Seismic bearing capacity of shallow foundations? 1

[drile007]

Is there a short way to calculate it and (or) is there any (simple) correlation between NonSeismic and Seismic?

Drile007

 

[BigH]

Try searching google or answers.com for "Choudhury Rao Seismic Bearing Capacity" - you'll get a few hits. Check out site below and then go through references for recent papers in ASCE, Geotechnique etc.

http://dspace.library.iitb.ac.in/dspace/bitstream/10054/45/1/ASCE2.pdf

 

[Mccoy]

drile 007,
as far as I know there is no simple way, nor reliable correlations.
I once tried to develop a correlation proposed by Paolucci & pecker (1997), which turned out to be totally and unrealistically conservative.

So you have to factor in structural inertia and, sometimes but not often, soil inertia.

The seismic wave hits the foundation. The structure sways according to its vibrational modes. The main mode can be computed by a simple correlation, sometimes it is enough to take, very simply, T = 0.1*# floors (period in seconds).

Then you should use such period to find the base shear, or soil reaction against the structure, I use the dissipative spectra calculated according to regulations, as an input you need the structural factor of dissipation.

The structure sways, dissipates the seismic energy in microfractures at plastic hinges, then imposes back such signal to the soil. This is structural inertia, which tends to dampen the soil seismic signal.

If W = weight force, Kh*W = H, the horizontal component of seismic force, and you can use H as an input for the inclined loads formulas in Hansen, Meyerhof, vesic (details in Bowles).

Foundation eccentricity must be given by the structural engineer. Then you have (at last) all which is needed to figure out seismic bearing capacity.
Richards et al. proposed a slightly different method, featured by Das (shallow gfoundation handbook) and such method is similar to the article suggested by BigH.
Soil inertia should be calculated only with very large foundations, tall & slender structures, very soft soils.
I use the correction factors given by Poolucci & Pecker (1997).

Hope my summary does not sound too discouraging...

 

[BigH]

Mccoy - let your fingers play the tune by giving the details of your Poolucci and Pecker (great name ) reference!!

 

[Mccoy]

BigH,
Paolucci (i mispelled him) and Pecker (you may not believe me but right now I'm laughing in front of the screen, sorry but you started that out!).

Let's play the keyboard, then (the pianist must be somewhere back in the shade, isn't there a smiley for a jazzband?)


There are actually two similar articles, I know them by third party references and material:
Paolucci R., Pecker A.
Seismic bearing capacity of shallow strip foundations on dry soils
Soils and Foundations, vol.37, n°3, pp. 95-105, September 1997


Paolucci R., Pecker A.
Soil inertia effects on the bearing capacity of rectangular foundations on cohesive soils
Engineering Structures, vol.19, n°8, Elsevier Science Ltd., pp. 637-643,1997



Paolucci & Pecker () also produced another 'messy' formula which includes 33 or so coefficients and is reccomended in the Eurocode8 appendix (the Eurocode on seismic foundations design).
If you are curious you may take a look at it at page 28/55 of teh following PPT presentation:

http://www.dica.unict.it/users/aghe...errenoFondazioni/2-FondazioniSuperficiali.pdf

by Maugeri, one of the Italian foremost seismic engineers (sorry, only in Italian).
I never had the courage to put teh EC8 formula on spreadsheet

Sorry folks for the unflinching facetiousness, BigH just woke up my sleeping knowledge of american slang

 

[drile007]

I'll try to put the EC8 formula in Excel but I don't know a few terms used in EC8. Don't laugh as I'm not a geotechnical eng. So, can you help me with these terms (where are the limits):
1) Medium-dense sand?
2) Loose dry sand?
3) Loose saturaded sand?
4) Sensitive clay?
5) Purely cohesive soil?
6) Purely cohesionless soil?
Thank you in advace.

PS: In which depth should I capture these soil parametrs.


Drile007

 

[Mccoy]

drile,
no more laughing I promise! If you have the patience to complete the excel sheet please send me a copy (hope units are metrics)!

Since EC8 gives no other indications,as far as I see, in the first stage the engineer should decide which main group the soil belongs to: frictional behaviour or cohesive behaviour. This controls which of the two 14-elements coefficients vector shall be used. They are discrete and not continuos values, again, they say nothing about intermediate values (unless otherwise specificed in the original paper but since everyone is ignoring this aspect I'll reasonably assume there are no intermediate values)

A very simple algorithm in pseudo-code could look like the following:

If (soil = frictional and soil = dense
then vector = B; gamma_RD = 1;
If (soil = frictional and soil = loose_dry)
then vector = B; gamma_RD = 1.5;
If (soil = frictional and soil = loose_wet)
then vector = B; gamma_RD = 1.50;
If (soil = cohesive and soil = sensitive)
then vector = A; gamma_RD = 1.15;
If (soil = frictional and soil = not_sensitive)
then vector = A; gamma_RD = 1;
else output: 'error - please check the input data!'

where vector A is of course the set of 14 coefficients to be used in the case of cohesive, non-frictional material, and vector B conversely the related set for frictional soil.

hope it's clear enough.

The depth should be the influence depth of the potential failure surface, 1B to 2B (B = foundation width). I do not think the formula is applicabile to markedly dishomogeneous profiles.

Also, it is valid only for strip footing.

 

[drile007]

I think there is a misunderstanding. I don't know what kind of soil is loose sand, sensitive clay... not the limits for gamma_RD. Where can I find any basic words (pictures) of these soils. How can I classify my soil (from which test, etc.).
If I understand right, Purely cohesive soil (cohesive behaviour) are those with [φ]=0,c[≠]o and Purely cohesionless soil (frictional behaviour) are [φ][≠]0, c=0! Am I right?


PS: When I'll finsh the sheet you'll get it.


Drile007

 

[Mccoy]

If I understand right, Purely cohesive soil (cohesive behaviour) are those with ?=0,c?o and Purely cohesionless soil (frictional behaviour) are ??0, c=0! Am I right?

basically you're right, in purely cohesive and saturated soils if not overconsolidated a phi=0 condition is usually assumed. Also there may be some soils (the silty ones) which are not easy to classify. Clean sands and gravel are usually c'=0, even though if capillary pressures are present, they may display a c'>0.

Loose sand is a sparsely packed sand prone to settlement, a dense sand conversely is thighly packed and shall settle little with not abnormous loads.
Sensitive clays possess a metastable structure which will collapse above a certain stress treshold.