# Wharf 3D Example - System of Units

Hi,

While calculating the x-area of piles in the example why are we using the moment of inertia calculation formula?

``````> ; Include end-bearing effect
> struct link create side=2 target zone group 'End' range position-z 5.9 6.1
> struct link attach x=normal-yield y=free z=free range group 'End'
> struct link attach rot-x=free rot-y=free rot-z=free range group 'End'
> struct link property x area=[math.pi*d^4/64] stiffness=1e6 ...
>        yield-compression=0 yield-tension=1e20 range group 'End'
``````

Also in the documentation, it is mentioned that the bulk modulus of water used 2e05 Pa but In fact, it is 2e06 Pa. This should be a typo, right? It should be in kPa.

The fluid modulus is assigned a non-zero value 2e5 Pa so that the deformation from the soil matrix can cause the change of pore-pressure. Note that a smaller water bulk modulus 2e5 Pa rather than the real water bulk modulus 2e6 Pa is assigned to compromise the effect the free surface of water table (water is not completely confined).

According to that Pore Pressure values in the graph should be in kPa’s?

Hello @musicinquark,

I am not sure why the area of the end-bearing link is set to the moment of inertia. Typically, for end-bearing effects the area should be set to 1.0 and the stiffness calculated accordingly (see example - Axially Loaded Pile — Itasca Software 9.0 documentation). I’ll look into this.

For the water bulk modulus you are correct - it’s a typo when saying, “the real water bulk modulus 2e06 Pa …”, should be kPa. And the y-axis in the plot should be kPa. I’ll correct this in our documentation.

Thanks for your response @dblanksma . But in the provided example, stiffness is calculated at the node, and the stiffness derived from this calculation indicates that the ‘1’ value used in this context is irrelevant. So like springs soil dominates the yield condition.

Thanks!