How to use performance-based design for fire protection systems
The fire protection engineer must work within the prescriptive constraints of the building codes and standards while applying best engineering practices.
- Examine prescriptive and performance design criteria for fire and life safety systems.
- Show how the building’s use directly affects the design of the fire system.
- Explore practical applications for performance-based design.
Over the course of the fire protection engineer’s career, it’s common to have frequently been asked “What does a fire protection engineer do?” It is one of the more specialized engineering career paths that one can take—and also can be one of the more fulfilling.
Whether you work for a consulting engineering firm, for a manufacturer developing the latest technology to be used in buildings, or for an authority having jurisdiction (AHJ) like a building or fire department, it all boils down to one simple phrase: A fire protection engineer helps ensure the safety of the public in buildings. From the consultant’s standpoint, we design safe buildings. This may sound like a simplistic description, but this phrase is at the core of the consulting engineer’s job duties.
The trick is working within the prescriptive constraints of the building codes and standards, while applying best engineering practices to help achieve the vision of the project that the owner and architect envision. This is what is referred to as performance-based design.
Prescriptive code criteria
All projects are required to be designed using a specific prescriptive code, whether it’s the International Building Code (IBC), NFPA 101: Life Safety Code, or some other code as approved by the AHJ. It is within this document that criteria are specified where certain restrictions to the building design are applicable.
For example, in a fully sprinklered Group B office building, the maximum travel distance to the nearest exit is 300 ft, per the 2015 edition of the IBC. By setting a maximum travel distance, the International Code Council (ICC) has established a threshold by which all buildings are to be designed, based largely on previous-loss history and research.
This can be somewhat problematic, as not all buildings are intended for this one-size-fits-all approach. Architects are typically hired for their creativity and design, which cannot always be accounted for in the code. Likewise, in the case of a renovation, such as a historic building, often there is a push to maintain the integrity of the original design. This is where designing to the intent of the code is critical for the fire protection consulting engineer. The model codes also recognize this approach and include provisions for alternate materials and methods designs. This allows the design professional to propose an alternative approach to meet the requirements of code.
Understanding the “why”
Code development is a long, iterative process that happens typically on a 3-year cycle. Modifications can be made to the code by pretty much anyone, provided there is justification for whatever change is being proposed and the code-making body agrees to the change. As mentioned previously, the code is developed in part based on previous-loss history and research. For example, the 2009 IBC included new criteria for photoluminescent egress markings in high-rise buildings, additional exit stairways in what is termed as a supertall building (more than 420 ft in height), and different thresholds for spray-applied fire proofing material based on the height of the building. These provisions were added to the code because concerned citizens and government officials were passionate about a change they thought would improve the code and make buildings safer.
Another example would be modifications to egress-width factors. The 2006 edition of the IBC allowed for reduction in egress-width factors if a building was provided with an automatic sprinkler system designed in accordance with the requirements of NFPA 13: Standard for the Installation of Sprinkler Systems. The 2009 edition removed the allowance for this reduction, as the proponent of the change argued that most commercial buildings constructed nowadays require the installation of a sprinkler system; therefore, there shouldn’t be a benefit to the designer to reduce egress-width factors.
The following code cycle, the 2012 IBC, was updated to include the reduction once again, with a new wrinkle: that buildings were required to install an automatic sprinkler system and be equipped with an emergency voice-communication system. The justification here was that an emergency voice system provides building occupants with specific instructions and increases the level of safety to a building versus a traditional audible fire alarm system.
Understanding the “why” behind the code will assist the fire protection engineer in arguing intent and developing a cohesive performance-based approach to building design. By understanding the intent of the code requirement, an alternate design approach can be developed to meet the code intent while not the specific prescriptive-code requirement.
No matter the project type—whether it’s new construction, historical renovation, or tenant fit-out of an existing shell space—, understanding how the space is intended to be used and function is critical in developing an approach to fire and life safety. It’s critical to work with the project stakeholders to help achieve the project vision. Historical renovations typically require maintaining the integrity of the space, while a new high-rise building poses different challenges.
A fire protection report, or code analysis, can be beneficial for outlining the design criteria for the building. It often includes the types of construction, whether it’s a new building or renovation. Items to consider include the ratings of the walls, ceilings, floors, columns, and roof; the exit requirements; and criteria for the sprinkler, fire alarm and detection, voice alarm, smoke control, and emergency power systems.
The prescriptive codes can be used as a guide to the designer, outlining what should be included in the design. Where prescriptive criteria will not quite fit the design intent or cannot be met, there are options available to the designer. Specifically, the IBC allows for the use of an alternative means or method, sometimes referred to as an “equivalency” or “variance,” to be developed and ultimately approved by the AHJ. This is one form of performance-based design.
Alternative means and methods
Section 104.11 of the 2015 edition of the IBC details the following:
The provisions of this code are not intended to prevent the installation of any material or to 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, at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability, and safety.
This language permits the designer to use an approach outside the prescriptive requirements of the code, provided the building official deems it in compliance with the intent of the code. At its core, performance-based design is about alleviating some of the prescriptive requirements of codes and standards and allowing for design flexibility. Generally, the designer is obligated to demonstrate how the proposed alternative is equal to certain aspects of the code, which can include fire safety compliance and structural integrity.
The fire protection engineer will encounter many potential practical applications for performance-based design, whether as a result of prescriptive codes being impractical to the situation or a better solution existing outside the constraints of the code.
One of the more common applications is smoke control system design, specifically for large voluminous spaces. IBC Section 909 details requirements for smoke control design. One of the potential methods of smoke control is the exhaust method. Criteria for the exhaust method includes exhausting the smoke from the space at a rate to maintain a tenable environment for building occupants. Prescriptively, the smoke layer is required to be maintained at least 6 ft above the highest walking surface within the zone. This could prove to be problematic, as large volumes or occupied areas above the ground floor that are open to below can require significant smoke-exhaust rates on the order of magnitude of a million cubic feet per minute (cfm). This is where performance-based design could come in.
With the help of a computer model—a computational fluid dynamics model—reducing the quantity of exhaust is achievable. The engineer goes through a series of steps to determine fire size (which in itself is a performance-based design, as the code stipulates a minimum heat release rate of 5,000 Btu/second), the time the fire alarm system is activated, the time it takes for occupants to be notified and begin to exit, the rate at which smoke is produced based upon the heat-release rate specified previously, the time to sprinkler activation, and the time it takes to fill up the large volume above.
With consideration of all of these factors, and with the help of calculations for egress time, it’s conceivable that the exhaust rate could go from more than 1 million cfm to less than a quarter of that quantity. Various factors contribute to the final quantity, as evidenced above. Provided the engineer uses sound judgment and documents his or her thought process along the way, significant savings can be achieved through not only the reduction in fan size, but also the associated equipment that would go along with it and the ongoing maintenance and testing associated with smoke control systems.
Other applications of performance-based design may include the use of a clean agent suppression system for a data center or high-value commodity space, such as a museum. The IBC requires buildings over a certain square footage to be provided with an automatic sprinkler system. For unique hazards, such as a data center or museum, IBC Section 904 allows for alternative suppression systems.
It is important to note that the use of an alternative suppression system does not allow the designer to use the exceptions located throughout the code for a fully sprinklered building. This is another instance within a prescriptive code that allows the designer to use a different approach, as the prescriptive approach—in this case, water—is not desirable to be applied to computers, servers, and invaluable pieces of art. Clean agent systems are intended to extinguish the fire without the use of water and without damaging the contents of the space.
Another area where alternative methods are used in fire protection is the application of interior finishes and unique interior designs. The code has specific design criteria for application of interior finishes that often conflict with the interior design criteria. Using an alternative method to reduce the impact of interior finish applications can be of a benefit to the design while also maintaining fire safety measures. The main concern with interior finishes is the spread of fire and smoke should the finish ignite. Alternative measures to mitigate these concerns could include increased sprinkler protection for the space.
Cost savings is not the only reason to consider performance-based design, although this can be a significant driving factor, especially in the case of smoke control system design. Historical renovations may cost more when using performance-based design to maintain the integrity of the structure.
However, when considering what the goals of the project are, a higher priority may be placed on the aesthetics of the building over cost. Structures built in the 17th century were commonly constructed of heavy timber and masonry. Structures built in the 21st century are mostly steel and concrete. How do you compare what was built several centuries ago to the prescriptive codes of today?
There are inherent fire-resistance ratings of heavy timber and masonry. This is part of the equation. Additionally, the sprinkler system that is now required by code could be enhanced to offset the prescriptive requirements for the fire-resistance rating of the structure. To maintain the aesthetics of the masonry or heavy timber, it is likely not desirable to apply intumescent paint or spray-applied fire-resistive material, like a cementitious coating. The inclusion of other active and passive fire protection and life safety features may help mitigate the concern associated with a prescriptive design requirement.
It’s not enough for a fire protection engineer to be knowledgeable and experienced. The best are out-of-the-box thinkers as well, able to provide demanding clients with creative yet safe solutions for buildings and attractions that are designed in ways that the code-writing bodies never considered. This requires a certain amount of creativity.
When working with architects or other engineers, it is not the job of the consulting-specifying engineer to regurgitate what the code says. It is their responsibility to understand the intent and work within that framework to achieve the vision for the project.
To be a good fire protection engineer, one must understand the intent of code to allow it to fit the design of a building, and not just design to meet the code requirements. Clients often want help finding a way to meet the code while allowing them to create the design visions they want. Everyone wants safe buildings; the manner by which that is demonstrated is what differs at times.
Case study: Playground meets fire code
A different engineering approach to a playground structure allowed the owner to open the building’s playground with code-compliant fire protection systems.
JBA Consulting Engineers, an NV5 Co., was recently retained to review and address concerns associated with a children’s playground structure within an existing building. The owner submitted building plans to the authority having jurisdiction (AHJ) and had assumed that everything was in order. It wasn’t until an astute plans checker noticed that the playground structure was greater than what the code allowed, which was when JBA Consulting Engineers, an NV5 Co., was called in to help.
International Building Code (IBC) Section 424 limits the size of children’s playground equipment within a building to 300 sq ft in area. Upon the initial site visit, it was clear that the playground structure was significantly greater than the prescribed 300 sq ft as outlined in the code. The structure comprised multiple levels and was approximately 5,000 sq ft in area. The engineering team engaged with the owner to understand what their objectives were for the space, and suggested an approach via the IBC’s alternative means and methods process. This would include an analysis of the existing sprinkler and fire alarm systems, meetings with the appropriate project stakeholders, and negotiations with the building and fire code officials.
The owner was most concerned about opening the space. The company had signed a multiple-year lease and was at an impasse with the permit; therefore, they were paying rent for a space that was not generating any revenue.
JBA Consulting Engineers’ analysis of the sprinkler system included assessing the hazard and using the existing pipe network as best as possible. By increasing the design density and design area to coincide with the hazard of the plastics in place, the engineers addressed the concern of having a large combustible structure within a building. In addition, adding in an otherwise non-required smoke-detection system could reduce time to notify the occupants within the space; therefore, reducing evacuation time should an event occur within the space.
A site visit was suggested with the building and fire code officials so that everyone could comprehend the magnitude of the structure, but also get an idea of the operations of the space. During the visit, it also was suggested that a piece of the foam that wrapped the noncombustible frame be burned, to see how bad the hazard was. To most everyone’s amazement (especially those that are familiar with foam plastics under fire conditions), the foam hardly produced any smoke, there were no visible drips associated with it, and the plastic melted in a small area only. The flame did not propagate across the material as most expected it to.
By working with the owner and the AHJ, understanding the limitations of the code, and being a little creative, a solution was achieved that satisfied all parties concerned.
Nicholas A. Moriarty is executive director of fire protection at JBA Consulting Engineers, an NV5 Co.