How NFPA 101 defines building egress
NFPA 101: Life Safety Code provides fire protection engineers many resources for designing safe egress, along with several other life safety options
- Grasp the basics of the Fire Safety Concepts Tree to understand egress requirements.
- Know how NFPA 101: Life Safety Code affects building design as a whole.
- Understand that new technologies are redefining life safety systems.
Up until the 1966 Edition NFPA 101: Life Safety Code was called the “Building Exits Code.” The title was changed because, since its inception, the code was being expanded by adding requirements for protection features beyond exits. While exits from a building certainly contribute to life safety, there are many building features that contribute to providing adequate life safety in buildings. As our buildings become more complex, there is an increasing need for engineering solutions to provide life safety.
In simple terms, safe egress from buildings can be expressed in the following terms:
ASET > RSET
where ASET is the “available safe egress time” and RSET is the “required safe egress time.” NFPA 550: Guide to the Fire Safety Concepts Tree identifies what must be done to safely move people who are exposed to a fire. Another branch of the concepts tree addresses the defend–in–place concept used in some occupancies, such as health care occupancies, but this article will focus on providing a safe means of egress. Using Figure 18.104.22.168 of NFPA 550, to move people who are exposed to the fire, one must:
- Cause the exposed people to move, which includes detection, signaling and providing instructions.
- Provide movement means such as issues regulated by NFPA 101, including egress capacity and protecting the egress path.
- Provide a safe destination that is typically the public way.
Causing the exposed to move
For the occupants to egress the building or move to another safe destination, one must first cause the exposed occupants to move. To do that, the fire event must be detected. There needs to be a signal indicating the need to move and instructions need to be provided.
With respect to detecting the event, NFPA 101 recognizes both automatic detection and manual detection. Automatic detection typically is required when the occupants of a building may not be in a condition to detect the fire themselves in a timely manner, such as residential occupancies in which people may be sleeping. It also should be noted that the scope of NFPA 101 has been expanded to address other emergency events such as detecting the presence of carbon monoxide. As such, the detection systems that need to be designed may be more than just a fire detection system.
Once the event is detected, the occupants of the building must be notified of the need to move. With respect to fire events, the code requires occupant notification systems based upon the occupancy classification, the number of people in the building and the complexity of the building (e.g., high-rise). The design and installation details for such systems are found in the code as well as the reference standard NFPA 72: National Fire Alarm and Signaling Code.
While the code contains the prescriptive requirements for audible and visual notification there are a number of options available to the design professional as to how this is done. High-rise buildings and some occupancies require an emergency voice communication system. However, the design professional may determine that voice communication may be the best solution for the other projects as well. The design professional also may determine that selective notification of specific areas of the building is preferred, as compared to a building-wide general notification to all occupants.
In certain occupancies, such as health care and detention and correctional occupancies, private mode notification may be the more desirable approach. The design professional also should evaluate the advantages of newer technology or approaches such as addressable alarm notification appliances and narrow–band signaling for high–noise areas. Narrow–band signaling is an approach that requires an analysis of the various frequencies associated with the background noise and the alarm signal, then focuses on a bandwidth where the noise level is lower than the overall average. An example is that pesky bug that you hear even though the overall noise level may be quite loud.
The above items are design options permitted by NFPA 101 or the reference standard and it is the design professional’s choice as to which approach and technology are to be used. In many instances, the choice is based upon what best suits the owner’s project requirements and needs.
Proper design of fire alarm systems does not involve simply placing a note on the drawings, indicating that a fire alarm system shall be provided in accordance with NFPA 72 and to consider that the engineering design for the system. NFPA 72 contains a list of information that is to be provided on the design documents that are typically prepared by a registered design professional.
As noted above, the notification may be for events other than just fire. The code now requires that certain occupancies or building uses, such as university classroom buildings, require a risk analysis to determine the need for a mass notification system. The fire protection engineer is one stakeholder that should be involved in the preparing the risk analysis and, in many instances, will be the lead person developing the risk analysis. Guidance regarding how to prepare the risk analysis can be found in NFPA 72 and the Society of Fire Protection Engineers’ Engineering Guide: Fire Risk Assessment.
Once the occupants have been alerted, it may be necessary to provide additional instructions to the occupants, which are typically provided using an emergency communication system capable of transmitting voice messages to the occupants. Textual alarm notification appliances also may be required so that occupants with a hearing impairment are able to receive the information. Where a mass notification system is provided, there are additional performance requirements to be met.
Providing a safe means of egress
While some of the requirements related to providing a safe means of egress are more architectural in nature (e.g., travel distance, egress capacity, arrangement of means of egress), there are many engineering aspects as well. These range from normal building systems such as the heating, ventilation and air conditioning system and the need to prevent the recirculation of smoke to special systems, such as smoke control systems, designed to prevent smoke from adversely impacting the means of egress. Many smoke control systems will involve passive features (such as smoke barriers) along with the active smoke control system.
The requirements for the HVAC systems are typically found in the applicable mechanical code or in NFPA 90A: Standard for the Installation of Air-Conditioning and Ventilating Systems, which is a reference standard in NFPA 101. The special systems regarding smoke control are addressed in the applicable building and fire codes and in NFPA 92, which is also a reference standard in NFPA 101.
As shown in the Fire Safety Concepts Tree, providing a safe means of egress includes factors such as defending against the fire products, structural stability and maintaining a safe environment. These factors are addressed in NFPA 101 using a combination of active and passive fire protection features and systems, along with requirements related to products that may be used in the egress path. Starting with the products permitted in the egress path, the major code requirement is related to performance of interior finish materials with respect to flame spread and smoke development.
Providing structural integrity typically starts with the structural engineer identifying the structural elements required to protect against reasonably credible fire scenarios. The protection is typically provided with some passive features such as membrane protection, spray-applied fire protection and thin–film intumescent products. As noted above with interior finish materials, structural integrity also can be addressed by limiting the contents, especially within the egress path.
Historically, structural fire resistance has been determined after the structural system was designed. Optimizing the cost of the structural member can lead to higher costs for fire protection design. As members of the design team work together, it is possible to optimize the overall cost of the structural system along with the required fire resistance.
Automatic sprinkler systems designed in accordance with NFPA 13: Standard for the Installation of Sprinkler Systems can address all three of the factors listed above. By establishing fire control, the fire products also are controlled and structural integrity is impacted by the reduced fire size. While some consider sprinkler systems as only providing property protection, the systems also provide a life safety benefit.
Traditional sprinklers, now referred to as standard–response sprinklers, have been documented to prevent fire spread, especially when combined with passive fire protection features. However, fast-response sprinklers have been shown in fire tests and computer fire models as being able to maintain tenability, even in the room of origin, during certain fire scenarios. The benefits associated with fast-response sprinklers are recognized by NFPA 101 by requiring such sprinklers in certain areas such as health care patient sleeping and treatment areas.
The life safety benefit of fast-response sprinklers is also recognized by NFPA 13 by requiring such sprinklers in all areas classified as a light hazard occupancy. The proper classification of the occupancies and hazards being protected by an automatic sprinkler system should be identified by the appropriate engineers of the design team.
In addition to sprinkler protection, engineered smoke control systems often are used to protect the means of egress in terms of maintaining a tenable environment. Smoke control systems are required by NFPA 101 in certain instances such as smokeproof enclosures — typically pressurized stairs — in high-rise buildings and when the engineering analysis requires a smoke management system in atria. Smoke control systems are an example of an integrated fire protection system, the design for which typically includes electrical, fire protection and mechanical engineering considerations.
Part of the design process for such systems also should consider how the system will be commissioned. NFPA 101 now references NFPA 4: Standard for Integrated Fire Protection and Life Safety System Testing for the testing of integrated fire protection systems, such as smoke control systems in high-rise buildings and certain occupancies.
Even in buildings without an engineered smoke control system, there are fire safety considerations in the design of the building’s HVAC system. For example, the corridor of certain occupancies is not to be used as a supply or return serving adjoining spaces. In addition, there are requirements to shut down supply and return fans to prevent the recirculation of smoke. Engineering decisions need to be made as to whether these controls are handled by the fire alarm system or the building management system.
The ever-expanding role
As buildings become more complex and new materials are being used, the need for the right engineering to provide for the life safety of the occupants is expanding. There is considerable attention being given to the global warming potential for products being used in buildings. New products are being introduced to reduce the global warming potential or carbon footprint and some of these new products offer new challenges to the engineering community. For example, some of the newer refrigerants are classified as mildly flammable refrigerants whereas historically the refrigerants used in buildings were all nonflammable.
Another example of the expanding role of the engineer is to consider the total cost of ownership of the building. Solutions that may have a lower initial cost also may come with a higher operational cost. Similarly, there are products that offer lower operational costs, such as remote or automatic testing of a device, but have a higher initial cost.
In addition to total cost of ownership, the new technology may offer other operational benefits. For example, the previously mentioned addressable alarm notification appliances have a remote testing feature. When the audible notification aspects are tested, the alarm will sound for a short time (possibly less than three seconds) resulting in less disruption to facility operation.
New products, new technology, environmental considerations and new challenges posed by the design community will increase the role of the design engineers, not just fire protection engineers, in providing life safety to our building occupants. Codes, such as NFPA 101, attempt to keep up with the changing risks and technological advances. The resulting changes often involve engineered solutions to the challenges posed. Where change occurs more quickly than the codes can adjust, the alternative approach or equivalency options can be used. However, the NFPA 550 Fire Safety Concepts Tree can continue to serve as a fundamental basis to meeting the fire safety objectives for the project.