Building and fire codes prescribe requirements for egress and evacuation that must be incorporated into facility design in order to provide a minimum acceptable level of protection. The prescribed requirements dictate limitations on travel distance, common paths of travel, and compartmentation.
They also set criteria for determining the occupant load and thus the number and size of exits required from the building as well as from each space inside a building. These requirements can sometimes interfere with facility aesthetics or may impose requirements that restrict the desired flow of the space or negatively impact the use of a space. However, there are avenues accessible through the fire and building codes to achieve the minimum required level of protection through a performance-based approach.
A performance-based approach in facility design is where the required level of protection is achieved through some means other than the means prescribed by the code. While the idea of applying a performance-based protection option is not a new concept and the approach has been widely embraced by jurisdictions and building authorities across the world, applying performance-based options to egress and evacuation requirements is often overlooked.
Part of the confusion stems from the requirement that the level of protection be somehow quantified in order to allow for a comparison between the level of protection that would be achieved through prescriptive requirements and the level of protection provided through a performance-based option. How does one quantify a level of protection for egress and evacuation? This article will seek to explain some of the most popular methods for applying performance-based design options to egress and evacuation as well as what tools can be used to quantify the level of protection for comparison to the prescriptive requirements.
Reasons for Considering a Performance-Based Approach for Egress and Evacuation
There are many reasons why compliance with the prescriptive egress and evacuation requirements may be undesirable. For example, in a historic facility, adding additional exits may damage the historic fabric of the building. In extremely large buildings, the limitations on travel distance may define the maximum floor area. In high-rise facilities, tenants that wish to maintain communicating openings between multiple floors occupied by the same tenant to achieve an open concept may fly in the face of compartmentation requirements that require floors to have fire resistance rated construction separating them from adjacent stories. The desire for these open communicating spaces may also be driven by environmental goals to reduce energy consumption by maximizing daylight in work areas. In some facilities, including industrial and manufacturing, egress and evacuation requirements may require facility layout decisions that could negatively impact workflow. Even aesthetic considerations can conflict with prescriptive egress requirements. These are just a few examples of why performance-based egress and evacuation options may be considered.
Model Building and Fire Codes
In the US, the most widely adopted model building code is the International Building Code (IBC) issued by the International Code Council (ICC). This article will focus primarily on the IBC but it is important to note that model building codes worldwide have similar language allowing the application of performance-based design approaches. Model building codes in Australia, the United Kingdom, Hong Kong, Japan, Sweden, and Canada all have similar allowances for application of performance-based design in their model building codes. This is not an exhaustive list so the first step in considering a performance-based design approach is to consult the applicable building code for the requirements specific to the jurisdiction. While we will be specifically focusing on the IBC, the procedure will be similar for any model building code that allows a performance-based design approach.
In the 2018 IBC, Section 104.11 specifically states the following:
“Alternative materials, design and methods of construction and equipment. The provisions of this code are not intended to prevent the installation of any material or prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, not less than the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety. Where the alternative material, design or method of construction is not approved, the building official shall respond in writing stating the reasons why the alternative was not approved.”
Similar language also exists in the International Fire Code (IFC) issued by the ICC as well as the Life Safety Code, NFPA 101, issued by the National Fire Protection Association which are both widely adopted in the US for egress requirements.
The important things to note are that
- Performance-based design options require approval of the Authority Having Jurisdiction (AHJ)
- The proposed performance-based design approach is required to provide protection that would be equivalent to or better than the protection provided by following the prescriptive code requirements.
The evaluation of the design to determine if it is equivalent to or better than the prescriptive approach requires an experienced professional Fire Protection Engineer. This experienced professional will be responsible for quantifying the level of protection to show equivalent or better protection. The experienced professional will need to select the appropriate tools to achieve this quantification of protection and translate the results of the analysis they conduct into reports with data that the AHJ can evaluate for approval.
The Performance-Based Egress Approach
The first responsibility of the fire protection professional is to identify the egress and evacuation requirements that are not feasible or practical. The next step is to and define the performance criteria to be satisfied and potential alternative approaches. Next, it is necessary to develop a strategy to quantify the performance criteria for comparison and select the proper tools to achieve this quantification. The tools are applied to analyze the performance-based approach. Based on the results of the analysis, it may be necessary to revisit and revise the design if the results aren’t satisfactory. Once a design illustrates that it can meet or exceed the required level of protection, the results must be documented and compiled into a format that can be submitted to the AHJ for review and approval.
Quantifying Egress and Evacuation Performance
When we apply prescriptive code requirements for travel distance and the number and capacity of exits, the focus of the prescriptive requirements is to provide a tenable environment during a fire for the entire required duration of the facility evacuation. To quantify egress time and tenability, we compare the Available Safe Egress Time (ASET) to the Required Safe Egress Time (RSET). The ASET is the length of time the facility is tenable to allow safe evacuation. The RSET is the length of time it takes to evacuate the building or area. If the RSET is lower than the ASET, we can generally conclude that we have achieved the minimum required level of protection for egress and evacuation.
To determine the ASET, the most commonly used approach is to create a computer fire model such as Fire Dynamics Simulator. It is important that the design fire used in the computer modeling approach represents a worst-case scenario fire. Therefore, the expertise of the fire protection professional is of paramount importance in ensuring that the design fire is appropriately selected. The computer modeling program will provide data on the rate of smoke production including temperature, visibility, and the concentration of toxic gasses over time. As products of combustion are buoyant, the tenability will begin to decrease at the top of a fire compartment first and will bank down as the fire progresses. Generally, model building codes anticipate that a smoke layer that is maintained at least 6 feet above the highest walking surface in a fire compartment represents a tenable environment. Therefore, the time it takes for the smoke layer to bank down to a height of 6’ above the finished floor is generally accepted to be the ASET. However, it will be important to confirm this height with the adopted model code and the AHJ.
To determine the RSET, a computer egress model can be employed. There are different egress modeling programs that are optimized for different kinds of facilities and scenarios and the fire protection expert will play an important role in ensuring the appropriate egress modeling tool is applied. The egress model will show the length of time it takes to evacuate the building or area in consideration to determine the RSET.
As with most engineering design calculations, an appropriate safety factor should be applied once the RSET and ASET are determined to account for unknown factors that may impact the ASET and RSET. Once the facility layout including any offsetting features, such as smoke control, can achieve an RSET that is lower than the ASET, it can be presumed that the minimum code prescribed level of protection has been achieved. It may be necessary to design a smoke control system to increase the ASET where building geometry and facility design cannot achieve desirable results in the comparison of ASET to RSET.
Real Life Application
In a facility in the US, an event space was planned for the top floor of an office building. Because of the increased occupant load for the event space, the number of required exits would have eaten up valuable real estate area on the office floors below. Based on the occupant load, the minimum required number of exits could be satisfied, but the egress capacity could not be satisfied with the size of the exit stairs. In this case, a fire model was developed using Fire Dynamic Simulator with Smokeview. The design fire was based on stacks of upholstered chairs to be used in the event space for seating to determine the ASET. Then an egress model was prepared using Pathfinder. It showed that the RSET exceeded the ASET meaning that the space could become untenable before all occupants would be able to evacuate the area.
To resolve the issue, a smoke control system was designed for the event space to exhaust smoke from the event space in the event of a fire to extend the time that tenability could be maintained. The smoke control system was designed to maintain the smoke layer at a minimum of 6 feet above the finished floor for the entire REST plus an additional 20 minutes as a safety factor to account for unforeseen conditions. The approach was presented to the AHJ for review and approved as an acceptable performance-based egress approach that provided an equivalent or better level of protection when compared to the prescriptive egress requirements.
Conclusion
Egress requirements are prescribed by the model building and fire codes to ensure that the occupants of a facility can evacuate the building before a fire renders the building untenable. Often, prescriptive code requirements related to egress such as exit capacity, travel distance, and compartmentation can be impractical or impossible to satisfy. In some cases, the prescriptive egress requirements may negatively impact flow or aesthetics or undesirably limit the options to maximize energy efficiency with natural daylight and communicating openings between adjacent floors. In cases where prescriptive egress requirements cannot be reasonably satisfied, a performance-based egress approach may be a useful approach. In a performance-based egress approach, it is necessary to define to quantify the level of protection. Tools available to quantify egress protection include computer fire models and computer egress models. Where assessment determines that the minimum level of protection cannot be met through a proposed design, it may be necessary to employ the use of engineered systems such as smoke control to offset the reduction in protection as part of the performance-based approach. Such performance-based approaches must be approved by the AHJ. However, there are strong precedents around the world for the application of performance-based design approaches for egress and evacuation. Such performance-based approaches are condoned by most model building codes both in the US and abroad.
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About the Author
April Musser, PE, CFPS, MBA is Regional Practice Leader, Fire Protection Engineering at Telgian Engineering & Consulting LLC.
