Footing Uplift Resistance 1

[JStructsteel]

Checking a footing for uplift (19K DL, 43.3K LL, -47.8K WL). How much soil over the footing would you use (30 deg flair out from footing is what I am using). Would you use more slab weight than the projected area that makes due to reinforcing?

Its a metal building interior footing. So far I am 7.5' square, 2' thick, 2' below slab. Still only 1,18 FS

Any thoughts?

 

[JStructsteel]

Gotach, that makes sense, so for that LC just make sure I am 1.0.

For the 1.0D + 0.6W have a FS of 1.5 then?

 

[DoubleStud]

or the 1.0D + 0.6W have a FS of 1.5 then?

That will not control for uplift. May control if you do downward wind for column load.

 

[JStructsteel]

@ Doublestud, i am seeing that now, but for reporting in calcs want to show its covered.

 

[EngDM]

In the past, when doing interior footings as a last resort for uplift I have checked the max span of the floor slab on grade as a cantilever, and taken that amount of slab in a square around my footing. Doesn't typically add too much but if you are trying to get something to work it could help.

I do not take any flare out for the soil, only on the immediate footprint.

You might consider thickening the footing for dead load. The bearing surface is typically specified by the geotech at a depth, so adding more soil above isn't always an option.

 

[Aesur]

Aesur, the uplift ends up being 17.34 K, and DL I get 26K. Footing weight alone for a 10'-9" is 20.1K. Are you including the other DL's?

There is some rounding in my spreadsheet, however I think someone tampered with my formulas as my spreadsheet on my personal drive gave a smaller footing.

 

[phamENG]

JStructsteel - 1.0D+0.6W wouldn't apply. You use that when the wind load is down, and you use the 0.6D+0.6W when the wind is up. You use more dead load when it is destabilizing, and less when it is stabilizing.

 

[phamENG]

however I think someone tampered with my formulas

And heads will roll...

 

[DoubleStud]

Aesur, where was the password protection? SMH

 

[Aesur]

Aesur, where was the password protection? SMH

There was not, I keep backups on a personal drive, but we try to let our employees have access to the files to edit if needed for specific reasons. This may need to change and I will look into saving as template files that cannot be easily overwritten. I do however have a few scripts that will unlock most older format spreadsheets.

 

[Aesur]

Found the issue - Someone added a newer building code to the dropdown, but didn't update the associated formulas that control for load cases, so it defaulted back to 1.0W instead of 0.6W.

 

[JLNJ]

I will use the frustrum of the cone with a 1:2 slope. Most interior footings are pretty shallow so that just gets you a little extra slab resisting the uplift.

If you have a big, thin, deep footing, don't forget top reinforcement or to check it for plain concrete flexure.

 

[ChipB]

Long ago....
The load combination used to be 0.9DL + 1.5WL for the 1.5 factor of safety.
The thought behind the 0.9DL, is, we, as engineers, may put a little too much load into the DL so it was reduced by 10%

So.....
ASD load combination of 0.6DL + 1.0WL
is really:
(2/3)(0.9DL + 1.5WL) => 0.6DL + 1.0WL

There you have it. The factor of safety is still 1.5

The 0.6WL is to bring the wind down to the ASD force.

 

[Eng16080]

ChipB's comment also explains why the associated LRFD load combination is: 0.9D + 1.0W