Lateral Pile Analysis: Episode 6 - Can you tell if a pile head is fixed or free?

One of the questions I always ask in my lateral pile analysis course is this:

If I give you only the deflection profile with depth, can you tell whether the pile head behaves as fixed or free?

Most engineers pause for a moment

Then the fun begins.

The answer is yes.

And it comes directly from the basics.

To understand the pile head condition you do not need the full set of outputs. You only need to remember the chain that links everything together:

1. deflection

2. slope or rotation

3. bending moment

4. shear

5. soil reaction p

Everything comes from the deflection curve.
Slope is simply the derivative of the deflection.

So if the slope at the pile head is zero, then rotation is zero, which means the pile head is fixed. 

In the deflection shape shown in the figure, you can see that the tangent at the pile head is vertical.
A vertical tangent makes zero angle with the y axis (and ninety degrees with the x axis).
That is the signature of a fixed head.

Once you understand this relationship, lateral pile analysis becomes a lot more intuitive. You stop looking at the software as a black box and start reading the behaviour directly from the curves.

This is exactly what I focus on in the lateral pile analysis course: connecting the dots and reading the story behind every curve.