How often has a life safety code requirement mandated a design change that impacts your vision for your design project? It happens often. When those requirements relate to egress size and location, there are alternative approaches that can help preserve the vision and still provide the code-mandated level of protection, and most importantly, ensure the occupants are safe. Performance-based egress solutions are a tool that all design professionals and owners should have in their toolbox.
I recently attended an event that included three speakers each presenting a recent adaptive re-use project. Each presenting firm indicated that one of the challenges they faced was egress as each buildings’ existing egress was inadequate for the proposed new occupancy. One project in particular had an especially big egress challenges because of the proximity to the property line and adjacent existing structures that limited the locations where they could provide additional exits. The locations where they were able to provide additional exit doors didn’t meet the prescriptive limitations on travel distance. As a result, they were forced to create additional exit doors and satisfy the travel distance requirements but the locations they had to use created some structural issues and ultimately added significant time and expense in structural modifications necessary to support the required exits.
At the conclusion of the presentations, I asked each architecture team if they’d considered any performance-based approaches to resolving their egress issues. Not one team had realized that there were alternatives to the prescriptive egress requirements. An opportunity to add value was overlooked because these teams weren’t familiar with performance-based egress options.
Differences Between Prescriptive Code Compliance and Performance-Based Egress Solutions
Before exploring the options for performance-based egress solutions, it is important to understand what the differences are between prescriptive code compliance and performance-based alternatives. The fire and building codes adopted by a jurisdiction set the requirements for a minimum acceptable level of protection. For example, in considering egress, the code provides limitations on travel distance to exits, common paths of travel, and dead end corridors. In addition, there are requirements for the number and size of exits available based on the calculated occupant load. The intent of these requirements is to ensure that occupants can safely escape the building in the event of a fire before the facility becomes untenable. The required number of exits and the travel distance and other limitations are the prescriptive code requirements intended to provide this minimum level of protection. However, there may be alternative approaches to the prescriptive code requirements that are capable of providing an equivalent or better level of protection. These alternative approaches are called “performance-based” because instead of focusing on following the code-prescribed checklist of requirements, they focus on the required level of protection and seek to provide it using alternative means.
This kind of performance-based approach is condoned by both the International Code Council (ICC) and the National Fire Protection Association (NFPA). The 2018 Edition of the International Building Code specifically allows performance-based approaches in the text of Section 104.11, Alternative materials, design and methods of construction and equipment. This section indicates that alternative materials, methods, design, and construction methods may be employed provided they provide an equivalent or better level of protection when compared to the level of protection that would be achieved if the prescriptive code requirements were applied. Additionally, all such alternative means and methods must be approved by the authority having jurisdiction (AHJ). Similar wording exists in the other ICC code books as well as in NFPA codes.
This requirement that performance-based approaches be equivalent to or better than the prescriptive code requirements makes it necessary to quantify the minimum level of protection intended by the code. In evaluating egress, we quantify the level of protection by considering the required safe egress time (RSET) and the available safe egress time (ASET). The RSET is the amount of time required for occupants to evacuate the facility. It is dependent on many factors including number and size of exits available and the travel distance occupants must cover to reach the exits. The ASET is the amount of time available between the detection of a fire and the onset of conditions that are untenable. When the RSET is less than the ASET (including a safety factor), the minimum required level of protection has been satisfied.
One of the most reliable methods of determining the RSET is through the use of computer egress models that use computational fluid dynamics to simulate the movement of occupants during evacuation. These models are highly sophisticated and can even be modified to account for mobility impairments and other human behavior characteristics that may impact crowd movement in egress.
It is possible to calculate the ASET by running a fire dynamics simulation model of a fire in the facility. By selecting a worst-case scenario fire based on the anticipated fuel packages in the facility, it is possible calculate how long a facility will remain tenable in the event of a fire.
However, in some cases, it may not be necessary to do a full FDS simulation to determine the ASET. It is possible to create an egress model that meets all the prescriptive egress requirements and use that model as a baseline for the minimum egress time necessary to achieve to ensure that a proposed egress layout meets or exceeds the prescriptive code requirements.
If the RSET exceeds the ASET, it then becomes necessary to make accommodations to reduce the RSET until all occupants can evacuate within the ASET. The recommended approaches vary based on the building geometry and occupancy. Some options for decreasing the RSET include early fire detection technologies, additional or upsized exits, horizontal exits, and smoke control.
This type of egress evaluation and performance-based approach can be extremely beneficial in projects such as the adaptive reuse project discussed above. In addition, this approach can be applied to many different cases where the prescriptive egress requirements might be difficult or impossible to satisfy. A historical facility where additional exits could damage the historic fabric, new tenant fit-out projects in existing high-rise buildings where the existing stairs are not sized per the current code, building renovations and additions, and complex manufacturing operations where equipment layout extends travel distances beyond those permitted by code are just a few examples.
Egress challenges can have significant impact on a facility design project. This can be especially challenging in renovations and additions to existing buildings where existing egress components may not satisfy currently adopted prescriptive code requirements. In such cases, performance-based egress approaches may offer more flexible and higher value solutions to achieving the required level of protection.
Telgian Engineering & Consulting is committed to helping our clients realize their vision. As teammates applying code-condoned alternatives to the prescriptive requirements for egress challenges in adaptive reuse and other renovation and addition projects, our value is in finding the most cost-effective means to realize that vision. How can Telgian support your vision and make your next project a success?
About the Author
April Musser, a certified professional engineer, is Southeast Regional Practice Leader of fire protection engineering for Telgian Engineering & Consulting (TEC). She has more than 15 years of experience in fire protection engineering and consulting, including code consulting and design. Musser has designed fire and life safety systems for large commercial and industrial facilities and has managed projects across a wide range of market segments across the Southeast and internationally.
She has also presented at numerous well-known conferences including the NFPA Annual Meeting and Conference, SFPE Southeast Annual Fire Safety Conference and Georgia Fire Safety Symposium, and the Campus Fire Safety & Emergency Management Conference. In addition, Musser is the author of numerous articles on building fires and their impact on fire and building codes.
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