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.

05/30/2017


This article is peer-reviewed.Learning objectives:

  • 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.

Figure 1: JBA Consulting Engineers’ fire protection and mechanical engineers worked with the City of Las Vegas to provide a performance-based approach to the 9-story atrium within an existing building scheduled to house city employees. All graphics courtesy: JBA Consulting Engineers, an NV5 Co.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.

Project approach

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.

CSE1705_MAG_FFIRE_02.jpg Figure 2: A clean agent system is a type of suppression system and can be a part of a 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.


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