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?

 

[phamENG]

I only use soil immediately above.

I try to engage the perimeter footing by designing the reinforcing to resist the moment. Slab is a last resort but I'll use it sometimes. There's no rule of thumb for me. It's all analytical and the reinforcement is proportioned to resist the loads induced by the uplift.

 

[JStructsteel]

Thanks. This is an interior footing, so no perimeter fooitng to use, thus why I was thinking slab.

 

[phamENG]

Sorry. I'm not used to those. I was thinking not corner. So use the slab but reinvigorate it for the uplift moment.

 

[DoubleStud]

I usually size the footing size based on the DL and LL. So assuming the soil is 2000 psf, you need about 5.5 x 5.5 footer. How did you come up with 7.5' square? Did you just make it bigger for this uplift? Just go deeper. 120 pcf vs 150pcf, you are not gaining much with concrete. Going deeper is cheaper because you use soil's weight.

 

[JStructsteel]

Doublestud, yes I went with footing weight, but perhaps your idea makes sense. Either way they have to put a pedestal on it to get to the slab elevation, so that would be less cost.

 

[Aesur]

For me, often times the top of footing is at 3 feet below surface to grab as much soil as I can. I use a 30 to 45 degree angle of soil. Based on the loading you have given, I come up with 24" x 10.75' square using soil above. I am using 0.6D for footing weight (as it's the load combination) and 110 pcf soil weight with 45 degree angle all 4 sides.

 

[DoubleStud]

Aesur, but if you use .6D, you try to make that equal to 0.6 wind correct? (assuming ASCE7-10 or later). 0.6D is technically 1.67 FS?

 

[Aesur]

0.6D would be using 0.6W for the load combination. I assumed the OP provided loads in nominal (ultimate) state under ASCE 7-10 or 7-16.

 

[BAretired]

If groundwater level is or can be higher than footing level, don't forget to consider the buoyancy effect.

 

[wth]

If it's an interior column it's probaly a bracing? For large uplift I'll combine the columns in the bracing in a combined footing - that way the sum of vertical wind (lateral) forces = 0.
Still have to deal with the eccentricity, gliding and wind suction on the roof, so you'll still need some depth.

I also only use the soil directly above, I've had projects where we've used an extended area of the slab - but documenting it was a PITA and suboptimal for the slab reinforcement.

 

[JStructsteel]

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?

 

[BridgeSmith]

Have you considered drilled shafts (AKA caissons)? That's our go-to for bridge abutments subjected to net uplift.

 

[JStructsteel]

WTH - This is a metal building frame, interior column. Uplift from wind, not any braching

Bridgesmith, contractor densest even want to pay for spread footings, drilled shafts would cause him to stroke out.

Instead of me reducing the footing, I put a bug in the contractors ear that perhaps the metal building supplier could reduce his loads

 

[phamENG]

Instead of me reducing the footing, I put a bug in the contractors ear that perhaps the metal building supplier could reduce his loads

Unlikely. Even if they could they probably won't. They have no motivation to do so. Unless you can show that they ignored your specified wind speed and exposure category and went with something more conservative, the uplift load is more likely to go up. They ignore my specs all the time and say that a metal building on the coast in hurricane country only needs to be designed for 115mph Exp. B.

Your original footing size works assuming you provided unfactored loading:

Building dead load = 19K
Footing dead load = 16.9K
Soil dead load (only direcly above) = 13.5K
Total Dead Load (ignoring slab) = 49.4K

Wind Uplift = -47.8K

0.6D+0.6W = 49.4K-47.8K = 1.6K ...net downward pressure so you're good.

Don't try to achieve a 1.5 factor of safety...that doesn't work with modern codes. Just use the load combinations and be done. Safety factors are built into them. If you really want to prove a 1.5, though...

Total Dead Load = 49.4K
Total Wind Load = 0.6(-47.8K) = -28.68K

49.6K/28.68K = 1.722 > 1.5 => OK

 

[WinelandV]

I put a bug in the contractors ear that perhaps the metal building supplier could reduce his loads

That is simultaneously mean and hilarious.

My experience with these center footings is that it will be quicker and easier for the contractor to just place a big ol' block of concrete and move on. Wouldn't be the first time that a contractor didn't realize he was putting up a kite, and thought that he'd just be able to land the center columns on a 3x3 pad.

Please note that is a "v" (as in Violin) not a "y".

 

[JStructsteel]

phamENG, was joking about the loads, contractor always complains footings are too big, some petty revenge is to say the metal building reactions are too big

For my loads: Isnt the 0.6D + 0.6W a ASD load combination, thus need a factor of safety?

 

[WinelandV]

JStructsteel,

The 0.6 load factor on the dead load is the 1.5 F.S. -> baked right into the load combination.

Please note that is a "v" (as in Violin) not a "y".

 

[EZBuilding]

The 0.6 factor on the dead load is the factor of safety.

1/0.6 = 1.6667

The 1.5 factor of safety got built into the load combinations a while back, but has remained as part of engineering folklore...

 

[phamENG]

WinelandV and EZBuilding covered it. The safety factor is built into the combination. If you were doing a steel building using ASD, would you add extra bolts to a roof girder connection over and above what the 0.6D+0.6W combination told you (assuming uplift controls)?