Net Allowable Soil Bearing 1

[Neauxla]

I'm reading the soil report for the area where my job is located and it refers to the net allowable soil bearing value of a footing foundation and a mat foundation.

I need to design a foundation for a small, skid mounted horizontal storage tank (approx 5'x15'). The foundation needs to be wider than the tank itself in order to house some pumps as well, so the foundation dimensions are 10'x20'. Would this foundation be considered a footing foundation or a mat foundation?

Also, the soil report gives values for net allowable soil bearing which it says includes a factor of safety of 3.0 against actual failure of the subsoils due to shear. So does this mean that this is my allowable bearing capacity and my ultimate bearing capacity would be the allowable divided by 3? I'm a young engineer and am having trouble understaing the relationships between allowable, ultimate, and factor of safety.

 

[motorbiketocrank]

The allowable bearing capacity is the ultimate bearing capacity divided by the factor of safety. Basically, the ultimate bearing capacity is the pressure at which failure would be expected to take place and therefore a factor of safety is applied to reduce this value to the allowable used for structural design.

 

[mar2805]

One thing that I never understod.
Terzagis formula for determining ultimate bearing pressure based on earth parametars, foundations width and depth of foundation is given by:
qult=c*Nc+qo*Nq+0,5*gama*B*N(gama) - strip footing
After various reaserches gave formulas that incorporate square footings, rectangular and circular footings.
But what aboth raft foundation?
How to deal with them?

 

[Neauxla]

So basically, the weight of the footing plus the weight of the tank divided by the area of the footing just has to be less than the net allowable soil bearing that is givin in the soil report?

 

[mar2805]

Seems like an aproximate method wich can be used for by-hand calulation-yes.
Question comes to mind. If your foundation solution is a raft, and your know frost-depth is around 0,80m below ground surface.
What to do?
Should this frost limit be considered as with other types of foundation or not?
Outer ground perimeter will be most exposed to this temperature differences, and vertical perimetar line load will be highest at this position.
Should one bulid raft beams at this point that have a depth of 0,80m...

 

[jdgengineer]

I could be wrong on this but I would read the wording of the geotechnical report closely. I think a lot of the time when they ,emotion "net allowable" you don't need to include the weight of the foundation. It's almost offsetting e went of the soil that was there and is therefore sometimes allowed to be neglected.

 

[PEinc]

jdengineer,
Please correct you posting to make it more understandable.

http://www.PeirceEngineering.com

 

[jdgengineer]

Sorry -- I posted that using my iPad, as you can tell I am terrible at typing using a touch screen with autocorrect.

I was trying to mention that I would read the geotechnical report closely. In my understanding, when they mention "net allowable" bearing pressure they are typically allowing you to neglect the weight of the foundation. As the weight of the foundation almost offsets the weight of the soil that was in place prior to the foundation, the weight of the foundation is often neglected.

 

[mar2805]

Im not sure I get you point...

 

[jdgengineer]

So basically, the weight of the footing plus the weight of the tank divided by the area of the footing just has to be less than the net allowable soil bearing that is givin in the soil report?

I was merely saying that a lot of the geotech reports I have used for relatively shallow foundations do not require you to include the weight of the footing in the bearing capacity checks.

 

[BigH]

Net pressure is the "new added" pressure over and above the "pre-existing pressures" that is added on by the new loads. Say, for instance, you have a footing 4 ft deep, install a square footing and then backfill to the original level. While not theoretically correct to say that the concrete of the footing and pedestal that replaces the "original soil" is of the same unit weight, it is generally taken as so - given the accuracy of material properties. Therefore, the footing weight has no influence on the pressure - the net pressure is then the new loadings above the original ground level. If the footing is placed directly on the soil with no embedment, the footing weight would be important.

When I use net allowable bearing pressure, I mean that pressure that can be used to meet serviceability requirements.

 

[mar2805]

Guys can oyu help me out a bit.
I curently reading "Foundation Analysis and design"-Bowels book.
I came across Hansen formula for calculating ultimate bearing capacity. Im having a hrad time inderstanding the B`,L` dimesnions that author uses.
If the footing is loaded with inclined force you calculate inclination factors. Later on when inserting values for B`,L` real values are multiplyed with inclination factors. in every term for shape factor formulas.
Is this correct?

If scenario is that I have a vertical force and a moment acting on a footing, B´=B-2*ex, L´=B-2*ey and no inclination foctors. Basicly they are all equal to =1

 

[grabens]

I agree with Big H,

In my reports I proved a net allowable soil bearing pressure which means the "new added" pressure over and above the "pre-existing pressures" that is added on by the new loads.