Spread footing overturn + uplift (Calculation vs Enercalc) 5

[StrEng007]

What is your approach to determining the factor of safety and the overall bearing stress for a spread footing subjected to overturning & uplift loads?

Overturning Factor of Safety Check
Resisting Moment = 25k (2ft) = 50 k-ft
Destabilizing Moment = 5k (2ft) + 10k (2ft) = 30 k-ft
Overturning Moment FOS = 50 k-ft/ 30 k-ft = 1.67

Now, if you consider the resultant load eccentricity and the relationship, Overturn Factor of Safety = L/2e (where L if the footing length, 4ft):
e' = 4ft - (50k-ft - 30k-ft)/25k = 3.2 ft
Eccentricity, e = 3.2ft - 4ft/2 = 1.2 ft
Overturn FOS = L/2e = 4ft/(2 x 1.2ft) = 1.67, OK
When I calculate the bearing stress, my approach is, qmax = (2 x 25k) / [(3 x 4ft)(4ft/2 - 1.2ft)] = 5.20 KSF

However, if you enter the same criteria in Enercalc, you get approx. 1.875 KSF, achieved by:
e' = 4ft - (50k-ft - 30k-ft)/(25k - 10k) = 2.67 ft
Eccentricity, e = 2.67ft - 4ft/2 = 0.667 ft
Overturn FOS = L/2e = 4ft/(2 x 0.667ft) = 3.0, DOES NOT EQUAL 1.67
qmax = [2 x (25k-10k)] / [(3 x 4ft)(4ft/2 - 0.667ft)] = 1.875 KSF

My question is, what is the best way to handle the destabilizing uplift load? And why does Enercalc's method not maintain the relationship where OT FOS = L/2e ?

 

[Settingsun]

Hi Josh, I understand what RISA is doing with the point load 'applied' at a location outside the footing footprint, but why? Is it a specific code provision that stability doesn't need to be satisfied under this loading, or is this RISA's own interpretation?

 

[IDS]

To get back to basics, the Factor of Safety (FOS) against overturning is the factor on the external loads that will cause the structure to overturn.
It follows that the factor should be applied to all the external loads, including the wind uplift.

Three analysis methods have been presented in this thread:
1. The RISA "True Safety Factor" method
2. The RISA "Traditional/Simplistic" method, from the same document (linked in 2nd post)
3. The method used in the Opening Post, which in effect ignores the uplift force, but increases the horizontal force to maintain the same nett overturning moment about the toe.

Method 2 treats the vertical wind load as a reduction of the dead load, which is not factored, so results in a higher and incorrect FOS.

Method 3 maintains the actual overturning moment, so finds the correct overturning FOS, but effectively increases the horizontal force, which increases the eccentricity of the resultant reaction force, and hence finds a much higher (and incorrect) bearing pressure.

Method 1 returns the both the correct FOS against overturning, and the correct bearing pressure.

The FOS formula given in the opening post, FOS = L/2e, assumes that the nett overturning moment is proportional to the reaction eccentricity, e. This is true if the external loads are all horizontal, so the reaction force doesn't change when external loads are factored, but when external loads have a vertical component it is no longer valid.

So if you want the correct FOS and bearing pressure, calculate the overturning moment with the vertical wind load included as an external load, calculate the bearing pressures using the actual forces, and don't use L/2e.

See calculations for the problem in the OP below:

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[bones206]

Note that ASCE 7-16 has new sections on Strength Design for Foundation Geotechnical Capacity in 12.13.5 and 12.13.6 (Seismic).

 

[CConrad]

ENERCALC technical support can certainly assist with your question. Please send your project file, software build number and user registration number to support@enercalc.com.

The question about loads “reducing overturning” or “increasing resisting” has been around for decades. The software applies the loads according to the sign entered and leaves the specific application of the loads to the user who can modify the signs as needed.

Director of Technical Services
ENERCALC, Inc.
Web:

http://www.enercalc.com

 

[JoshPlumSE]

Hi Josh, I understand what RISA is doing with the point load 'applied' at a location outside the footing footprint, but why? Is it a specific code provision that stability doesn't need to be satisfied under this loading, or is this RISA's own interpretation?

Footing Stability (as far as I understand it) is a service level failure criteria. So, I don't think it needs to be satisfied for strength level load combinations. As long as the total shear and moment demand on the footing are properly satisfied, I think you're ok. When I was at RISA I don't think I ever heard a user challenge that aspect of the program. If I was still working there, I might be able to go back through the "user request" system to see if there was a reference for that change. If I remember correctly, it was a request that had a lot of users behind it. Though it's long enough ago that I can't be sure.

Now, I have seen other engineers react to the this situation differently. By looking at the service level soil bearing and factoring it up. But, that never felt right to me and is unconservative in comparison.

 

[bones206]

Josh, see my post above. ASCE 7-16 has added some strength level stability provisions for foundations in Chapter 12.

 

[canwesteng]

I think that is just a misunderstanding of old ASD design. Footing stability should always be checked for strength combinations - under ASD this was done with a FOS, under LFRD you can use the load factors provided. For soil bearing (which is different than footing stability), geotechs in the US only use ASD with a FOS, so you really shouldn't be factoring your loads.

 

[JoshPlumSE]

Josh, see my post above. ASCE 7-16 has added some strength level stability provisions for foundations in Chapter 12.

I see what you're saying. Though, in my defense:
a) The portion of RISAFoot that I am talking about probably was added 10+ years ago. Long before these code provisions were in ASCE 7.
b) I believe the Strength Design for Foundation Geotechnical Capacity only applies when you (as the engineer) CHOOSE to use strength design as the basis for your soil pressure and overturning checks. But, you are not required to do so.
c) There really isn't much about overturning in that section.... likely because they know that when foundation design is done via ultimate strength level loads, the soil bearing will control.
d) I left RISA at the end of October 2017. So, ASCE 7 -2016 was relatively new. If I remember correctly, it wasn't actually release until well into 2017.

It is possible that RISA has added something for this "12.13.5" section on strength design of foundation geotechnical capacity.... I have not used their foundation programs since I left the company.

 

[canwesteng]

If you use ASD instead of LFRD for overturning, in ASCE-7 you arrive at almost the same factor of safety. There is a small (~10% for wind only, same results for seismic) edge to ASD right now, but I assume they will correct that in the future. For bearing pressure you are constrained to what capacity your geotech gave you, generally in ASD, in which case it is incorrect to use LFRD combinations.