When there is fire in a building, our instinctive reaction is to escape. However, the general public is less aware of the risks associated with smoke. First of all, smoke limits visibility, making it difficult to escape from a building in flames and also hindering rescue efforts by the NYC Fire Department. Many people are also unaware that smoke can kill just like fire, due to the toxic substances it contains; carbon monoxide is of special concern, since it can cause unconsciousness within seconds and death within minutes when highly concentrated.
Section BC 909 of the NYC Building Code provides the requirements for smoke control systems. These are categorized as life safety systems, and their main purpose is to provide suitable conditions for escape and rescue when there is fire inside a building. Smoke control systems should not be confused with post-fire smoke purge systems, which are not considered life safety systems, and have the goal of exhausting indoor smoke and restoring suitable conditions for occupancy after a fire is controlled.
Smoke control systems can be either mechanical or passive, just like ventilation systems. A mechanical smoke control system uses fans to establish an airflow that prevents migration of smoke into means of egress or other key building areas, while passive smoke control features use physical barriers to prevent smoke migration between different zones.
Importance of Pressure Differences in Smoke Control
Smoke control systems achieve their purpose by creating pressure differences between indoor spaces. If key areas such as elevator shafts and staircases are kept at a higher pressure than rooms containing smoke sources, it is impossible for smoke to move against the pressure difference by itself. There are three main ways to achieve this effect:
Injecting fresh and clean air into the space where positive pressure is required.
Exhausting air from the room containing a smoke source, creating negative pressure with respect to the smoke-free area.
Combining injection at the smoke-free enclosure with extraction at the source.
General Design Requirements for Smoke Control Systems
It is important to note that smoke and fire cause several physical effects that are not present during normal building operation, and they must be taken into account when designing a smoke control system. The minimum design requirements specified in the NYC Building Code are summarized in the following table:
1) Stack Effect
Conditioned indoor spaces have a temperature difference with their surroundings, and this induces bulk air movements that increase in magnitude according to building height.
Indoor air is warmer in the heating season, and it tends to leak out of upper floors, while outdoor air infiltrates lower floors.
The opposite effect occurs during the cooling season, when the temperature difference between indoor and outdoor air is reversed.
2) Fire-Induced Heating
During a fire, the localized heating effect causes air expansion and buoyancy, and this behavior interacts with the stack effect. Smoke control systems must also account for this.
3) Wind Effect
Smoke control system design must consider the effect of outdoor air patterns on air injection and extraction systems.
4) HVAC Operation
HVAC systems influence the spread of fire and smoke throughout indoor spaces. Depending on operating conditions, the effect of HVAC systems during a fire can be beneficial or detrimental, and this must be analyzed when specifying smoke control systems.
5) Local Climate
Air inlets and outlets used by the smoke control system must be located in a way that prevents obstruction by ice or snow during cold weather.
6) Operating Schedules
The NYC Building Code requires smoke control systems to be operational for a minimum period after fire detection. The required time is 20 minutes or 1.5 times the estimated evacuation period, whichever is longer.
The ducts used by smoke control systems must be capable of withstanding the expected temperatures under a fire, and tested at 1.5 times the maximum design pressure. In addition, the maximum leakage allowed is 5% of design flow.
Fans used in smoke control systems are required to have 1.5 times the number of belts needed to operate at design conditions (at least two). The motors used to drive these fans must have a minimum service factor of 1.15.
In general, smoke control systems must be designed to withstand seismic loads according to the ASCE 7 standards. Mechanical smoke control systems must be fully automated, but also provided with a manual override at the Fire Department control panel.
Power Requirements for Smoke Control Systems
The NYC Building Code also establishes minimum power supply requirements for smoke control systems. The primary power supply should be the electric service entrance, and a standby power system should be used as a secondary supply.
Smoke control system elements that use memories or other sensitive electronic components must be equipped with an uninterruptible power supply, of enough capacity to tolerate 15 minutes of primary power interruption. The system must also include adequate surge suppression, and power conditioning if needed.
Testing Requirements for Smoke Control Systems
The NYC Building Code also establishes the periodic testing requirements for smoke control systems:
Weekly verification using the automatic control system.
Yearly inspection of sensors, dampers, fans and controls.
Full testing every 5 years, by an inspector qualified by the NYC Department of Buildings.
Smoke and Heat Vents
There are specific types of occupancies where the NYC Building Code requires the installation of smoke and heat vents on the rooftop.
Occupancy Group F-1 (Moderate-Hazard Industrial) with a floor area above 50,000 ft2.
Occupancy Group S-1 (Moderate-Hazard Storage) with a floor area above 50,000 ft2, excluding aircraft repair hangars.
Buildings and building portions with high-piled combustible stock or rack storage, regardless of their occupancy classification.
These smoke and heat vents must be labeled and listed according to the UL 793 Standard for Automatically Operated Roof Vents for Smoke and Heat. These vents must be automated just like smoke control systems, but provided with a manual override as well.
Smoke control systems play a critical role in ensuring occupant safety. Uncontrolled smoke can increase the difficulty of escaping from a building in flames, it becomes life-threatening when large amounts are inhaled. Given that NYC fire protection requirements are among the most demanding in the world, working with experienced design professionals from the start of the project is strongly recommended.