A fire cut is a diagonal notch made in the top of a joist to reduce the risk of a masonry wall collapsing during a fire. It allows the joist to sag instead of putting pressure on the wall. The structure is designed to favor inward collapse to protect people outside. Fire cuts must be carefully designed and placed.
A burn cut is a cut made into the top of a joist used with a masonry wall such as a brick or stone wall. This cut is designed to act as a safety device that will reduce the risk of the wall collapsing in the event of a fire burning through the joist. If the wall does collapse, the nature of the fire cut will encourage it to collapse inward, protecting people who may be on the outside, such as firefighters and rescue workers. Fire cuts are one of many safety features built into the design of structures to reduce the risk of losing life in a fire.
Joists are horizontal load-bearing beams that are used to distribute the weight of a structure across the walls so that it can be distributed down to the foundation. They may support floors or joists, depending on where they are located in a building, and are usually large and somewhat redundantly laid out so that if a single joist fails, the supporting joist network can still function. When joists fail in a fire, they add stress to existing joists, and joists that are burned can add significant horizontal pressure to the walls they are attached to.
The fire cut consists of a diagonal notch. The notch is not deep enough to compromise the integrity of the joist, but if the joint burns, it will allow the joist to sag, rather than putting high levels of horizontal pressure on the wall. While it might seem like a bad idea to have a sloped joist out of the wall, fire shear is actually beneficial because it relieves the horizontal pressure, which causes masonry walls to collapse during a fire.
The structure of the firebreak is also designed to favor the collapse of the walls inwards, should they collapse. Collapses can occur when the building is destabilized by an intense, widespread or hot fire and pose a risk to people working outside. If walls collapse outward, they can eject debris that can injure rescue workers, and materials that may be on fire can also be spread around the collapse site, putting nearby structures at risk.
Fire cuts need to be carefully designed and placed to ensure they are the right size. An experienced bricklayer, contractor or engineer can confirm that a cut is in the correct depth and position.
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