Overturn Moment FOS when Uplift is involved.
Overturn Moment FOS when Uplift is involved.
(OP)
When checking the overturn factor of safety provided in Enercalc vs. the method used in “Foundation and Anchor Design Guide for Metal Building Systems” by Alexander Newman, I found some discrepancies. Rotation is considered around point A.

Enercalc, Case 1:
Resisting Moment, Mres = L/2*(F2+w1+w2)
Applied OT Moment, Mot = F1*H
Enercalc, Case 2:
Resisting Moment, Mres = L/2*(w1+w2)
Applied OT Moment, Mot = (F1*H)+(F2*L/2)
-------------------------------------------------------------
Text Ref., Case 1: (Same as Enercalc)
Resisting Moment, Mres = L/2*(F2+w1+w2)
Applied OT Moment, Mot = F1*H
Text Ref., Case 2:
Resisting Moment, Mres = L/2*(w1+w2-F2)
Applied OT Moment, Mot = (F1*H)
My biggest concern is from Case 2. Each method gives different answers and will provide a different factor of safety against overturning. This isn’t covered in any references I have for retaining walls since we don’t typically have uplift loads on them. For any of you who do PEMBs, what do you use?
I know some of you might comment that the load directions wouldn’t make sense for typical PEMB reactions. My question is more to do with calculating the overturning and resisting moments, so don’t get mixed up about load directions.

Enercalc, Case 1:
Resisting Moment, Mres = L/2*(F2+w1+w2)
Applied OT Moment, Mot = F1*H
Enercalc, Case 2:
Resisting Moment, Mres = L/2*(w1+w2)
Applied OT Moment, Mot = (F1*H)+(F2*L/2)
-------------------------------------------------------------
Text Ref., Case 1: (Same as Enercalc)
Resisting Moment, Mres = L/2*(F2+w1+w2)
Applied OT Moment, Mot = F1*H
Text Ref., Case 2:
Resisting Moment, Mres = L/2*(w1+w2-F2)
Applied OT Moment, Mot = (F1*H)
My biggest concern is from Case 2. Each method gives different answers and will provide a different factor of safety against overturning. This isn’t covered in any references I have for retaining walls since we don’t typically have uplift loads on them. For any of you who do PEMBs, what do you use?
I know some of you might comment that the load directions wouldn’t make sense for typical PEMB reactions. My question is more to do with calculating the overturning and resisting moments, so don’t get mixed up about load directions.






RE: Overturn Moment FOS when Uplift is involved.
RE: Overturn Moment FOS when Uplift is involved.
My opinion is that Enercalc is doing the check appropriately.
RE: Overturn Moment FOS when Uplift is involved.
RE: Overturn Moment FOS when Uplift is involved.
The applied OT moment and the resisting moment as evaluated above are not equivalent for both cases, neither is the FOS... but yes, the net moment will be the same.
Winelandv,
I agree with your conservative approach. Have you seen this discussed anywhere?
RE: Overturn Moment FOS when Uplift is involved.
thread507-343140: Footing Design Guide
RE: Overturn Moment FOS when Uplift is involved.
I think physically, the text reference's method is more representative of what happens and the practical FOS.
However, the overturning FOS is typically defined as the sum of resisting moments divided by the sum of overturning moments, so Enercalc's method is technically correct per our engineering convention.
Assuming you're checking the FOS against the conventional limit of 1.5, you should probably follow the conventional definition.
RE: Overturn Moment FOS when Uplift is involved.
RE: Overturn Moment FOS when Uplift is involved.
However, when you look at it from a point of view of the "Enercalc" method, this uplift is indeed an applied overturning moment.
This means that that we're considering the force distribution of the heel (or toe) as a single tipping point of infinite compression resistance, which I don't entirely agree with. In reality, we're going to have distributed compression force acting to resist overturn. Is the FOS check an old off the cuff check from the past?
RE: Overturn Moment FOS when Uplift is involved.
I tend not to label the methods as "incorrect" vs "correct". Rather I tend to refer to them as a "Traditional / Simplistic" method vs a "true safety factor" method.
Now, it should be clear that I prefer what I call "true safety factor" method.
If anyone is interested, below is a link to write-up in the RISAFoot help file which explains how that program works (which appears to be similar to Enercalc) and compares the results of the two methods.
http://risa.com/risahelp/risafoot/Content/Stabilty...
RE: Overturn Moment FOS when Uplift is involved.
Note that failure could be by vertical uplift as noted above, could be taken as a loss of contact on the light side, could be taken as excessive bearing on the heavy side, etc.