Uplift in spread footings

[bridgeengineer2007]

Looking for other interpretations of AASHTO LRFD 10.6.1.3 "Effective Footing Dimensions"...

It appears AASHTO allows a reduced footing for geotechnical purposes, but this will eliminate any uplift calculation...(since the concentrated force is now centered on the footing in the effective area)

When uplift on a spread footing is encountered, all AASHTO says in 10.6.1.7 is to "investigate for resistance to uplift and structural strength". So if the footing is structurally capable of resisting the uplift pressure and the footing is investigated for resistance to uplift (i.e. overturning is satisfied???) then is some uplift in your footing acceptable?

Thoughts?

 

[WARose]

Theoretically, the footing shouldn't see any uplift because the SF keeps the net bearing pressure always downwards.

However, in practice, I have typically just distributed the hold down weight on the top of the footing and designed the top steel (in the footing) accordingly. It basically looks like a typical bearing pressure diagram.....only rotated 180 degrees.

Granted this is pretty conservative......but especially in a industrial setting.....the last place I want to tie their hands at is the foundations.

 

[BridgeSmith]

Yes, I think. If the uplift resistance exists (overburden soil weight, anchorage to a drilled shaft or pile, etc.) and the footing is structurally adequate for that uplift, then it can be accounted for in the design. The area that is in uplift is obviously not bearing on the soil below it, resulting in an 'effective' geotechnical bearing area that is smaller than area of the footing. That reduced area must be accounted for when calculating the bearing capacity of the foundation soil.

If the footing uplift is resisted, you cannot ignore the effect that has on the bearing pressure on the soil or the loading demands on the footing in shear and positive and negative moments.

 

[Ron247]

I do not work with AASHTO, but from a purely structural analysis standpoint, a 1,000 pounds of uplift needs at least a 1,000 pounds of resistance to have a FOS of 1.0. We generally use 1.5 for working stress loads.

 

[skeletron]

Highter and Anders (1985) presented some useful charts for 2-way eccentricity and effective area. I found them in Braja M. Das' textbook and it was a neat exercise to implement for spreadsheet use.

 

[JStephen]

https://www.structuremag.org/wp-content/uploads/2014/10/C-Engineers-Notebook.pdf