Stenbrook
Structural
- Dec 5, 2014
- 125
So I have a Geotech Engineer who has given me an uplift capacity equation for the drilled piers on a project I am working on. The equation is:
Qu=5.5*Cu*(B-b)
Qu=ultimate uplift capacity, pounds
cu=average soil shear strength above the footing (use Cu=900psf)
B=underream diameter, ft
b= shaft diameter, ft
Use a factor of safety of 2.0.
He also states that the pier will see 0.9 tsf ( 1800 psf) of adhesion for the top 9 feet of the shaft. Based on those numbers, if I were to use a 20/60 pier size the uplift load on the pier would be 65,973 lbs and the uplift resistance would be 8250lbs. Leaving me left with 57,723 lbs of net uplift left to resist. Even with 20 feet of grade beam, a 2'-8" tall CMU wall,plus the self weight of the wall, I still have 17,884 lbs of uplift left to resist.
My loading from the building is light as it is a PEMB structure. For instance I have a column with a load of 4.4 kips. Which would have to be factored by 0.6 still.
The bottom line is that if I were to just drill the pier and put it in the ground, it would not work with no load applied to it. So my question is, am I looking at the analysis here wrong? Do I not include the adhesion in my uplift calculations but only in the design of the reinforcement of the pier? Also, is it possible that the uplift capacity equation is incorrect? (cohesion (psf) multiplied by (B-b) (feet) would give me a pounds per foot, not pounds. Granted I don't know what the units might be on the 5.5 factor.)
If anyone can help me that would be awesome!
Qu=5.5*Cu*(B-b)
Qu=ultimate uplift capacity, pounds
cu=average soil shear strength above the footing (use Cu=900psf)
B=underream diameter, ft
b= shaft diameter, ft
Use a factor of safety of 2.0.
He also states that the pier will see 0.9 tsf ( 1800 psf) of adhesion for the top 9 feet of the shaft. Based on those numbers, if I were to use a 20/60 pier size the uplift load on the pier would be 65,973 lbs and the uplift resistance would be 8250lbs. Leaving me left with 57,723 lbs of net uplift left to resist. Even with 20 feet of grade beam, a 2'-8" tall CMU wall,plus the self weight of the wall, I still have 17,884 lbs of uplift left to resist.
My loading from the building is light as it is a PEMB structure. For instance I have a column with a load of 4.4 kips. Which would have to be factored by 0.6 still.
The bottom line is that if I were to just drill the pier and put it in the ground, it would not work with no load applied to it. So my question is, am I looking at the analysis here wrong? Do I not include the adhesion in my uplift calculations but only in the design of the reinforcement of the pier? Also, is it possible that the uplift capacity equation is incorrect? (cohesion (psf) multiplied by (B-b) (feet) would give me a pounds per foot, not pounds. Granted I don't know what the units might be on the 5.5 factor.)
If anyone can help me that would be awesome!
