Building codes require that resistance to fire be considered in the design for protection of both life and property. The degree of fire resistance required depends on the type of occupancy, the size of the building, its location, and in some cases, the amount and type of fire detection and extinguishing equipment available in the structure. Precast concrete members are inherently noncombustible and can be designed to meet any degree of fire resistance that may be required by building codes, insurance companies, and other authorities.
Both fire resistance and containment must be considered when building. Building codes commonly assign fire resistance ratings based on results of standard fire tests. In recent years, there has been a trend toward calculating the fire endurance of building components, rather than relying entirely on fire tests. This is where concrete shines.
What Is Fire Endurance?
Fire endurance is defined as the period of time elapsed before a prescribed condition of failure or end point is reached during a standard fire test. The major “end points” used to evaluate performance in a fire test include:
- Structural End Point: Collapse of loadbearing specimens (structural end point).
- Flame Passage End Point: Formation of holes, cracks, or fissures through which flames or gases hot enough to ignite cotton waste may pass.
- Heat Transmission End Point: Temperature increase of the unexposed surface of floors, roofs, or walls reaching an average of 250 °F (122 °C) or a maximum of 325 °F (163 °C) at any one point.
- Hose Stream Test: Collapse of walls and partitions during a hose -stream test or inability to support twice the super-imposed load following the hose stream test.
Fire Resistance Rating
In IBC 2012, there are two ways to define the required fire-resistance rating. The first is based on type of construction, the second on separation distance, with the more restrictive of the two applying to the job being rated. Performance is defined by the authorities (regulatory and insurance) as the time for which each component would reach its controlling end point if it were subjected to a standard test.
Designing for Fire Safety
A key goal for the design team and the client of any given project is to protect the building from the many risks and potential losses that can be caused by fire. A common misconception is that fire destroys by flames, which can be suppressed by sprinklers. In practice, this oversimplification can leave both property and human life vulnerable during a fire.
Among the goals that must be achieved when designing for fire safety are the following:
- Contain high heat, which can melt or ignite materials or kill in one breath.
- Contain smoke that can blind, choke, and ruin building components and contents. Smoke is often generated by the sprinkler suppression process, and it is unavoidable.
- Contain toxic gas, which is given off when plastics, synthetics, and chemicals burn. They can be deadly at any temperature.
- Confine the fire event to its place of origin and prevent it from spreading.
- Reduce the fuel content of the building by using non-combustible building materials whenever possible.
- Avoid the potential for structural collapse during the fire by protecting all structural framing elements that support the building.
- Create a passive fire-protection strategy for the building that will enable it to survive should arson, low water pressure, or a delayed fire-department response occur.
- Divide the building into several noncombustible compartments that will help achieve solutions for all the aforementioned hazards. This is the most important aspect of all.
- Recognize that building codes provide the minimum protection allowable and may not be enough to achieve the fire protection the building and its occupants will need. Each risk exposure requires a defense.
Using precast concrete in your building solves many of these problems.
Using Precast for Fire Safety
As previously stated, precast concrete is naturally noncombustible, and can help contain a fire within minimal boundaries. As a separation wall, precast concrete helps to prevent a fire from spreading throughout a building or jumping between structures. During wildfires, precast concrete walls help provide protection to human life and the occupant’s possessions. As an exterior wall, concrete that endures a fire can often be recycled and reused when the building is retrofitted.
Noncombustible compartmentalization, combined with an inherently fire-resistant/tolerant structural frame, provide the best combination of economics and protection that owners and users seek. When this passive design combines with other safety measures, including sprinklers and early-warning detection systems, a balanced design approach is achieved.
A variety of precast concrete components can be used in creating a complete passive-design system for a building:
- Hollowcore planks, which can serve as a combined floor/ceiling system and can also be used as wall panels in either vertical or horizontal configurations.
- Wall panels, which offer high fire ratings and work with other components to create a noncombustible envelope. Insulated sandwich wall panels can also be used.
- Double tees, which can be used similar to hollow-core planks for roofs, ceilings, floors, or wall panels.
- Columns and beams, which create a framework that will resist intense heat and will not add fuel to a fire.
As you can see, a total-precast concrete system provides an effective design for minimizing fire damage and containing the effects within the smallest space possible for the longest time. It is the most economical way to ensure your next project meets all of the fire resistance requirements of building codes, insurance companies, and all other authorities.
Fire Resistance of Architectural Precast concrete Envelopes
Vice President of Project Management