Pile Fixity

[nbr1]

I am at a loss as to the term 'pile fixity' and its importance relative to a laterally or axially loaded pile.

My understanding is that it is a depth at which there is no lateral deflection. Why would this be important in pile design?

 

[SlideRuleEra]

The point of fixity is a simplifying assumption that allows the complex wave shape of a laterally loaded pile to be represented by a basic (vertical) cantilever beam. If this hypothetical point of fixity is correctly established then the upper portion of the pile is accurately modeled by the cantilever beam. The lateral load deflection for a vertical cantilever beam is easy to calculate.

See "How To Determine Lateral Load Capacity Of Piles" and "Laterally Unsupported Timber Pile Study" on this page of my website

http://www.slideruleera.net/AWPI-Page.html

http://www.SlideRuleEra.net

 

[msquared48]

It establishes the theoretical point of maximum moment in the pile due to cantilever beam action, as with a soldier pile with no tiebacks.

From this moment, the size of the pile, also considering maximum allowable deflection, can be determined. A pile that resists lateral load is merely a pole footing, and subject, for the post part, to the equations to design the embedment depth - also subject to the design recommendations of the Geotech.

For only axially loaded pile, the point of fixity is not so much a concern, but definitely is for any pile that resists any lateral load.

Mike McCann
McCann Engineering

 

[nbr1]

Wouldn't LPile or similar programs consider the 'complex wave form' shape of a laterally loaded pile?

It sounds like the 'fixity' condition above does not consider the soil interaction??

 

[SlideRuleEra]

Yes, Lpile does take into account the shape of the loaded pile. This is possible because of the low cost and ready available of computing power today (compared with a few decades ago). There is no doubt that accurate modeling of the "real" pile is the best way to solve the problem. Even the engineers "back then" knew that. They just did not have any way to do it at a reasonable cost; therefore a simpler roughly equivalent model was used.

The concept of fixity does take the soil interaction into account, but not directly. The computed depth to the point of fixity for the same pile / loading condition varies depending on the soil properties, as judged by the Engineer. This is a good example of why "engineering judgment" was a valuable commodity in days gone by.

http://www.SlideRuleEra.net

 

[nbr1]

The last post has cleared my confusion on this concept. Thanks for the references............

 

[MCP0915]

I have a question regarding point of fixity when using the L-Pile program. I have been told numerous different things about what is the correct point of fixity to use. Some engineers have said that it is the point of zero deflection. Others say it is the second point of contraflexure of the displacement curve (if the pile is fixed at the top) and others say it is the second point of curvature of your moment curve. Still others say it varies depending if you are evaluating the pile for moment, unbraced length or deflection. Help! Which is it?

 

[Panars]

Because it is theoretical, there is no one correct answer. It depends on what you are using the point of fixity for.