A fire cut joist is designed and installed the same as a typical floor joist with one exception. At a supporting wall a regular joist has a square cut end, whereas a fire cut has an angled cut on the end. The cut angles away from the outside wall so that in the case of a fire severe enough to cause a floor collapse, THE JOIST CAN ROTATE OUT OF ITS BEARING, LEAVING THE WALL INTACT. If the joist has a regular square cut end, as the floor collapses, the end of the joist can push up on the wall above it, causing that wall to tip outward and endanger people outside of the building.
The detail below is conceptual only, showing the potential joist end cut geometry. Other considerations (by others) may be necessary, such as the design of the wall, diaphragm attachment detailing/design, etc.
Fire cuts are permitted on TJI joists provided:
- The TJI® joist has sufficient bearing,
- The cut does not extend past the inside face of the wall.
- The joist has full depth lateral support.