Foundation Rigidity and Settlement

[NBRY1]

Is it reasonable to use rigidity factors? I would like to hear back from engineering community on this.
Some texts say they can be used if the site is well understood; but most engineers ignore for conservatism.
I haven't seen anything stating they should not be used.

 

[oldestguy]

I've been around for a long time and that term doesn't ring any bell for me.

 

[NBRY1]

Here is the image I meant to add.

 

[Ron]

NBRY1...what is the source of that reference? It seems to conflict itself and uses the term "rigidity" loosely. As an example....a mat foundation is clearly more rigid than a shallow footing, either isolated or a strip.

I understand the concept you're trying...essentially that if a foundation has a greater stiffness, the potential for differential settlement is reduced because the foundation bends less. To me that's a non-conservative approach and if soil stresses reach a point of allowing the foundation to bend (break), you'll get a sudden settlement.

For me, I would ignore the premise. Keep the structural and geotechnical as independent parameters and handle each separately with consideration of their influence on each other.

It's a bit like measuring with a micrometer, marking with a crayon and cutting with an axe.

 

[NBRY1]

Ron,

The use of rigidity factors seems to be similar to the use of a characteristic point as described in
'Basics of Foundation Design'; by Fellenius. The concept seems appropriate and not overly conservative
in my opinion unless you can convince me that there will be more settlement at the center of a rigid
foundation than at the edge. That said, I am curious if anyone uses either technique to reduce estimated foundation settlements estimated under the center of a rigid foundation. The assumption would have to be
that the soils are fairly uniform in the influence zone; which is why Coduto indicates extensive site info. is recommended prior to using this technique.

 

[NBRY1]

I am curious if anyone uses these routinely for settlement calculations.
If someone needs more context, I'm guessing they are not using very much...….which answers my question.

The concept seems appropriate; just not sure why I don't hear of many who use this technique???

 

[NBRY1]

Characteristic Point_Red Book

 

[NBRY1]

Rigidity Effects_Coduto

 

[r13]

Now it is clear. I am not familiar with the methods, but I believe it is the typical practice of the geotechnical engineer to provide guidance on foundation design based on the loading and type of foundations. So for pile, spread footing, mat foundation, the structural guy will have different bearing strength for design, which has accounted for the settlement characteristics of the soil. I hope our geotechnical guys can chime in here to provide insights.

 

[EireChch]

Schmertmann 1978 method was developed for flexible footings, as such rigidity factors cant be applied. I use Schmertman for foundations with width less than 5m and general elastic theory for rafts with width an area greater than 5 x 5m.

 

[NBRY1]

EireChch,
I am unclear on why rigidity factors cannot be applied if using the Schmertmann method.
Wouldn't that be the point, to convert from flexible to rigid behavior??
Please provide rationale.

 

[Ron]

Agree with EireChch

Go back to my second sentence:

if a foundation has a greater stiffness, the potential for differential settlement is reduced because the foundation bends less. To me that's a non-conservative approach and if soil stresses reach a point of allowing the foundation to bend (break), you'll get a sudden settlement.

This is Schmertmann's premise, somewhat along the lines of his strain influence factor. He did all of his testing in sandy soils in Florida.

 

[NBRY1]

I'm not sure this answers the specific question. Are rigidity factors appropriate?
The answer is no if using schertmann's method...reason why (not sure), method is for flexible footings; why not reduce settlements for 4x4 rigid ftg using this method?
Not sure why rigidity factors would not apply to most footings; I can't envision a 4x4 ftg flexing that terribly much; if at all. So back to original question.

 

[SlideRuleEra]

I'm not familiar with rigidity factors. For certain foundations at our electric generating stations, having a rigid slab is essential for maintaining operating equipment alignment. Optimumal soil loading is secondary.

http://www.SlideRuleEra.net

 

[NBRY1]

Settlements estimated below the center of a flexible footing will be more than for a rigid footing. The rigidity factors account for this since the rigid footing cannot settle as much at the center, although it wants to. More info. on rigidity factors is attached.

I am still curious to know if anyone uses this technique to reduce settlement values estimated at the center of rigid foundations...mainly footings....not large mats.

 

[Ron]

NBRY1....keep in mind that structures act and react to loads without regard to our modeling them. By modeling or analysis, we cannot control the performance of structures, just give our best guess as to what they will do. That's why we use relatively conservative methods and we get by with them. If you think you can completely and accurately model how a foundation will perform in every case, you will be significantly disappointed.

Fine tuning foundation performance is not a ladder I'm willing to climb. That's why we use large factors of safety for bearing capacity and other soils parameters. Is there any advantage to you as an engineer to reduce the projected settlement of a footing? No. The standard of care you need to meet is what other engineers do.....the consensus here is NO for the factors.

In short, I would not apply rigidity factors. Have not in the past, won't in the future.

 

[NBRY1]

Thanks Ron,

I appreciate all of your posts. I also agree that trying to get accurate with settlement predictions is probably a lost cause.
With regard to rigidity factors, it doesn't appear that most are familiar with this concept; which is what I kind-of suspected.
They are presented often in texts and the theory appears valid.

 

[r13]

it doesn't appear that most are familiar with this concept; which is what I kind-of suspected.

Everybody has been through that curve and walked past it. Valid does not make it practically worthy, though important sometimes.

 

[ATSE]

Rigid factors and other coefficient methods are really only appropriate for initial design. Fellenius is a legend, but I would not use a coefficient method for fndn analysis.
For soft to stiff clay, assuming rigid is not conservative for contact soil bearing pressures and related total settlement (assuming your fndn aspect ratio (length / thickness) is greater than 5 or so).

To get more accurate structural results (one-way shear, two-way shear, flexure, and displacements), instead of using a coefficient method, consider using a simple linear FEA with vertical area springs that vary in stiffness (SAP2000 or RISA or STAAD). Springs at the center are weaker (more settlement).

For a clay with a concentrated loads near the center of the fndn (column on large spread or elevator core on large mat), the fndn will smile. One or two iterations will allow you to compare your FEA results with a bounded deflection shape (that is, you are curve fitting). Unless you have a sensitive model, changing the spring stiffness by 2x or 0.5x will not result in more than 10% change in results.

 

[NBRY1]

I am not chasing accuracy here.

I am trying to determine if the use of rigidity factors for reducing the settlement of a rigid foundation (where settlement was estimated at its center assuming the foundation was perfectly flexible)
is appropriate. Assuming somewhat uniform soils, the settlement of a perfectly flexible foundation will be more at the center than at the edges.
A rigid foundation does not behave in this manner; thus it seems logical to reduce the calculated settlement some if the settlement was estimated at the center assuming it was flexible.

There is no argument in this thread that to date that convinces me that the use of rigidity factors is unacceptable; unless someone can mathematically prove to me that a rigid foundation
will settle the same as a flexible foundation at the center.