When designing a structure it is important to consider the fire rating of the walls. Fire ratings are particularly important for structural walls essential to support or bracing the structure and fixed partition walls surrounding stairs and elevators. The term "fire-resistive rating" is the amount of time, in hours, a component resists a standard fire test without failure. Fire rating for different wall types are governed by the local building code. Some building codes give engineering and architects the option of procurement of a Fire Underwriter’s label for their overall design.
There are 3 fire conditions that generally are important for the fire resistive rating:
- High Temperature Structural Failure: The structural must maintain its structural integrity during a fire and resistance against burning, melting and deformation.
- Transmission Failure: Hot gases are released during a fire. The tendency for hot gas is to rise or move away from the flames. Transmission of these gases can result in failure of a structural member through holes or cracks.
- Stream of Water Failure: Make the structure capable of withstanding a stream of water from a fireman’s hose. This includes during a fire event and after the fire.
Good fire rating: Concrete block wall without insultation panel
Excellent fire rated wall: Reinforced concrete wall
The best wall materials to resist against fire all contain non-organic materials. Reinforced concrete, solid brick, brick plus block, glass block, and insulation-lined concrete block wall materials all provide excellent protection against fire inside a structure. All of these wall materials also are frequently used by designers as structural load bearing walls.
In terms of fire protection, the next tier of wall materials includes precast concrete panels, metal sandwich panel walls, corregated sheet steel on steel framing, concrete block without an insulating lining, structural clay tile, solid gypsum perlite plaster on metal lath, steel lath and studs with gypsum perlite plaster on both sides, and gypsum block with plaster on both sides. These materials are capable of providing good fire protection. The reason that gypsum building materials rate well in terms of fire rating is its natural chemical properties. Gypsum is the only known natural building material that releases water when heated to high temperatures. The release of this small amount of water delays the spread of fire in a structure.
There are wall materials that are fair to poor at resistance of fire. Some of these wall systems are commonly used in residential and commercial construction. Fair to poor fire rated wall materials include brick veneer on wood or steel framing, wood studs with gypsum drywall board on both sides,
Improvement in all structural materials is possible by using fire resistive materials (FRMS) or by applying spray-applied fire resistant materials (SFRM). As mentioned, low-density gypsum is a popular FRM. Cement materials with lightweight fillers or special fibers are also used to resist fire. These ingredients can be a FRM which is used to build the structural units or a FRMS that is sprayed on to the structure during construction.
Another new method of adding fire resistance in structural wall is the application of a intumescent coating. Intumescent coatings expand rapidly to a volume up to 40 times when heated to a high temperature. The intumescent material then acts like a sacrificial thermal barrier to allow the main structural members to survive during a fire.