Pile ôFixed Head Conditionö

[J1D]

There have been some discussions on this topic. What I wonder is the detail to be entitled as “Fixed Head”.

In my impression, it is generally required to have at least 12” pile embedment into concrete, and some reinforcement is also required in the 12” thick concrete. The main reinforcement is still above pile cut-off. But some engineer advises that the main reinforcement of the pile cap should be placed near the bottom of concrete to make it really “fixed”. This sound a bit odd to me, not only the rebar can not go through the piles, also the Strut-&-Tie model (for vertical load) will be off the “hook” with this detail.

This is a pile cap with two steel pipe piles and cap plates, plus strong anchor rods from plates into concrete.

What do you think?

 

[prsconsultant]

I think the fixed head condition has more to do with lateral stiffness of the pile / pile group. Typically geotechnical engineer would make this determination based on the lateral shear load at the foundation level and the corresponding lateral displacement of the foundation.

 

[haynewp]

Look at the relative stiffness of the piles to that of the cap and grade beams. I think I have read you should embed the pile 2 times the pile diameter and have around 10 times the stiffness of the cap to that of the piles.

 

[J1D]

10 times (or even higher) stiffness of the cap is fine to prevent rotation of the pile under lateral load.

In additional to the stiffness of the cap, fixty of the connection between pile and pilecap is another important measure to make the "Fixed-Head Condition".

I believe 12" embedment is conventional for "Fixed-Head", but I'm not sure what reinforcement detail in the 12" concrete is more practical. I remember some statement from Joseph Bowles' foundation design book, but I couldn't find it now.

prsconsultant, you're right, lateral stiffness (or allowable lateral load) is typically the business of geotechnical consultants. They advice the pile lateral load stiffness of "Free-Headed" piles and "Fixed-Headeded" piles. What I'm talking is about how to make the connection (or interface) fixed.

 

[msquared48]

Fixity of the pile to the pilecap will give you nothing extra laterally without a good grade beam system also "fixed" to the pilecap.

Mike McCann
McCann Engineering

 

[J1D]

The pilecap here is the grade beam. See the original description.