# Interface "contact" forces **URL:** \ **Category:** FLAC3D\ **Created:** [July 6, 2024, 1:27pm UTC](https://forum.itascainternational.com/t/interface-contact-forces/1572 "2024-07-06T13:27:09Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![mprevitali](https://avatars.discourse-cdn.com/v4/letter/m/47e85d/32.png) [@mprevitali](https://forum.itascainternational.com/u/mprevitali)\ **Post date:** [July 6, 2024, 1:27pm UTC](https://forum.itascainternational.com/t/interface-contact-forces/1572/1 "2024-07-06T13:27:09Z")
Hi, I am trying to calculate the “contact” forces between two bodies. Looking at the example “Axial and Lateral Loading of a Concrete Pile”, the best way to do so is using the interface function and doing the product of node area \* (normal stress \* axial component + shear stress component). Is it safe to assume that the shear stress components use the global coordinate system and not a local one based on the face orientation (i.e. as the contact shear does in PFC). Thank you ---
**Author:** ![dblanksma](https://avatars.discourse-cdn.com/v4/letter/d/858c86/32.png) [@dblanksma](https://forum.itascainternational.com/u/dblanksma)\ **Post date:** [July 8, 2024, 2:26pm UTC](https://forum.itascainternational.com/t/interface-contact-forces/1572/2 "2024-07-08T14:26:34Z")
Hello @mprevitali For interfaces, I believe the best way to calculate the normal and shear forces at the interface nodes would be like you mentioned - using the normal and shear stresses and multiplying by the node area. This really isn’t like PFC style contact forces. However, the new addition of **zone joints** in FLAC3D 9.0 and greater does include new joint logic that is based on PFC contacts - you can find more information on this new feature here - [Zone Joints — Itasca Software 9.1 documentation](https://docs.itascacg.com/itasca910/wad/wallzone/doc/manual/theory/theory.html?node2395)