November 6, 2019
Saving lives with smoke vents
by Thomas Renner
A General Motors plant in Livonia, Mich., was considered a state-of-the-art facility for the post World War II-era when it was built in 1949. Four years later, a fire destroyed the expansive facility, killing six people and throwing 45,000 workers out of jobs, at least temporarily. Only an office building and a smaller power plant were reported salvageable of the 139,355-m2 (1.5-million sf) facility.
Sparks from a welder’s torch started the blaze, and the building went up in smoke in a matter of minutes. By the time firefighters arrived, there was very little they could do. The roof had partially collapsed, making the building too dangerous to enter. The fire burned itself out the following day, but not before the smoldering wreckage spread out across 14 ha (34 acres) resulting in a $50 million (almost $500 million in today’s dollars). It is considered the most destructive industrial fire in U.S. history.
At the time of the fire, the building and production processes were deemed fully up to all established fire and safety codes, and the building itself was said to have “the most modern fire-prevention and fire-quenching equipment.” However, a Fire Engineering article published in September 1953 reported the building “was enlarged several times, and there were no effective firestops to divide up the large floor area. Also, the sprinkler system did not cover the entire plant.” Later reports said no more than 20 percent of the building was sprinklered, and there were no fire walls, partitions, or roof vents.
Without a way to escape, the smoke, heat, and fire were trapped inside the building, creating temperatures intense enough to melt and warp metal girders and cause structural damage to roof trusses, which ultimately led to the collapse of the building.
“Two of the biggest challenges we face in fighting any fire are heat and smoke,” said Steve Martin, battalion chief for the Columbus, Ohio, Fire Department. He watched a fire at Dick Cold Storage in Columbus destroy the building in August 2016. That building also lacked smoke vents. “The heat of the fire radiates on everything surrounding it, causing the flames to spread and rapid degradation of structural elements.”
Understanding smoke vents
In June 2018, Dick Cold Storage opened a new, state-of-the-art facility with automatic smoke vents. The vents, manufactured by The BILCO Company of Connecticut, allow for the escape of smoke, heat, and gasses in a burning building.
“Vents will allow for the removal of heat and smoke will potentially slow the spread of fire,” Martin said. “They will also permit firefighters to see and enter the building, to possibly extinguish the fire early, preventing the entire building from becoming a loss.”
After the fire at Livonia, building codes and common design practices changed. One of the new practices included the installation of smoke vents on ceilings of storage buildings, manufacturing facilities, and warehouses.
Regulations for smoke vents in commercial structures are outlined in the National Fire Protection Association (NFPA) 204, Standard for Smoke and Heat Venting. NFPA 204 provides calculations to determine the required dimensions and spacing of smoke and heat vents. The number of smoke vents depends on the size of the building or area protected, the height of the ceilings, and the depth of the expected smoke layer, according to Robert Solomon, PE, division director for NFPA.
The number of smoke vents required is also determined by local building and fire codes and by the type of commercial structure. Dick Cold Storage has 169,902 m3 (six million cf) of storage space, which is the usual unit of measurement for the cold storage industry.
“In the cold storage industry, it is not so much about square feet as it is about cubic space,” said Rob Adams, a partner at Tippmann Innovation who helped design the new building. “It is about getting pallet positions. You want to maximize square footage, so you generally build horizontal instead of vertical. You are refrigerating the same footprint. It is significantly more costly to build a refrigerated building.”
In 2017, architects designing the Hale Centre Theatre in Utah had workers install 20 acoustical smoke vents. The theater stretched out over 12,356 m2 (133,000 sf)—about 9290 m2 (100,000 sf) larger than the cold storage facility—and only required two more smoke vents. The acoustic vents were required, however, to limit noise intrusion from interstate roadways and a nearby airport.
“A cold storage facility may have a greater fuel load than a theater, which correlates to a design with a higher expected heat release rate, greater temperature outputs, and an increased smoke production rate, which may require larger, more closely spaced vents,” Solomon said. “The design influences the number, size, spacing, and activation requirements. The theater scenario is largely directed at stages and is essentially a key occupant life safety feature.”
Without vents to exhaust the smoke, the smoke is contained within the building, said William Koffel, PE, FSFPE, and president of Koffel Associates.
“This could impact egress, cause increased property damage, and hinder the efforts of first responders,” he said.
Koffel said if the incident commander determines that ventilation is necessary and appropriate, firefighters will manually cut holes in the roof to serve as roof vents.
“Many of these buildings are light steel construction and there may be concerns with the integrity of the roof upon which the firefighters need to work,” he said.
Clearing the air
When a fire develops, a smoke plume comprising hot gases and smoke rises upward into the space directly above the fire. When the smoke plume impacts the ceiling, the hot gases and combustion products spread out horizontally under the ceiling surface, quickly reaching areas of the building that are removed from the immediate location of the fire. The rapid flow of hot gas moving in a shallow layer beneath the ceiling surface is referred to as the ceiling jet. The smoke layer flows under the ceiling jet. As the fire continues to burn, more smoke and hot gases rise to the ceiling.
Ambient air is also entrained into the smoke plume. As a result, the layer of hot gas and smoke at the ceiling thickens, and the temperatures of the ceiling jet and smoke layer rise. The accumulation of smoke inside the building limits visibility, which can make it more difficult for occupants to escape and firefighters to gain access to the fire. The intense heat building at the ceiling will eventually begin to weaken the structure, increasing the potential danger to the people and firemen on the scene and the amount of damage caused by the fire.
Underwriters Laboratory (UL)-listed smoke vents are required to open automatically and be equipped with both an interior and exterior pull release that allows firefighters to open them manually. Automatic smoke vents can be actuated by fusible links or electrically by a heat or smoke detector or sprinkler water flow switch.
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 the development of the ceiling jet and smoke layers better controlled, occupants will have time to evacuate safely, and firefighters can enter the building and fight the blaze from the floor.
The automatic smoke vents installed at Dick Cold Storage include a Thermolatch® II positive hold/release mechanism that ensures reliable vent operation when a fire occurs. The vent automatically releases upon the melting of a UL-listed 74 C (165 F) fusible link. The curb-mounted fusible link housing allows the latch to be quickly and easily reset from the roof level.
It is also critical that smoke vents open in snow and wind.
“NFPA 204 requires opening mechanisms to remain free and clear, and require periodic inspection, testing, and maintenance,” Solomon said. “Additionally, NFPA 204 requires automatic vents to be fail-open, meaning that if there is a failure in the mechanism of the vent, it will automatically open.”
Acoustic vents work similarly, but they also have the added feature to guard against noise intrusion. Acoustic vents have a sound transmission class (STC) sound rating, which is a figure that rates how well a vent attenuates airborne sound. The rating roughly reflects the decibel reduction in noise that a vent can provide. Acoustic vents are ideal for concert halls, theaters, and other interior applications that require limited noise from the outside.
In the Hale Centre project, architects used BILCO smoke vents with an STC-46 rating. The theater is situated directly in the flight path of a nearby airport and a busy interstate runs next to it.
“The sound issues were pretty intense,” architect Lyle Beecher said. “We knew it was going to be an issue.”
One of the most important sections in NFPA 204 is a section that requires vents to be designed so that they can be inspected visually. Solomon said Section 5.2.4 of the standard requires the supporting structure and means of actuation have to be designed to be inspected visually.
“This is very important because too often mechanical equipment is installed in a manner which makes regular inspection, testing, and maintenance very difficult or infeasible,” he said. “The vent layout as presented on the contract plans is also critical. The vent size, arrangement, and geometry are often times based around the location of draft curtains and intended storage configurations. Deviations for the vent location may impact their effectiveness. Any changes from the original design need to be carefully evaluated.”
Solomon said smoke and heat vent requirements can be found in local building and fire codes. The decision to provide smoke and heat vents should be made by an engineer, as there are times when smoke and heat vents should not be provided.
“Vents should not be provided if the building is equipped with an early suppression fast response (ESFR) sprinkler system or if an alternative means of smoke ventilation is provided, such as an active smoke control or smoke evacuation system.” Solomon said. “The benefits of smoke and heat vents are also highly dependent on the size and geometry of the building and the expected size of the fire. They are most effective in large, open spaces with high ceilings with a significant fuel load.”
Difference between vents and sprinklers
Smoke vents and sprinklers serve different fire protection and life safety purposes. Sprinklers control or suppress a fire until the fire department can respond to extinguish it. Smoke vents allow for smoke and heat to leave the building, which can help increase the safe egress time for building occupants. They will also allow for increased visibility for both occupants and firefighters, and reduce interior temperatures for firefighting operations.
“Smoke vents are most effective in large, open areas with high ceilings and have limited benefit in a building which is highly subdivided,” Solomon said. “While the two can be used together, NFPA 204 requires an engineering analysis to make sure the design of the smoke vent system will not have a detrimental impact on the sprinkler system performance.”
“Proper operation of an automatic sprinkler system should result in a reduction in the amount of smoke produced,” Koffel added. “While the amount of smoke produced may be less, there are fire scenarios, especially in storage occupancies, in which a considerable amount of smoke is still produced.”
Solomon said there is some concern vents may create a vacuum effect and bring more outside air into the building or pull a fire in the direction of a smoke vent, which will help the fire continue to grow and spread.
While smoke and heat vents do increase entrained air into the building, the benefits from the significant reduction the amount of smoke and superheated gases in the space usually outweigh the possible fire growth due to the entrained air,” he said.
In June 2018, Dick Cold Storage opened a new facility with 15-m (50-ft) ceilings for expanded vertical storage, 169,902 m3 of storage space, 15,000 pallet positions, and seven multi-temperature storage rooms.
The new building also had many of the fire-protection technology and safeguards that were lacking in the original structure, such as horns and strobes, pull stations at doors, linear heat detection in freezers, and 18 automatic smoke vents.
The automatic smoke vents installed in the new facility feature a positive hold/release mechanism to ensure reliable operation. When the temperature at the ceiling of the facility reaches 74 C, the fusible link will melt and release the vent covers, allowing the hot gases and smoke to exit the building. Gas spring operators are designed to open the covers against snow and wind loads and include integral dampers to assure the covers open at a controlled rate of speed.
While devastating, the fire at the General Motors plant ushered in a new era in building design and safety. Automatic smoke vents are a powerful fire-response solution that can deliver significant protection to the people and property inside a burning building and help create an environment that allows firefighters to access and contain a fire as quickly and safely as possible.
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