Point of Fixity in Drilled Shafts 1

[mtbgal23]

I am doing an analysis of drilled shafts supporting a bridge pier using the L-Pile program. This program allows me to input loads and soil layers, and gives me output such as pile head deflection, maximum moment, maximum shear, etc. It also gives graphs of deflection v. depth, bending moment v. depth, shear v. depth, and soil reaction v. depth. My question is: Where is the point of fixity? Is there a definite answer or is there a guideline somewhere?

 

[Ron]

I think there are two graphic options (at least) in LPile that will show you the fixity depth. One is the lateral deflection plot of the whole pile (not just the head displacement). The point at which the lateral deflection first reaches zero should be the fixity. You can check with another graphic, the soil response plot. In this one the inflection point of the soil response should be the fixity point. If you check both graphs, they should correspond pretty close.

Ron

 

[Hariharan]

I have found that defining point of fixity as the point of zero
deflection leads to a conservative design. In case of pile design
for jetties etc, where the pile extends fairly high above the soil,
the K.L/r factor becomes unduly large if you consider point of
zero deflection. We have used point of maximum bending
moment as the point of fixity for this purpose.

Hariharan

 

[yakpol]

I always thought, the point of fixity is used to simplify the designs. If you know point of fixity, you don't have to know the foundation springs. So the pile segment from ground line to point of fixity is to have the same stiffness as the soil springs at groundline. Unfortunately the location of this point depends on the magnitude of applied loads and border conditions of pile (fixed head or cantilever), and now I use POF approach only for very preliminary design, when I don't have enough Info to run LPILE.

 

[aun]

Usually when I design drilled shafts and columns, I always add the additional length, from top of dr.s to POF, to the columns. And then do the frame analysis to determine the max moment. That way both columns and dr.s will resist more moment. ( I ignore all soil spring stiffness) any comment is very welcomed..