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1.1 Scope. 1.1.1* This standard shall apply to the design of venting systems for the emergency venting of products of combustion from fires in buildings. The provisions of Chapters 4 through 10 shall apply to the design of venting systems for the emergency venting of products of combustion from fires in nonsprinklered, single-story buildings using both hand calculations and computer-based solution methods as provided in Chapter 9. Chapter 11 shall apply to venting in sprinklered buildings. A.1.1.1 This standard incorporates engineering equations (hand calculations) and references models to provide a designer with the tools to develop vent system designs. The designs are based on selected design objectives, stated in 4.4.1, related to specific building and occupancy conditions. Engineering equations are included for calculating vent flows, smoke layer depths, and smoke layer temperatures, based on a prescribed burning rate. Examples using the hand calculations and the LAVENT (Link-Actuated VENTs) computer model are presented in Annex D. Previous editions of this document have included tables listing vent areas based on preselected design objectives. These tables were based on the hot upper layer at 20 percent of the ceiling height. Different layer depths were accommodated by using a multiplication factor. Draft curtain and vent spacing rules were set. Minimum clear visibility times were related to fire growth rate, ceiling height, compartment size, curtain depth, and detector activation times, using engineering equations. The following list provides a general description of the significant phenomena that occur during a fire when a fireventing strategy is implemented: (1) Due to buoyancy, hot gases rise vertically from the combustion zone and flow horizontally below the roof until blocked by a vertical barrier (a wall or draft curtain), thus forming a layer of hot gases below the roof. (2) The volume and temperature of gases to be vented are a function of the fire’s rate of heat release and the amount of air entrained into the buoyant plume produced. (3) As the depth of the layer of hot gases increases, the layer temperature continues to rise and the vents open. (4) The operation of vents within a curtained area enables some of the upper layer of hot gases to escape and thus slows the thickening rate of the layer of hot gases.With sufficient venting area, the thickening rate of the layer can be arrested and even reversed. The rate of discharge through a vent of a given area is primarily determined by the depth of the layer of hot gases and the layer temperature. Adequate quantities of replacement inlet air from air inlets located below the hot upper layer are needed if the products of combustion-laden upper gases are to be exhausted according to design. See Figure A.1.1.1(a) for an illustration of the behavior of fire under a vented and curtained roof, and Figure A.1.1.1(b) for an example of a roof with vents. ****Insert Figure A.1.1.1(a) Here**** FIGURE A.1.1.1(a) Behavior of Combustion Products Under Vented and Curtained Roof. ****Insert Figure A.1.1.1(b) Here**** FIGURE A.1.1.1(b) View of Roof Vents on Building. The majority of the information provided in this standard applies to nonsprinklered buildings. A limited amount of guidance is provided in Chapter 11 for sprinklered buildings. The provisions of this standard can be applied to the top story of multiple-story buildings. Many features of these provisions would be difficult or impracticable to incorporate into the lower stories of such buildings. 1.1.2* This standard shall not specify under which conditions venting is to be provided or required. A.1.1.2 The decision whether to provide venting in a building depends on design objectives set by a building owner or occupant or on local building code and fire code requirements. 1.1.3 Where a conflict exists between a general requirement and a specific requirement, the specific requirement shall be applicable.
This standard establishes guidelines for the design of venting systems for the emergency venting of products of combustion from fires in buildings. Both manual and computer-modeled solution methods are incorporated to help designers meet selected performance objectives related to a specific building and a specific set of circumstances.
Requirements cover design fundamentals, vents, air inlets, draft curtains, the design fire, sizing vents, mechanical smoke exhaust systems, inspection and maintenance, and design documentation. A separate chapter of venting in sprinklered building is also included.
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© National Fire Protection Association (NFPA) 2016