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Interpretation of Net Allowable Bearing Pressure

ptdgeo (Geotechnical)
13 Dec 07 19:48
I realize this topic has appeared and been discussed here many times, but for myself I would like to bring it up one last time to assure myself I’m thinking correctly when explaining this topic to fellow structural engineers that call regarding this question.  

Most recently I had a site that was in a valley setting where the soils at the site were alluvial deposits composed of a sequence of fat clay, lean clay and sand over shale.  The fat clay had formed a desiccated crust over the underlying lean clay, that was normally consolidated to just slightly overconsolidated, and the sand below was medium dense to dense.

The structures for the project consisted of a single story mechanical building and reinforced concrete sedimentation basin with 20’ tall walls and a concrete roof.  The floor slab for the mechanical building needed to be a minimum of 5 feet above grade for flood protection.

Long story short, total settlements needed to be less than 1 inch and an increase in the vertical effective stress greater than 300 psf calculated more than an inch of settlement.  The client did not want to use deep foundation or any type of ground modification technique because the “local” contractor wasn’t equipped to handle them.

We ended up giving recommendations for “contact” pressures for a mat foundation established at depths ranging from 5 to 10 feet below the existing ground surface with anticipated total settlements of 1 inch or less.  I used the term “contact pressure” because the structural engineer and I could not agree on the meaning of “ net allowable bearing pressure”.  

The structural’s argument was, that if bottom of the mat was at 10 feet below existing grade, then he could use the net allowable pressure I was giving and add the weight of the soil removed.  His explanation, “you give a net allowable pressure of 1500 psf for a mat at 10 feet deep, so to size the mat I can use at least 2500 psf, by taking advantage of the soil removed.”  My reply was, “No, not if you want to keep settlements to less than 1 inch.”  So I was informed that I was not giving him a “net allowable pressure” because a net allowable pressure is that pressure in excess of the overburden.  I agreed, with half of his statement, and explained myself by saying:

“A net allowable bearing pressure is that pressure in excess of the overburden and it does account for the depth of overburden but what that means is that this is the total change in effective vertical stress that the soil below the foundation can experience to keep settlements within serviceable limits.  The 1500 psf is the 10 feet of soil (gamma total 120 pcf) plus the 300 psf increase in effective stress at that depth.  So this pressure is what needs to be used to size foundations and should be the total of the structures dead load and live load divided by the given pressure.”

After a little more banter, back and forth, regarding the meaning and definition of bearing capacity versus settlement and that the strength of the soil was not the issue because from a bearing capacity standpoint the soil probably could support 2500 psf without a bearing failure but it was the soil’s compressibility characteristic that was governing, so we agreed to use the term “contact” pressure.

My question is, does that make sense?  Even if I were to have called the term a net allowable pressure of 1500 psf, technically wouldn’t that have been correct?  By the structural's interpretation the net allowable bearing pressrue should have been 300 psf., I didn't agree.

Sorry about the long message.
fattdad (Geotechnical)
13 Dec 07 21:34
Sounds like the structural engineer's picking a fight.  S/he had a question, you clarified your perspective and s/he didn't like the answer you gave, which was based on your experience, analyses and consistent with your recommendations.

Some people!

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

UcfSE (Structural)
14 Dec 07 8:45
Now, I'm no geotech, but doesn't Terzaghi's bearing capacity and the general modified bearing capacity equations account for the overburden at the level of the bottom of the footing with the qNq term?  It seems to me that subtracting the overburden again would not only be double sipping but would be doing it incorrectly.
fattdad (Geotechnical)
14 Dec 07 9:44
Areal loads do not conform to the design approach for  rational bearing pressure analysis.  Areal loading is typically governed by settlement and most often the design stresses are either less than the maximum pre-consolidatation pressure or evaluated in light of soil modulus/elastic theory.

f-d

¡papá gordo ain’t no madre flaca!

ptdgeo (Geotechnical)
14 Dec 07 10:54
UcfSE

I think this is where some confusion comes into play, and fattdad let me know if you don't agree because this ties in with my original question.

Yes the ultimate bearing capacity in Terzaghi's equation accounts for the overburden and even the allowable bearing capacity does after applying a factor of safety.  However, the way I understand by giving a net allowable bearing pressure in a report I am attempting to convey that the overburden does not need to be added in sizing footings, I have already accounted for it and the pressure given is what should be used to size a footing based on loading condition from the proposed structure. Sometimes that pressure needs to be adjusted below the allowable bearing capacity of the soil because settlements resulting from the change in effective vertical stress induced by a footing are greater than the serviceable limits, but I still think that reduced pressure can be termed a net allowable bearing capacity because that is the total pressure that should be applied to the soil from any new loading sources that were not present at the time of exploration

I can't tell if I'm just mudding the water or making sense, but that is how I have intended a net allowable bearing pressure to be used.

Thanks for the comments.

ptd.  
BigH (Geotechnical)
14 Dec 07 20:10
In my view - when you say capacity (gross, net, whatever) you are referring to a "pressure" that is based on shear.  When you say pressure (gross, net, whatever) you are, again in my view, giving that based on serviceability.  As said many times before, capacity seldom governs design as serviceability constraints are IT.
   capacity = shear controls
   pressure = settlement or other serviceability controls
JAE (Structural)
15 Dec 07 13:27
I'm not a geotech either....from a structural engineering perspective the important thing is for me to correctly understand what your 300 psf or 1500 psf means.

For the above example, digging down 10 feet and placing a mat footing, I will have removed 10 feet of existing overburden and replaced that with a mat dead load, some air, and some live load.  

I just want to know what pressure to check my new mat against.

What this requires is that

The structural engineer communicate to the geotech:
1.  The intented configuration of the new structure with respect to the existing ground conditions.
2.  The rough magnitude of the loads for use by the geotech in preparing the recommendations.

The geotechnical engineer communicate to the structural:
1.  What the hell "net bearing pressure" means in light of the proposed structural configuration.
2.  Lots of questions to we structurals making sure we correctly interpret your nomenclature  (we structurals have thick skulls sometimes)

What you two did was great - lots of communication making sure the calculations will be correct.

Don't get lost in semantics...just make sure the semantics are understood.

UcfSE (Structural)
15 Dec 07 19:40
pt, I would stick with what you said above.  To me, that makes sense and seems logical, and of course agrees with what I've already thought of as net pressure as a structural dude.  

Bottom line, you're the soil expert.  Stick to your guns.  If the SEOR decides not to listen, write a letter to the owner and copy your boss (after discussing with boss) and make sure you cya and let other parties know.  You can only do so much.    
DRC1 (Civil/Environmental)
15 Dec 07 20:32
I  am sorry, but my understanding of net bearing capacity is as the structural engineer explained. You are correct though, that settlement often controls, but as BigH explained, that is not bearing capacity. The difference between gross and net allowable bearing capacity has caused considerable confusion, and I think the way you handled it , by stating an allowable contact pressure that considered bearing & settlement at the given depth, is the best way to handle it.
JAE (Structural)
15 Dec 07 22:42
UcfSE - huh?

I read the original post as just being a difference of opinion between struct and geotech over terminology.

What's with a letter to the owner?  I don't see the point.

BigH (Geotechnical)
16 Dec 07 4:05
Structural types:  Is this all why the geotechs are being pushed to LRFD designs? - I think that in the end, there will be problems for quite a while in the transition.  
UcfSE (Structural)
16 Dec 07 10:49
Disregard - that's what happens when I don't read the entire short story in the OP.
ptdgeo (Geotechnical)
17 Dec 07 19:01
Thanks for all the discussion; I find this site to be quite insightful.

I was curious how other folks would view the situation in the OP and if technically (terminology) there was anything wrong with how the situation was resolved.  I've never been in court but have a bad habit of thinking how an attorney would argue a point if something were to ever go wrong on a project.

What I get out of the comments is that communication is the real issue. Best set of plans or report doesn't mean diddly if nobody can follow them.

Again, thanks for all the discussion.

ptd
JAE (Structural)
18 Dec 07 0:05
For ptdgeo (and other geotechs out there)...for we structurals, I wonder if some sort of means of clarifying terms would be helpful in your reports.  Many times I get reports from different geotechnical firms who use terms differently (at least I think they do).

Even a couple of design examples in an appendix, clarifying terms might help.

Alternatively, one thing I really value in geotechnical firms is if I have questions or need help understanding their report, they then are aggreeable to follow up with a written addenda letter to their report summarizing our discussions.  I know this takes a little more time but in the best interests of the project (and avoiding future mis-interpretations) I think its a good idea.

Panars (Geotechnical)
18 Dec 07 8:43
BigH-

The push towards LRFD is not caused by confusion about terminology, but clarifying the terminology we use is a side benefit (or only benefit depending on your point of view [grin]).  With LRFD, we talk about nominal resistance and factored resistance, instead of allowable capacity.  Since these are new terms, it is a lot easier to be consistent with how they are used.
fattdad (Geotechnical)
18 Dec 07 10:04
Uh oh. . . Do I have to go back to college?  I have no idea what LRFD, nominal resistance or factored resistance is about - sigh.

f-d

¡papá gordo ain’t no madre flaca!

J1D (Structural)
18 Dec 07 17:26
ptdgeo

From the post and the discussion, it seems clear that:

1.your given 1500 psf net allowable pressure is the allowable new load pressure. The foundation and the backfill soil to replace the original soil are not new loads. They have already been accounted while determining the 1500 psf;

2.The 1500 psf is derived and governed by settlement. (e.g. to keep settlement less than 1 inch.)

However, the sentence “The 1500 psf is the 10 feet of soil (gamma total 120 pcf) plus the 300 psf increase in effective stress at that depth” is a bit confusing to me. Is the 1500 psf somehow related to 10ft thick soil with 120 pcf unit weight?
fattdad (Geotechnical)
18 Dec 07 18:20

Quote:

The foundation and the backfill soil to replace the original soil are not new loads.

Technically, they are new loads - new in that after the foundation excavation is complete all you have below the foundation level is a preconsolidated elastic media, which will deform under any reapplied loading.  The reason (in my mind) that we discount the weight of the foundation and the soil backfill is that these loads (and any corresponding settlement) occur prior to construction and do not contribute to any construction- or post-construction-related settlement.  Considering that rarely does "bearing capacity" govern the design (and settlement does) the weight of the foundation and backfill is not relavent.

Just another perspective.

f-d

¡papá gordo ain’t no madre flaca!

ptdgeo (Geotechnical)
18 Dec 07 19:49
J1D

First, yes the pressure given was completely governed by settlement, which muddies the water a little bit when considering the difference between bearing CAPACITY, and how it is determined, and bearing PRESSURE, and how it is determined.  BigH defines these two well in the comment above.

In this case, it was a “contact” pressure that was given in the report, but that brings me to my question in the OP  - Could I of used the term net bearing pressure and still been technically correct.

To answer your question:
 
The 300 psf was the limiting amount of increase in the effective vertical stress that the underlying soils could experience to keep settlements below 1 inch.  The proposed mat foundation was to have minimal dimensions of 65 feet by 45 feet, so there was going to be very little decrease in the stress induced by the mat foundation at the depth of the underlying normally consolidated to slightly overconsolidated soils.

By placing the bottom of the mat 10 feet in the ground, the foundation was being designed as a partially compensated foundation.  A fully compensated foundation would be where the pressure induced from the foundation matches the weight of the soil removed, thus there is no increase in the effective vertical stress on the underlying soils.  The soils at this particular site could withstand a slight increase in stress, thus the 300 psf.  So, to give a pressure that needed to be used to size a partially compensated mat foundation I took the weight of the soil to be removed (10'* 120 pcf) to get 1200 psf plus the amount of increase that could be applied (300 psf) to get a contact pressure of 1500 psf.  Now, in determining bearing capacity in clays and assuming undrained conditions where phi = 0 the soils cohesion is a component and the overburden above the foundation is a component and then to determine an allowable bearing capacity that value is divided by a factor of safety, usually 3.  Then a net allowable bearing capacity would take into account the component from the overburden so you wouldn’t have to consider it in sizing your foundation.  You simple only need the total loading from the proposed structure.

In this case the pressure given was not determined from the soils cohesion, is was determined from data obtained from consolidation tests and the soils compressibility characteristics, but 1500 psf was still the pressure that should have been used to size the foundation based solely on the total loading from the structure.

So if a foundation is to be sized solely on the loads from the proposed structure couldn’t the pressure used to size that foundation still be called a net allowable pressure, regardless if it is governed by settlement or the soils c and phi?
ptdgeo (Geotechnical)
18 Dec 07 20:00
fattdad

Good points and that is why I'm not a fan of Fully compensated foundations or "floating" foundations, plus buoyancy affects come into play if there are groundwater issues.

ptd
haynewp (Structural)
18 Dec 07 21:06
Regarding misinterpretations of the geotech report; the geotechs in my area almost always say "allowable bearing pressure" and don't state if it is a "gross" or a "net". Usually when I ask they say they meant net, but I wish they would just state that to begin with.
BigH (Geotechnical)
18 Dec 07 23:20
panars - I'll be in Indiana in January - can we get together, maybe? with Jeff?  I understand your point for clarification - but note that you use the terms nominal resistance and factored resistance. Goes along with capacity.  So, I've calculated the LRFD resistances and all that - everything is copacetic. But, you do a settlement analysis (say using Hough if so inclined) and the settlement is 50% larger than permitted by the requirements of the structure.  What good is the factored and nominal resistances now? We must develop a safe pressure that the settlement doesn't exceed.  Does LRFD do this for me?  For all those, it is true that we geotechs should use net in our "allowable bearing pressure" wording.
Panars (Geotechnical)
19 Dec 07 8:13
BigH- Settlement in LRFD (Load and Resistance Factor Design) is a service limit state, and as such it uses unfactored loads.  As the geotech, you would calculate the bearing resistance for the service limit state (controlled by settlement) and the factored bearing resistance for the strength limit state (controlled by failure of the soil).  It will then be up to the structural engineer to determine which controls the footing design (or you can do it if the structural engineer will give you the dead load, live load, wind load, earthquake load, etc.).  I realize this overlooks the effect that footing size has on settlement.  This can be addressed by presenting a graph that correlates settlement with footing size for a given bearing resistance.

I'll contact you off-board about January.
miecz (Structural)
19 Dec 07 9:07
We don't need new clarifying terms.  The clarifying terms already exist.  See Article 7,3 in Bowles 4th Edition:

Net Pressure - pressure in excess of existing overburden pressure, based on settlement considerations.

Gross pressure - total pressure whch can be carried, based on bearing capacity.

Whether the pressure is a gross or net value should be stated in the geotech report, but often, is not.

In this case, the geotech report should have given the allowable bearing pressure as 300 psf net pressure.
ptdgeo (Geotechnical)
19 Dec 07 12:22
miecz

Interesting.  That is the same argument/disagreement interpretation issues I had with the original SEOR.  You’re not him are you???

Funny after all the above comments it comes around full circle again.
miecz (Structural)
19 Dec 07 12:41
ptdgeo

Well, I guess I'm in total agreement with your SEOR.  After reading the 9 pages (printed 'em) of comments, I don't think your original question

Quote:

if I were to have called the term a net allowable pressure of 1500 psf, technically wouldn’t that have been correct?   
was answered.  My position is no, it should have been a net allowable pressure of 300 psf.
miecz (Structural)
19 Dec 07 13:31
BigH

Yes, if the allowable pressure is based on settlement, then the soil report should say net allowable bearing pressure.  If the allowable bearing pressure is based on bearing capacity, then the soil report should say gross allowable bearing pressure.
JAE (Structural)
19 Dec 07 14:20
This is getting confusing - I have always understood the geotechnical terms "net" and "gross" to deal with the types of loads included in their use, not on what the limit state of the design was.

From my earlier post - the key is for you geotechs to simply be sure you communicate your intent, and that the intent is correctly understood by we thick skulled structurals.
ptdgeo (Geotechnical)
19 Dec 07 15:02
miecz
First I hope my comment, "you're not him are you??" was taken in good fun - that is the way it was intended anyway.

Second, you are correct that nobody, besides yourself, directly answered my question in the OP.

Third, and this ties in with JAE's latest post, I have always intended for a "net" allowable pressure to deal with the type of loads that should be included (i.e. for "net" no need to include the load from the overburden, only the loads associated with the proposed structure need to be used)
miecz (Structural)
19 Dec 07 18:41
ptdgeo-

Yes, your question made me laugh (and I need a good laugh).

From your latest post, it seems as though your definition of net pressure is the same as mine, i.e., it doesn't include the overburden (the excavation).  If I understand your original post, then, the net allowable pressure is 300 psf.
BigH (Geotechnical)
19 Dec 07 20:54
We did get off track - and if 300 psf additional loading over and above the effective overburden pressure at the invert of the footing at the gives you the 1 inch settlement and this is the operative serviceability limit, then 300 psf is the net allowable bearing pressure that would be given.  The designer could put a gross bearing pressure equal to the weight of the soil removed (divided by the footing area) plus 300 psf.
ptdgeo (Geotechnical)
19 Dec 07 21:16
okay.

but say you are to give a footing pressure for a fully compensated mat foundation 10 feet in the ground and the total unit weight of the soil removed is 120 pcf

How would you term that pressure in a report?

Net allowable bearing pressure of 0 psf?

Allowable bearing pressure of 1200 psf?

Contact pressure of 1200 psf?

or something else?
miecz (Structural)
20 Dec 07 8:48
ptdgoe-
I'm not sure what you mean by fully compensated.  If the allowable is based on settlement, 1 inch in your original post, then there must be some net pressure that would cause that settlement.

JAE-
In terms of "limit states", I think the terms "net" and "gross" are directly related to the design limit state; net loads are used for the deflection (settlement) limit state, and gross loads are used for the strength (bearing) limit state.
JAE (Structural)
20 Dec 07 10:08
miecz - yes both HAVE limit states, but the terms net and gross have always distinguished between whether you include existing overburden or not.

Net has always meant to me (and to most textbooks on foundations) as a difference in pressure that the soil feels due to the footing while Gross has always meant the total pressure, regardless of whether it is the original overburden pressure or the newly added foundation pressure.

From Ralph Peck's Foundation Engineering: "The net ultimate bearing capacity is defined as the pressure that can be supported at the base of the footing in excess of that at the same level due to the surrounding surcharge".

To say that NET means one type of limit state and GROSS means a different limit state isn't what I've seen and used for 30 years.  News to me.
ptdgeo (Geotechnical)
20 Dec 07 10:56
miecz

a compensated foundation, also referred to as a floating foundation, is a foundation that is designed and constructed by excavating a volume of soil with a weight nearly equal to that of the proposed structure and then constructing the foundation in the bottom of the excavation, thus the increase in vertical effective stress in the soil, is very small.  For a fully compensated foundation the loads from a structure would equal the weight of the volume of soil removed, resulting in a zero change in stress. However, there are rebound concerns, buoyancy concerns with groundwater issues, and the live load varies over time so the weights are not always perfectly balanced.

so from my latest post above, curious how folks would term the pressure to put in a report for a SE to use for sizing this type of foundation
BigH (Geotechnical)
20 Dec 07 11:44
for anyone's interst on buoyancy foundations - see Tomlinson's book on Design and Construction of Foundations and Chapter 12 by H.Q. Golder in Winterkorn and Fang's Foundation Engineering Handbook, 1st Edition.
   If there are any code specifiers out there and professors - perhaps you should all be more concerned with the concerns raised herein than by 4th order FEM analyses using 1st order soil parameters.
miecz (Structural)
20 Dec 07 12:29
I'm feeling really thick here, as I'm having trouble understanding where we disagree.  So let me back up a bit.  I don't believe that disagree on the definition of net pressure, or that net pressure is what causes settlement.  I don't believe we disagree on the definition of gross pressure or that gross pressure is what would cause a bearing capacity failure.

When the op asks:  "if I were to have called the term a net allowable pressure of 1500 psf, technically wouldn’t that have been correct?",  I don't think we disagree that a net allowable pressure of 1500 psf is incorrect.

So, I guess the disagreement is in the standard practice.  If you'r saying that geotechs don't specify allowable net pressures to direct the engineer to use net pressures, and allowable gross pressures to direct the engineer to use gross pressures, well, even the author (Bowles) says it's not often done this way, so I'd have to agree with you there.

What I was saying, is that it should be done that way, as, it's clear and easy, and, no other proposed system makes sense to me.
miecz (Structural)
20 Dec 07 12:51
Sorry, the above post should have been addressed to JAE.
DRC1 (Civil/Environmental)
20 Dec 07 21:52
Okay Bearing and settlement are like beer and ice cream, both are great, just not together.
Gross bearing pressure is the total pressure that will cause a shear failure of the foundation. Net bearing pressure is the gross bearing pressure less the overburden stress.
Settlement is due to additional contact or applied stress above the existing overburden stress and represents compession of the underlying soil due to elasyicity(sands) or drainage (cohesive soils) Peck devloped the charts that combined the two concepts and they have been hopelessly linked ever since.
To make the matter simple, tell the structures guy what he wants to know: the foundation is 10 feet deep and can have an applied stress from the structure of 300 psf.

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