# Using Coulomb Static Friction in Nodal Supports

### Tips & Tricks

000494 11 March 2013

Friction plays an important role in practice. Without friction, the brakes of cars would be useless, objects on inclined planes would just slide away, and prestressed bolt connections would be impossible. Static friction is to be considered in those cases in which two solid bodies transfer a compressive force at their contact area. The compressive force changes the shear force resistance at the contact area.

The forces are related as follows:

$${\mathrm F}_\mathrm R\;=\;{\mathrm m}_\mathrm u\;\times\;{\mathrm F}_\mathrm N$$

where

 FR is the force of friction mu is the coefficient of friction FN is the normal force

Since the shear resistance between two elements influences the interplay of forces in the entire model, we have implemented the application of friction in our programs. It can now be used in the definition of nodal supports. This approach simulates the friction by using an automated iterative calculation process which checks the static friction conditions after each iteration and applies according measures to maintain the equilibrium.

The function was implemented in such a way that the user directly specifies the direction of the friction force for the degrees of freedom of the support as well as the direction of the compressive force. The coefficient of friction can then be defined in the detail settings.

If the static friction is exceeded, a so‑called ‘yielding process’ is induced. That means that if a force greater than the force of the static friction occurs on the friction degree of freedom, the nodal load would take up only the static friction force and then start to deform freely.