Silly Question 2

[IYIeI2cY]

hi all,

sorry for that SILLY Question, but what is the releationship between Bearing capacity for a soil and the soil modulus?? are they the same or what !!

thx in advance

Pea$e

 

[Focht3]

They are not the same. Soil modulus has to do with stiffness; bearing capacity has to do with the load required to cause yielding -

 

[IYIeI2cY]

Very nice,

now, if i have the Bearing capacity, how could i get the soil modulus??

in other word, i know that when we are modeling a raft using sap2000, we multiply the bearing capacity(in t\m2 units) by 100 then multiply the result by the area of the shell carried by every joint and then assign a spring in that joint that has stifness equal to the final result.

isn't the spring stiffness we assigned is soil modulus or its some thing else ??

is modeling that raft right that way or not ??

sorry guys im very confused here !!

Pea$e

 

[Focht3]

You need to talk to your geotechnical consultant about this - it involves a lot more than just using the "bearing capacity" and calculating a soil modulus -

 

[npthao121]

I think your subsoil investigation report shows you all information such as soil elastic modulus, compression factor...ask your geotechnical engineer for those info.
I did not understand well what is your purpose. you want to design the concrete structure (rebar) for the raft or what?
If i understand you correctly i suggest the way like this. assign spring at the node of your model. the sprin stiffenes can be calculated as F/s, F is the load impact, s is probable settlement. To get the envelope of intenal force you should try s as some values 1cm, 2cm, ... until the maximum value specified at design code. but just try some values around the elastic settlement is enough (i assume the no consolidation settlemnt in this case). Elastic settlement can be calculated from load & soil data.

 

[Focht3]

Most geotechnical reports don't include soil modulus values. Why? Because soil modulus is not unique - it isn't a "testable" value. It has to be developed for a specific site, loading condition and structure. And you can't just pull it from a text book, or the last similar project.

[blue]npthao121[/blue]'s suggestion to trst your design against a range of soil modulus values is good, but you have to know where to start. Ask your geotechnical consultant.

 

[BigH]

Just a comment - I don't know where IYIe2CY is from, but there is always a chance that the geotechnical consultant, if there is one, is not of the calibre that you have in Canada/US/Europe. Just a thought based on sad experience.

 

[IYIeI2cY]

BigH,
i think u absolutly right, im from Egypt there is a great geotechnical consultants here, but as a lack of facilities, we need them only in the big projects, but for a small projects like a normal building just a economic check for the bearing capacity is enough.

but to solve a raft foundation using SAP2000, i need to define a spring stifness in the shell joints, its very common here to multiply the Bearing capacity in t/m2 units by 100 then multiply the result by the shell area every spring will carry.

i dont know whay we do that, i need to know if thats right and safe or not.

your help is very appreciated

Pea$e

 

[Focht3]

When you perform the calculation, what units does your answer have?



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[BigRedGeo]

As everyone else has said here already, modulus of subgrade reaction will vary depending on many things.....including the definition. For instance, I recall one standard method that used modulus values within the upper 10 feet of soils at a site (might be good for pavements, but not mats). However, if you are using a mat analysis for a large area, the upper 10 feet will definitely not represent what will actually occur at the site, as you depth of influence will be much greater than 10 feet.

On large mat projects, I use an iterative analysis to come up with spring constants(modulus of subgrade reaction), which I in turn supply to the structural engineer to formulate a pressure distribution. I then re-analyze the situation with the new pressure distribution to come up with revised spring constants. This continues until the spring constants reach agreement with calculated settlements. In the first step, I start out with a pressure distribution supplied by the structurual engineer which is based on an assumed spring constant. I split out the pressure distribution into representatively loaded individual elements, calculating settlements due to each of these elements, which then allows me to calculate modulus values. I provide the structural engineer the moduli across the mat, and the structural re-analyzes this data, and supplies me with another pressure distribution. Usually this only requires 1-2 iterations until reasonable agreement is made.

I have used this approach on a few projects, and my company has been doing this for a while. However, you will need very detailed subsurface information. On jobs like this, we do extensive CPT work, supplemented with consolidation testing and vane shear testing if we are in clays. Settlement monitoring of the structures indicates that our estimates are pretty reliable.

We don't have a lot of structures on mats in this area, but we do have a few. Mat settlements are usually high, which is one of our main reasons for providing this type of analyses. Obviously, there is a lot of work involved in this type of analysis, although we use computer programs and spreadsheets for most of the computations.

 

[Focht3]

[red]BigRedGeo[/red]:

Welcome to [blue]Eng-Tips[/blue]!

Your company is employing a technique pioneered by M^cClelland Engineers in Houston during the 1970's and 1980's. It's quite effective - and provides good results. Most of the high rises in downtown Houston were designed using this approach. M^cClelland referred to this as the "unit area" method.

It seems intuitive to me - and I am surprised at how few engineers make use an iterative design method.



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[BigRedGeo]

I was not aware of the genesis of this method, but, I guess it makes sense that it was pioneered by McClelland in Houston, as I work for both a previous McClelland employee, and someone out of Houston.

It does seem very intuitive, and can work very well. But very good, detailed subsurface information is key (then again, when is very good, detailed subsurface information not key?)