There is general agreement that the goals of structural design against fire are to limit risks to the individual and society, to directly exposed or neighboring property, and to the environment. To meet these goals, fire protection requirements use the prescriptive format, e.g., they specify the permissible materials for buildings, the thickness of insulation, or the minimum acceptable spacing between buildings.
This is the traditional approach that continues to this day. In the early 1970s, performance-based approaches were developed, following an evolution in the understanding of fire and building performance in fire. Performance-based methods allow the designers to account for the unique features and uses of buildings and promote a better understanding of how buildings perform in fire. Compared to prescriptive methods, performance-based approaches have a greater potential to promote innovation and cost savings, but require more expertise.
1.0 Prescriptive Design
Traditionally, building codes have specified prescriptive methods to improve fire safety in buildings.
These methods use:
- Fixed values, such as maximum travel distance, minimum fire resistance ratings and minimum features of required systems, such as detection, alarm, suppression and ventilation;
- Safety factors, typically historically based, used to account for uncertainties inherent in the data, and to achieve the desired excess capacity;
- Exposure to a standard fire, such as ASTM E119 (2012) or ISO 834 (2002). The standards that focus on fire exposure (time-temperature curves) and fire scenarios, such as NFPA 5000
Most prescriptive codes include an equivalency clause that allows the use of performance-based methods to satisfy the intent of the code. Fire resistance rating is determined in compliance with the test procedures set forth in ASTM E119 or UL 263. Specified in ASTM E119 or UL 263 should be used (Sections 703.2 and 703.3 of IBC 2012). The test specimen is deemed acceptable if it can sustain the applied load during the fire resistance test without passage of flame or gases hot enough to ignite cotton waste for a period equal to that for which classification is desired.
For walls or partitions, and restrained or unrestrained beams and floor and roof assemblies, transmission of heat should not raise the temperature on the unexposed surface more than 250℉ (139℃) above its initial temperature. The IBC references other standards, such as those of the American Society of Civil Engineers (ASCE), the American Concrete Institute International (ACI), The Masonry Society (TMS), the Precast/Pre-stressed Concrete Institute (PCI) and the American Institute of Steel Construction (AISC).
Other prescriptive standards used in the US include ACI 216.1-07/ TMS 0216-07 Code requirements for determining fire resistance of concrete and masonry assemblies, and PCI 3rd ed. 2011 Design for fire resistance of precast/pre-stressed concrete. ASCE/SEI/SPFE 29-05 Standard Calculation Methods for Structural Fire Protection show how to calculate the equivalent fire resistance, in terms of hours, of concrete, timber, masonry and steel members, that would be achieved under the standard ASTM E119 fire test.
External walls and roofs must be constructed to avoid vertical and horizontal fire spread. The necessary protection may be achieved by one or more of:
- Separation distance between buildings;
- Using building elements that have a fire resistance rating (FRR);
- Restricting the use of combustible surface finishes;
- Limiting the areas of external walls and roofs that are close to a title boundary and that do not have an FRR;
- Providing parapets, spandrels or aprons; and
- Protecting the building with an automatic fire sprinkler system.
Fire Resistance Ratings (FRR)
To prevent fire spread or structural collapse, the Acceptable Solutions require building elements to have FRRs. The level of FRR required depends on the risk group of the building. An FRR comprises three numbers, which give time values in minutes for structural adequacy, integrity and insulation:
- Structural adequacy is usually provided by primary elements within a fire cell. (A fire cell is any space including a group of contiguous spaces on the same or different levels within a building, which is enclosed by any combination of fire separations, external walls, roofs, and floors.)
- Primary elements include building elements which are part of the structure, and those providing support to other elements with an FRR within the same or adjacent fire cells. Examples are: columns, beams, floors and walls (which may also be fire separations)
- Integrity is usually provided by secondary elements. Examples are fire separations, which are internal partitions and floors. Primary elements forming an integral part of a fire separation are also rated for integrity
- Insulation applies to fire separations and is required where the transmission of heat through the element may endanger occupants on the other side or cause fire to spread to other fire cells or adjacent buildings. For example, insulation is necessary for fire Stability of building elements having an FRR
- For building elements required to have an FRR:
- Primary elements in a vertical orientation (e.g., walls and columns) shall be rated for structural adequacy under the design dead and live loads and any additional loads caused by the fire.
- Primary elements in a horizontal orientation (e.g., floors and beams) shall be supported by primary elements with at least an equivalent structural adequacy rating.
- Building elements required to have an FRR shall:
- Be cantilevered from a structural base having an equal or greater FRR;
- Be supported within the fire cell by other building elements having an equal or greater FRR;
- Be supported by primary elements outside the fire cell.
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