Footing uplift safety factor

[nrjdjjs]

Would appreciate some insight into uplift safety factor related to footings.

Here are the inputs:
Footing weight : 100 kN
Pedestal weight : 10 kN
Soil weight above the footing : 100 kN
Applied load on the pedestal : -25 kN
Buoyant load due to water pressure : -105 kN

What is the correct way to calculate the uplift safety factor using the 3 methods displayed below? Each method generates a different safety factor.

Method 1:
All upward acting total loads: U = -25-105 = -130 kN
All downward acting total loads: D = 100 + 10 + 100 = 210 kN
Uplift safety factor =D/(abs(U)) = 210/130 = 1.62

Method 2:
Total buoyant load: U = -105 kN
Total algebraic applied load at base of footing: D = 100 + 10 + 100 - 25 =185 kN
Uplift safety factor =D/(abs(U)) = 185/105 = 1.76

Method 3:
Total uplift load on pedestal: U = -25 kN
Total self weight at base of footing including buoyancy: D = 100 + 10 + 100 - 105 = 105 kN
Uplift safety factor =D/(abs(U)) = 105/25 = 4.2

 

[Struct123ure]

To me only the first method makes sense.
As for design you should follow load combinations which would factor down the deadload by 0.9 and factor up the buoyancy by 1.5.

 

[human909]

I don't see why you would factor buoyancy up by 1.5. But that might depend on:
-your assumptions regarding your buoyancy calculation
-the requirements of local codes

Load factors if LFRD is being being consistent should be base on the likely variance of the load. Both the dead load of the foundation and the buoyant force of the foundation are almost completely correlated. Factoring one up and the other down makes ZERO sense.

 

[HTURKAK]

I could not figure out your input figures ;
Assuming the worst scenario WL is at ground level ,interpretation of your data ,

Footing weight : 100 kN makes 4 cu-m of concrete will subject to 40 kn uplift
Pedestal weight : 10 kN makes 0.4 cu-m of concrete will subject to 4 kn uplift
Soil weight above the footing : 100 kN assuming submerged unit wt 20kN/m3 makes 50 kN uplift

Net wt of the foundation and overburden =210-94=116 kN
F.S against uplift =116/25

Pls post a sketch showing the dimensions , GWL etc and load factors if any to get better responds.

 

[JStephen]

I think the issue here is simply that "Factor of Safety" has to be defined clearly to have any meaning. So, for example, in your Method 3, the "Factor of Safety of actual pullout force relative to pullout force causing uplift" is indeed that 4.2 factor. But if you have multiple loads, multiple reactions, etc., and don't clearly define how that factor is being used, the term "Factor of Safety" just doesn't have much meaning.

 

[human909]

Agreed. "Factor of safety" is seemly the most obvious and clear margin of safety for a layman or even a engineer not wanting to do too much statistics.

On the other hand it is close to the least suitable metric for gauging reliability and safety. That is why LFRD exists. And despite many in this forums still in love with FOS, it is a much more robust approach.

(Not that I'm a perfect engineer. I use sometimes FOS either because it is code driven or just damn easier when variance is hard to quantify.)