on Dr. Fellenius' article!





RE: Foundation Design Approach of Past, Present and Future
http://www.fellenius.net/papers.html
I have not read this paper yet but I have read a lot of his other publications and have always found them to be very insightful and right on target.
RE: Foundation Design Approach of Past, Present and Future
I found the paper at this link:
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RE: Foundation Design Approach of Past, Present and Future
But how to change codes to reflect this?
As a practical matter, I think we'll have to continue to address foundation design in terms of allowable bearing capacity until the building codes no longer reference it.
RE: Foundation Design Approach of Past, Present and Future
Going to read his paper and I'll be back later
RE: Foundation Design Approach of Past, Present and Future
First let me say that I agree with the serviceability approach for certain soil types.
Our fascination with load based designs I believe are based on the structural engineer or architect needs. Since our main concern is transmitting load, we, as an industry, have come to speak directly in those terms.
When serviceability issues arise, again, the SEs or AEs wanted an answer in terms of settlement for a given load.
Naturally, if you do this long enough someone is going to write a code with it as the main crux of the matter.
With the many years I have I have also learned the codes are, believe it or not, geared to simplicity so as not to overwhelm engineers with cumbersome, highly technical content. In this light, code writers examined the single variable and try to write all equations and or code material with that in mind.
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RE: Foundation Design Approach of Past, Present and Future
One of the points that seem out of line is forcing one to perform calculations for 4 or 5 load cases where, when compared to experience/judgment and subsequent computations, they are not critical since serviceability governs. Seems like a lot of wasted time. For me, in most cases, I use 2*Su for clays and seldom bother for granular. Of course, I hope I have enough experience and judgment to know when I MUST delve into the bearing capacity in a more depth.
RE: Foundation Design Approach of Past, Present and Future
As an example, the governing factor for the foundation design for a 200-foot tall stack was based on differential settlement - the differential settlement of the supporting mat that would result in the public's perception that the stack was "leaning too much." The same is true for earth dams with slopes constructed flatter than necessary because the visual appearance of the steeper slope is worrisome to the casual observing public.
Define serviceability, then analyze the problem within that context.
. enforcement personnel in many instances have neither the experience of interest in delving into the background of the code to understand when the coreviewr's
RE: Foundation Design Approach of Past, Present and Future
Who does the most writing and therefore gets recognized as "engineer of the year" etc, due to this "professional" activity and giving seminars. Therefore it is only natural for code writers to assume they are experts and should be relied on.
In my experience I have been to many a seminar by these so called experts. In most cases I have walked away wondering why I went, having learned nothing new.
The moist I have learned from others came from practicing engineers like Rutledge, etc. However which of them, besides Osterberg, are given credit for their achievments in codes?
It's the old professors with their many papers and books. Hough had a term for this: "diarrhea of the pen".
RE: Foundation Design Approach of Past, Present and Future
RE: Foundation Design Approach of Past, Present and Future
http:/
Besides, everywhere he doesn't loose the chance to expose his 'heretic' ideas about bearing capacity and the fallacy of modern codes such as LRFD and the Eurocodes.
I'd say he's not wrong on one side, on the other side:
- what about the velocity issue? I don't know if I'm saying some foolishness, but we might as well call a very fast settlement a bwearing capacity failure since the structure has no time to accomodate itself to the movement. The velocity variable is not contemplated in the paper figures
- While we have miriads of lab tests where a clear failure surface is visible (I like the Hill's surface pictured in the Lamb & Withman book) we have on 2-3 real size tests to show that there is not a phenomen which can be called failure. A tad to little to draw drastic conclusions as those illustrated by dr. Fellenius
- We all know that there are some parameters like fooundation width which govern the limit state which governs design; in example, in narrow footings bearing capacity calculations often govern whereas in wide footing it's the other way around.
- There are some foundations types where a failure by bearing capacity would be almost unheard of, like in big slabs, hi-coverage foundation grids and similar. However, since bearing failure entail potential loss of human life and substantial economic damage, codes impose the relevant calcs. Rationale is that probability of failure must be lamost nihil, and we have to show in our calcs that it is indeed like that
- what about design in seismic conditions, more parameter, sometimes with complex interactions show up so it's not so simple to just say: settlements govern
Speaking of deep foundations, actually there is indeed this accepted, maybe warped logic where an index of bearing capacity failure is given by a settlement ratio (in relation to base dimensions). Again, maybe velocity of settlemnt might be the key factor of separation here, please tell me if what I'm talking nonsense.RE: Foundation Design Approach of Past, Present and Future
it is not totally true that the geotech community is still standing at the traditional, 'outdated' 3-terms formula.
Since about 10 years has been available an interesting solution, which also appears as an attachment to Eurocode8 - 5 (the seismic part of the European building code), known as the Paolucci-Pecker formula (I can see BigH smiling):
http:
also please see Gazetas on seismic bearing capacity issues
http://www.ecees.org/Paper/paper4007.pdf
Here, again failure is defined as 'unacceptable permanent displacements' so 2 conditions are required:
1- deformation must not be elastic, i.e.: irreversible
2- deformation must not be small compared to structural capability to withstand settlements
RE: Foundation Design Approach of Past, Present and Future