Footing uplift safety factor

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.

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.

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.

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.

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.)

human909 said:

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.

Click to expand...

Nuts I was going to make some smart comment about buoyancy forces being in the code and I struck out. Maybe that's in ACI 350 which probably doesn't apply here as it doesn't scream environmental engineering concrete structures. I believe other standard like, say, API (American Petroleum I-something, Institute?) include buoyancy for below ground tanks, those could be looked at for reference, although ACI usually includes extra load factors (or did back in 2004) to reduce and restrict cracking in the tanks that would not likely apply to a normal footing.

Granted a fluid load doesn't have all that much variability, unless somebody sabotages the river with a billion gallons of mercury, (or molasses) but you apply the factor in the code to arrive at a standard of care design, rather than debating the variability of the density of water. They might be giving it a factor greater than 1 to provide some measure of reserve strength for overtopping/freeboard/flood or something similar.

A blanket (ahem) load factor) on a pressure load allows a certain amount of over-pressure, it does not provide a consistent design margin/safety factor/whatever based on an explicit "two feet extra of hydraulic load" would provide.