Base Bearing Pressure from Subgrade Modulus vs. Net/Effective Bearing Pressure

[33mmrrdd]

I am new to flexible foundation analysis.

My question is when using the subgrade modulus, the pressure gives by the SAFE would be compared to the net/effective bearing pressure provided in the soil report in determining the footing size? or you have to divide it with safety factor?

I'm confused, because I just noticed that in using stronger soil with higher subgrade, it gives higher bearing pressure, exceeding the allowable nearing pressure.

Also, any reference book that I can read to understand this? Thanks

 

[Okiryu]

33mmrrdd, see attached.

Also, I considered that the modulus does not do anything with the allowable bearing pressure. The modulus is used for calculating moments and deflections in a slab for example and may cover the subgrade soils at shallow depths. However, allowable bearing pressures are governed by settlement considerations which depends on the size of the structural element which will affect the depth of influence of your analysis.

Here is other link about modulus of subgrades :

http://www.structuremag.org/?p=7840

 

[33mmrrdd]

Okiryu

So if that is the case, how would you know if the size of the footing is adequate when you are using subgrade modulus. Especially when you are designing a small mat footing acting like semi flexible semi rigid foundation.
This kind of footing is quite challenging if you analyze it with rigid analysis (with lateral loads) using allowable bearing capacity.

I have already read that article and still have questions.

It says that "The base pressure values reported by the FEA analysis cannot be directly compared with the bearing capacity. Max base pressure should be multiplied by the safety factor and then compared with the allowable bearing capacity."

Isn't the max base pressure should be divided by safety factor? Because the value you used in the formula of Bowles Ks = 40*SF*qa is already ultimate. It is like you are multiplying twice the safety factor.

Lastly, if I want to compare the base pressure reports from the SAFE to the bearing capacity, can I approximately use the basic formula Ks = pressure / settlement?

Thanks for your response.

 

[33mmrrdd]

Okiryu

After reading the Foundation Analysis and Design of Bowles, I think I got your point. For large footings, allowable bearing capacity is governed by settlement and not by the shear failure, am I right? But I still have questions in mind.

Anyway, thanks for your response.

I am planning to read more references.

BYE

 

[Okiryu]

You can check old threads in this forum and you will note that there were a lot of discussions from very experienced geotechs(BigH, Oldestguy, etc) talking about the differences between bearing capacity and bearing pressure. Bearing capacity is governed by shear and bearing pressure by settlement. Please make a search and you will find very interesting stuff in these forums.

 

[fattdad]

subgrade modulus informs the design of the concrete reinforcement, based on deflections in the concrete.

Deeper soil properties inform both the deeper soil settlement from the entire loaded area and the determination on shear stability in these soils.

(As an aside, use of subgrade modulus has taken a life of its own - almost to simplify the mindset of structural engineers, but it's frequently used incorrectly.)

f-d

ípapß gordo ainÆt no madre flaca!