Fighting Fires and Saving Lives in Large, Single-Story, Undivided Buildings

A closer look at the need to incorporate automatic smoke vents into these designs
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Sponsored by The BILCO Company
By Jeanette Fitzgerald Pitts

Introducing Automatic Smoke Vents

There are, generally, two categories of automatic smoke and heat-removal systems: mechanically opened and gravity opened.

Photo courtesy of The BILCO Company

Mechanically opened automatic smoke vents in double-leaf models ventilate larger areas.

Mechanically opened automatic smoke vents consist of covers that are over-sprung by a mechanical lift assist and held closed by a latching mechanism with a fusible link. The fusible link—the same technology commonly used as the trigger mechanism in fire sprinkler systems and automatic fire doors—melts at a specific temperature, opening the vent doors when temperatures at the ceiling reach a certain level. When released, engineered lift assistance opens the steel or aluminum covers and locks them in an open position, allowing the heat, smoke, and toxic fumes gathering at the ceiling to escape the burning building. Once the fire has been put out, the vents can easily be reset from the roof level. These automatic smoke vents are the most common solution used for smoke and heat removal in large, single-story, undivided buildings.

Gravity-opened vents, also referred to as melt-out domes, do not open by mechanical means. When the heat from the fire reaches the dome material, it shrinks the material until it falls into the building, providing a hole in the roof through which the heat and gas can escape. There are a few notable disadvantages in selecting this technology over the automatic smoke vents. The melt-out domes cannot be designed to activate in response to a fire alarm or smoke detector. They cannot be reset, and the dome material could contribute to the fire as it falls into the building. Melt-out ventilation is less common but offers a cheap alternative to mechanically operated smoke vents.

When a fire-venting strategy is implemented in a large, single-story, undivided building, the progression of a fire event is dramatically different, and the damage can be much more contained. The automatic smoke vent or vents located closest to the source of the fire open first, providing the most direct exit for the hot gases and smoke produced in the combustion zone. This reduces the spread of hot gases and smoke throughout the facility, and it slows the thickening rate of the ceiling jet and the smoke layer. With sufficient venting area, the thickening rate can be arrested and even reversed.

With the development of the ceiling jet and smoke layers better controlled, occupants will have time to evacuate safely, and firemen can enter the building and fight the blaze from the floor.

The Evolution of Automatic Smoke Vent Use and Regulation

After the GM fire in Livonia, in recognition of the catastrophic damage that could occur when fires start in large, single-story, undivided spaces, building codes and common design practices changed. There were new restrictions on roof tar buildup, a required separation of hazardous operations, sprinkler requirements in industrial buildings, a mandated fire coating for steel-frame trusses, and new requirements around the use of automatic fire doors. It also became a widespread practice to install automatic smoke vents into the ceilings of storage buildings, manufacturing facilities, and warehouses. To provide guidance on the design of these life-safety systems, the Standard for Smoke and Heat Venting, authored by the National Fire Protection Association (NFPA), was also created. The standard is referred to as NFPA 204.

NFPA 204: Standard for Smoke and Heat Venting, 2018 Edition

Initiated in 1956, the first guide dedicated to smoke and heat venting was officially adopted by NFPA in 1960 and has undergone regular revisions since then. The 2002 edition was converted from a guide to standard, and the 2018 Edition is its latest iteration. As it describes in Chapter 1, “This standard shall apply to the design of venting systems for the emergency venting of products of combustion from fires in buildings.” Provisions are included to address appropriate designs for both non-sprinklered, single-story buildings and sprinklered buildings.

Photo courtesy of The BILCO Company

Smoke vents are available in single-leaf models for smaller, more confined areas like stairwells or elevator shafts.

The standard identifies design objectives for smoke and heat venting systems and explores vent design constraints, methods of operation, and vent dimensions and spacing, as well as information on air inlets, draft curtains, mechanical smoke exhaust systems, and required inspection and maintenance. Ultimately, NFPA 204 helps designers meet selected performance objectives related to a specific building and a specific set of circumstances. Those performance objectives include designing a vent system that will “slow, stop, or reverse the descent of a smoke layer produced by fire in a building by exhausting smoke to the exterior.” By controlling the smoke layer, a well-designed vent system will provide occupants with a safe path of travel to a safe area, facilitate manual firefighting, and reduce the damage to buildings and contents that results from exposure to smoke and hot gases.

Although NFPA 204 is technically referred to as a standard, it does not specify under which conditions venting is to be provided or required. The decision whether to provide venting in a building depends on the design objectives set by a building owner or occupant and on local building and fire code requirements.

2018 International Building Code (IBC) Section 910

While NFPA 204 guides the design of automatic venting systems where they are required, the 2018 International Building Code (IBC) Section 910: Smoke and Heat Removal addresses the specific types of buildings that require these automatic smoke and heat vents.

910.2 Where Required

Smoke and heat vents or a mechanical smoke removal system shall be installed as required by Sections 910.2.1 and 910.2.2. Exceptions include:

  1. Frozen food warehouses used solely for storage of Class I and Class II commodities where protected by an approved automatic sprinkler system.
  2. Areas of buildings equipped with early-suppression fast-response (ESFR) sprinklers.
  3. Areas of buildings equipped with control-mode special-application sprinklers with a response time index of 50 (m · s) ½ or less that are listed to control a fire in stored commodities with 12 or fewer sprinklers.

 

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Originally published in Architectural Record

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