Embedment Length: Short vs Long Piles / Drilled Shafts
Embedment Length: Short vs Long Piles / Drilled Shafts
(OP)
I am designing a drilled shaft foundation for a high mast lighting tower. There are some differing opinions around the office about the required shaft length and I am seeking some more opinions.
The HML tower is 120' tall and the soil is sand with phi = 0.33. The applied service loads at the pile head are as follows: Axial = 5kip, Shear = 10kip, and Moment = 300kip-ft. The shaft diameter is 4.5' and controlled by the geometry of the anchor bolt connection to the tower.
I originally determined the depth using Broms' Method and came up with a length of 25'. I then input the information into LPile to see what the deflection curve looked like.
Here is my question. Is a short shaft adequate in this situation or is a long shaft required for stability? When a shaft length of 25' is input into LPile, the shaft is rigid and rotates in the soil like a "fence post". Is this acceptable as long as the deflection is within allowable limits (1/2")?
Some people I have discussed this with feel the shaft needs to be lengthened. They feel the shaft needs to have a minimum of two points of zero deflection for stability so the shaft does not behave like a "fence post" and tilt under load. In order to acheive this condition, the shaft has to be 48' long. Is doubling the shaft length to satisfy this requirement necessary or is a short shaft okay?
Any comments are apprectiated. Thanks.
The HML tower is 120' tall and the soil is sand with phi = 0.33. The applied service loads at the pile head are as follows: Axial = 5kip, Shear = 10kip, and Moment = 300kip-ft. The shaft diameter is 4.5' and controlled by the geometry of the anchor bolt connection to the tower.
I originally determined the depth using Broms' Method and came up with a length of 25'. I then input the information into LPile to see what the deflection curve looked like.
Here is my question. Is a short shaft adequate in this situation or is a long shaft required for stability? When a shaft length of 25' is input into LPile, the shaft is rigid and rotates in the soil like a "fence post". Is this acceptable as long as the deflection is within allowable limits (1/2")?
Some people I have discussed this with feel the shaft needs to be lengthened. They feel the shaft needs to have a minimum of two points of zero deflection for stability so the shaft does not behave like a "fence post" and tilt under load. In order to acheive this condition, the shaft has to be 48' long. Is doubling the shaft length to satisfy this requirement necessary or is a short shaft okay?
Any comments are apprectiated. Thanks.





RE: Embedment Length: Short vs Long Piles / Drilled Shafts
RE: Embedment Length: Short vs Long Piles / Drilled Shafts
If the shaft is made long enough, then it will bend before the soil at the base of the shaft yeilds and there will be zero deflection at the base.
Is this a necessary requirement for a foundation of this type or is tilting of the foundation allowed?
RE: Embedment Length: Short vs Long Piles / Drilled Shafts
I'd do some altenative design procedures to get you in the ball park.
Does your computer program allow for submergence of the soil?
RE: Embedment Length: Short vs Long Piles / Drilled Shafts
That paper is titled "Tapered Steel Poles - Caisson Foundation Design", Prepared for USS Corp., Teng and Associates, July 1969
For the structural design portion, refer to "Resistance to Overturning of Single Short Piles" - by Eli Zerniak,
ASCE Structural Journal, Vol. 83, March 1957.
RE: Embedment Length: Short vs Long Piles / Drilled Shafts
My concern about using a rigid, short shaft is that the foundation will rotate and the light pole would be tilted. Based on the LPile model, the foundation will rotate about a single point and tilt like a fence post. A 1/2 inch deflection at the top of the foundation would be about 1 foot deflection at the top of the light pole.
A tilted light pole could be perceived as a safety issue with the general public.
RE: Embedment Length: Short vs Long Piles / Drilled Shafts
RE: Embedment Length: Short vs Long Piles / Drilled Shafts
It states that capacity for "Short" piers are governed by soil failure and "Long" piers are governed by structural failure of the pier.
Prior to that, it also states that static methods (i.e. Broms Method) are primarily for small diameter piles (although it does not specifically define "small diameter piles"). And that static methods can be employed for preliminary design or as a check of the computer output in simple cases.
All of that considered, I would recommend using some type of computer analysis that considers p-y curve anlysis of the soil like L-Pile or something similar, and then using Broms Method as a comparison.
RE: Embedment Length: Short vs Long Piles / Drilled Shafts
tdunaway, AASHTO also says to use Broms Method for preliminary analysis only. That is one reason why I used L-Pile to compare the results. However, when using L-Pile I am unsure of exactly how to determine required embedment length and none of the books I have reviewed or post on here give any requirements.
I have reviewed previous designs for HML lighting foundations and the drilled shaft lengths compare favorably to the lengths calculated using Broms method. I know this length is long enough to prevent an ultimate collapse, but is it long enough for all serviceability issues?
Still uncertain on whetere or not a long pile is required for this case.
RE: Embedment Length: Short vs Long Piles / Drilled Shafts
If your deflection is permanent, then your system probably does not meet the stability requirements of the code (resisting moment > 1.5 times overturning moment)
It has been quite a while since I've used LPile, but check the point where the moment in the shaft goes to zero. That should be your point of fixity.
I haven't run the numbers, but the 20-25 foot depth seems about right.