soil s

[atika]

Hi everyone:
during my design of a foundation with high eccentricity ( eccentricity out of middle third ). I tried to calculate soil pressure manually and then by SAFE software. But I couldn't justify the large difference in soil stress results. I have attached a PDF file showing the two results. can anyone share his/here thought about this discrepancy?

where thickness of footing = 200 mm
modulus of subgrade reaction = 18000 kN/m3 ( calculated based on allowed settlement = 8.33 mm )
allowable bearing capacity = 150 kN/m2
width of footing = 0.8 m
length of footing = 1.95 m
total service load = 51.6 kN
total service moment = 40 kN.m
thanks.

 

[Splitrings]

I just glanced quickly but I would guess it is because your software package is allowing negative pressures. In your hand calcs you are not. Can you force your software not to use negative pressures?

 

[atika]

this is what I'm suspecting but I don't know how to force it to ignore negative soil stress

 

[Splitrings]

I am familiar with RISA software. In this software you can use compression only springs to eliminate the issue you are having.

 

[atika]

having said that. shouldn't I check stresses at top surface of foundation resulting from gravity ( kindly see the attached )

 

[Splitrings]

Yes. You forgot you attachment.

 

[JAE]

I think this question and thread would be better over at the Foundation Engineering forum - found here: forum256

While it is semi-structural it is really dealing more with a foundation design issue.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[IDS]

The only way I can get 90 kPa with a linear-elastic distribution is to allow tension and take the footing width as 1.0 m, rather than 0.8 m.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[atika]

sorry guys I forgot the attachment

 

[DamsInc]

Plugged your numbers into my mathcad sheet and had got the same result as you (maximum stress = 215 kPa), assuming no tensile stresses allowed. With tensile stresses, my maximum is 112 kPa. I concur with IDS, I get 90 kPa with a width of 1.0m and allowing tensile stresses. Never used SAFE software, but that could be your problem.

I'm glad your search bar didn't reveal what search terms came after "ariana grande"

 

[atika]

SAFE is one of CSI software and it is specialized in designing footings and slabs. I found the option in version 12 to remove tension stress in soil support ( in the attached).
but still, if I calculate the center of soil stress it won't coincide with the eccentricity of external applied force. does anyone know the reason ? I've been told that the reason of this: that foundation should be sized in a way prevent any tension at soil i.e. maximum eccentricity should not be out of middle third.
I'm asking why not? does it mean that the footing will undergo excessive rotation in order to meet the large eccentricity ?