I know I'm not the only one who walks into a building and immediately starts looking around to see what kind of life safety features a building has. When I’m looking around, I’m usually comparing what I see to what I know is required per NFPA 101, Life Safety Code. Sometimes, I have to remind myself that although what I see doesn’t align with what is required by Code, the building may still be compliant. Determining compliance isn’t always just following the applicable occupancy chapter requirements. Rather, two compliance options are recognized by the Code; prescriptive-based and performance-based. Both options offer equivalent levels of protection and one method is not preferred to the other. While we will focus on performance-based code compliance as it applies to the Life Safety Code, other codes, such as NFPA 5000, Building Construction and Safety Code and NFPA 1, Fire Code, also allow for performance-based designs.
The prescriptive-based option is what most people associate with compliance, looking through the Code and determining what requirements apply to your specific situation. However, sometimes this option is too limiting, and the performance-based option will be applied. This is especially true for complex buildings or buildings with unique functions and features. The Stratosphere Tower in Las Vegas, Nevada (pictured above) comes to mind. The tower rises over 900 ft (274 m) above grade with ten floors and outdoor amusement rides in the upper portion of the tower “pod”. At the base of the tower is a casino building. The occupant load of some of the floors in the pod would have required three remote exit stairs to go from the top all the way to the base of the building. The physical area of the structure isn’t big enough to provide remotely located stairs and the height of the building makes the use of stairs as a means of evacuation somewhat impractical. Through the performance-based design compliance option, the Stratosphere Tower uses typical exit stairs for the occupied floors (floors 3-10) discharging to areas of refuge on the lowest two floors of the pod as part of its primary evacuation method. The areas of refuge serve no other purpose and consist entirely of noncombustible construction. The floors serving as areas of refuge are open to the surrounding exterior environment so that natural ventilation occurs, and a mechanical ventilation system is not required to keep the areas free of smoke. While a single stair is provided from the area of refuge to grade, the primary evacuation route from the area of refuge involves the elevators. The elevator evacuation system is capable of moving the maximum 2600-person occupant load of the pod to the base building in under 1 hour. As you can see, this fire protection strategy departs from the typical approach. Another example is the crown of the Statue of Liberty where visitors can now go up and look out onto New York City. For these unique situations, the prescriptive-based compliance option would have eliminated the design flexibility, that was imperative for the design of these buildings. Therefore, the performance-based option was applied. In addition to the two compliance options recognized by the Code, there is also an equivalency clause found in Chapter 1 that allows alternative systems, methods, or devices to be used when they are approved as equivalent by the authority having jurisdiction.
Goals and Objectives
In order to allow for two equivalent compliance options, a common understanding of the minimum level of life safety needed to be established. This allows for performance-based designs to be evaluated against similar criteria that a prescriptive-based approach is assumed to meet. This is done through the goals and objectives, found in Chapter 4. The primary goal of the Life Safety Code is to keep occupants reasonably safe from fire and in addition to fire, to keep occupants safe from comparable emergencies (such as explosions), hazardous materials, and crowd movement.
The objectives compliment the goals and strive to provide more quantitative expectations than that of the goals. For example, to help clarify the expectation around the primary goal of life safety from fire, the first objective of the Code states that “a structure shall be designed, constructed, and maintained to protect occupants who are not intimate with the initial fire development for the time needed to evacuate, relocate, or defend in place.” Without these objectives, the goals could be interpreted differently. Perhaps someone thinks occupants should be reasonably protected from fire for 10 minutes while someone else thinks 2 hours. The objectives play an important role in providing appropriate context for the goals.
Option #1 Prescriptive-Based Code Compliance
The prescriptive-based option is the method people are most familiar with. In this approach, the design is in accordance with the core chapters and the appropriate occupancy chapter(s). The requirements outlined in these chapters and the resultant level of life safety is deemed to meet the goals and objectives of the Life Safety Code. For most buildings, this is the approach taken because the requirements are practicable to apply. However, there are situations where the structure is so unique, or the functionality of the space is so unusual that the prescriptive-based approach is too limiting. In this case, designers can use the performance-based option.
Option #2 Performance-Based Code Compliance
As mentioned above, the intent behind the performance-based option is to provide design flexibility. Designs utilizing this option must comply with Chapters 1 through 5. Chapter 5 states that if a design, for each design fire scenario, assumption, and design specification, meets the performance criterion, then it shall be considered to meet the objectives. The performance criterion states that any occupant who is not intimate with ignition shall not be exposed to instantaneous or cumulative untenable conditions. The annex material provides four different methods that could be used to show a design meets this performance criteria. One of the methods described is to perform calculations for each design fire scenario proving that each room or area will be fully evacuated before the smoke and toxic gas layer in that room descends to a level lower than 6 ft.
The performance-based design needs to successfully handle different fire scenarios. NFPA 101 provides 8 specific scenarios, covering a wide range of situations, that must be assessed. One design fire scenario that must be considered is a fire that starts in a normally unoccupied room and addresses the concern of a fire in such an area migrating into the space that potentially holds the greatest number of occupants in the building. In addition to the eight specified design fire scenarios, there is a requirement that the design fire scenario be as challenging as any that could occur in the building, but shall be realistic, with respect to any of the following: initial fire location; early rate growth in fire severity; or smoke generation. This usually requires at least one, if not more design fire scenarios to be added to the eight already required for evaluation. The intent of including these nonspecific scenarios, as opposed to only the specified scenarios found in Chapter 5, is to capture those scenarios in which initial fire location, early rate of growth in fire severity, or smoke generation poses a greater problem than those conditions captured by the required scenarios.
Performance-based design requires that the designer and authority having jurisdiction (AHJ) agree that the goals and objectives of the Life Safety Code have been met and that the desired level of safety is provided. The AHJ may require an independent third-party review of the performance-based design. As you can see, the selection of design fire scenarios as well as the evaluation of the scenarios is not a simple process. Therefore, performance-based design is usually reserved for unique situations where design flexibility is important.
The equivalency clause found in Chapter 1 permits alternative systems, methods, or devices as approved as equivalent by the authority having jurisdiction to be recognized as complying with the Life Safety Code. It is not intended to serve as a waiver of compliance, but instead requires that a level of safety is provided that is equivalent to that required by the prescriptive-based provisions. When employing the equivalency clause, it is important to clearly identify the prescriptive-based code provision being addressed, to provide an interpretation of the intent of the provision, to provide an alternative approach (proposed design), and to provide appropriate support for the suggested alternative (evaluation of proposed designs). One example where equivalency may be granted is using a newer edition of a standard. If your jurisdiction follows the 2018 edition of NFPA 101, then the 2016 Edition of NFPA 13, Standard for the Installation of Sprinkler Systems, would be the referenced standard for the installation of sprinklers. As a designer, you may want to use the 2019 edition, and could ask the AHJ to approve an alternative design as equivalent that uses the 2019 edition of NFPA 13.
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