By Andrew Bowman, P.E.
How safe is a building? When discussing fire safety, most people can’t provide a solid answer. They think that if a building is protected with a code-required fire protection system it is safe. Unfortunately, as we discovered on September 11, 2001, compliance with the building code is not a guarantee of any level of safety. While the scale of the terrorist incidents went well beyond the scope of any building code or any imagined fire protection scheme, there is an important lesson to learn that you can apply to more common situations.
Compliance with code requirements, while certainly a reasonable assurance of some level of fire and life safety, does not always address the more unique situations. Therefore, it may be more appropriate to ask, “In what types of situations are your building and its occupants safe from fire?”
ENTER THE FPE
To answer that question, you need to consult a fire protection engineer (FPE). An FPE is a practitioner of fire protection engineering—the application of fire-related engineering principles to protect against the destructive effects of fire. The application of these principles during building design and construction, where a great percentage of the architectural code requirements are fire and life safety related, illustrates the importance of fire protection engineering. Simply stated, fire protection engineering in building design and construction (much like other disciplines such as mechanical, electrical, and structural engineering) is the harnessing of scientific principles to provide better and safer buildings.
Typically, an FPE has a degree and license in the field of fire protection engineering, is versed in the nature and characteristics of fire, and can apply this knowledge to a wide variety of practical buildings or processes. A fire protection engineer can help by designing cost-effective and efficient fire protection and alarm systems that protect people and buildings. He can also evaluate the condition of a building and provide recommendations to improve the level of fire safety. Additionally, FPEs are heavily involved in writing the many codes and standards that affect nearly every corner of the building construction industry. This valuable experience, combined with a fire protection engineering education, allows an FPE to guide a project through the maze of fire protection and life safety requirements you see today.
FPEs are trained to address the many components associated with fire protection in buildings but also deal with the inevitable crossover of disciplines. FPEs design mechanically based systems such as automatic sprinklers. They design fire alarm systems based on sophisticated electrical circuitry. They specify the fire resistance necessary to protect the structural components in a building, and they are versed in the seemingly arcane fire safety related architectural requirements that often drive the layout of the building.
In most current building codes, the requirements are set forth in a manner that dictates precisely how a building will be built. This is called the prescriptive approach to building construction. Essentially, this entails following a one-size-fits-all approach to construction. The prescriptive approach is useful in that the requirements are easily enforceable and fairly obvious to those tasked with designing a building (although many would strongly disagree that the requirements are obvious!). The downside of the prescriptive approach is that most buildings have unique design features or required uses that the code does not properly address. This is the equivalent of fitting a square peg into a round hole and usually comes at the expense of functionality, aesthetics, or cost with little or no additional benefit. Additionally, building codes are typically developed in three-year cycles. Therefore, a new technology, approach, or design may not be usable because of prescriptive code limitations until several years later. Add in the typical delay for adoption of new codes, and it is apparent the building codes are not the most dynamic of documents. Unfortunately, as with most industries that rely on technology, the past few years have brought an explosion of new technologies and analysis tools that are difficult to use if they are not formally recognized by the prescriptive methods in the building code.
Today’s FPEs understand that evaluating the level of safety within a building is not necessarily limited to verifying the compliance with the building code or relying solely on techniques and solutions found in possibly outdated codes. A new generation of analytical tools allows us to evaluate a building for its own unique capability of withstanding a fire and discard the notion that all buildings must be treated by a cookbook prescriptive approach. This is called performance-based design.
The theory behind performance-based design is that you can obtain an appropriate level of fire and life safety by evaluating and designing a building based on its inherent merits instead of following a prescribed set of requirements intended to apply to a narrow range of occupancies. Performance-based design can be used to explore unique architectural designs while tailoring the fire and life safety systems to the building. This can help to reduce the costs of unnecessary fire protection features by identifying and protecting against the actual potential hazards in a building. However, reducing the cost of construction is not always the result. In many instances, the use of performance-based design increases the costs of fire protection systems in an effort to produce the building the owner desires. Furthermore, the performance-based design approach can be used for the entire building or on a specific area or system in coordination with the prescriptive requirements. Regardless of the scale of the engineering effort, it is through the use of performance-based design that FPEs can provide a significant positive impact on most building design and construction projects. It is imperative, however, that local code officials and firefighters thoroughly understand the implications of the performance-based building design being presented to them.
FPEs who apply performance-based design give architects and planners the freedom to develop unique and creative structures. With prescriptive codes, these techniques may be suppressed, since they do not fit any of the definitions previously conceived of in the code. This design freedom can often provide greater functionality within a building while maintaining the level of safety intended by the code. You can apply performance-based design to virtually every type of building and occupancy—for example, in historic buildings to preserve their unique architectural features. Monumental buildings can also benefit from the use of performance-based design since, by their very nature, these types of buildings are not capable of being effectively regulated by a prescriptive approach.
Examples of performance-based design are found everywhere and are rarely very apparent. Atria in large hotels and office buildings are commonly designed with a performance-based design approach. Fire alarm and protection systems in K-12 schools are routinely combined with security systems in a performance-based manner. Industrial facilities with high-hazard operations are often protected using a performance-based design approach when the prescriptive approach does not properly address the hazard being protected. These are a few examples of the types of projects that have benefited from the participation of FPEs and the implementation of a performance-based design approach.
The current prescriptive building code is based on multiple redundancies to provide a satisfactory level of safety. This seems to be a very reasonable approach. However, as the quality of construction materials, fire suppression systems, and fire alarm systems and our overall knowledge of fire have grown, it appears that in certain situations a paradox occurs where fire protection is too conservative and too relaxed at the same time. This occurs when multiple fire protection features are in place but the systems were not designed or intended to protect. Fortunately, better fire protection technologies (addressable fire alarm systems, quick response sprinklers) and new analysis techniques (computer-based fire modeling), in combination with the advent of the performance-based design approach, have opened the door to a wide range of design and construction opportunities that are not available through the traditional prescriptive code approach.
The complexity of the performance-based design itself is directly related to the complexity of the issue being assessed. Performance-based designs can take the simplistic form that is currently called a waiver, a variance, or an alternate method of construction or be a complex, formalized analysis that addresses a wide range of potential issues. Either way, a performance-based design is used to evaluate a specific configuration and is not typically bound by prescriptive code requirements. The time and costs associated with a performance-based design typically exceed that of traditional prescriptive code compliance. However, these costs can often be more than offset by efficiencies that are created through the results of the analysis.
Consider a K-12 school in an old inner-city building. The school has been in operation for more than 50 years. The building has been maintained but never really updated. Now the building is going to be completely renovated and modernized to keep pace with new schools being built in the neighboring suburbs. Unfortunately, now it will have to comply with all of the requirements of the modern building codes—a seemingly impossible feat since the building has several unenclosed grand stairs, no sprinkler or fire alarm system, insufficient exit capacity, and many dead-end corridors.
There are several options. The first is to proceed with the renovation and hope that none of these deficiencies are identified by the building code official or fire marshal. This solution only appeals to those who feel that unsafe buildings are acceptable.
The second option is to comply with all requirements of the building code regardless of cost and impact on aesthetics and functionality. This will result in a building that meets the code-intended level of protection but lacks the efficient use of space demanded in today’s schools. The impact of this potential solution becomes painfully obvious when enclosing all of the stairs, creating new corridors, and providing additional stairs and exits reduces the functionality of the building by eliminating several classrooms to accommodate all of the required features. And there is no assurance that the new features are effective fire safety measures. In many renovations, this type of approach defeats the supposed benefits of the renovation.
The third option is to evaluate the building using a performance-based design approach. This establishes a baseline level of safety within the building for a wide range of fire scenarios. You can then determine which additional fire protection features (sprinklers, automatic fire detection, enclosure of specific critical stairs, and so on) are necessary to ensure that the code-intended level of safety is reached without providing unnecessary fire safety measures that produce little or no benefit.
You can apply this example of performance-based design to most types of occupancies, including new buildings. Assume that a new school was being planned with a few features that did not comply with the code but were essential for the building’s functionality or aesthetics. Should these features be immediately dismissed and removed from the design merely because they are unique? The answer is yes if the building is to comply with the prescriptive requirements of the building code. The answer is no if a performance-based design is used.
APPLICATION TO COMPLEX BUILDINGS
While many examples of performance-based design involve obtaining waivers or variances, there is often a need to evaluate a building through the use of more rigidly quantifiable measures. These evaluations are often conducted through the use of a comprehensive and formalized performance-based design process. In brief, this process documents the fire and life safety goals and objectives for the building, which are then measured against simulated design fires within the building. The use of computer-based fire modeling and evacuation modeling helps quantify whether the building can meet the previously determined goals and objectives as originally defined.
An example of the use of a formalized performance-based design is that of the Smithsonian Institution’s Arts and Industries Building on the National Mall in Washington, DC. The project consisted of a complete interior renovation of this historic museum. Because of the historic nature of the building, a major focus of the project was to restore and retain the historic features while providing mechanical, electrical, and fire protection systems required in modern buildings. This created issues, since the original interior configuration could not easily accommodate many of the characteristics of modern buildings. The original configuration of the Arts and Industries Building can be likened to today’s covered malls. The 90,000-square-foot first floor is nearly entirely open with various occupancies located on the outer perimeter. The second story is largely open to the first, also with office spaces around the perimeter. A network of balconies serves the entire second floor.
The approach used in developing the performance-based fire and life safety analysis for the building was based on the formalized guidelines identified in the Engineering Guide to Performance-Based Fire Protection Analysis and Design of Buildings, published by the Society of Fire Protection Engineers. This guide outlines a structured approach to proceeding through a performance-based analysis.
To conduct the analysis, computer-based fire and evacuation models were used. The fire model used for the analysis, Fire Dynamics Simulator (FDS), represents a significant advancement in modeling the effects of fire in complex buildings. FDS, developed by the National Institute of Standards and Technology (NIST), is a computer-based computational fluid dynamics fire model capable of describing the transport of mass, momentum, and energy from fire-induced flows across hundreds of thousands of separate volumes within a building. FDS was used in conjunction with the development of realistic design fire scenarios to quantify the impact of fire within the Smithsonian’s Arts and Industries Building. The selection of design fires included determining locations and fire characteristics that not only were based on the expected fuel loading but also took into account several criteria that exceeded the current building code requirements.
In addition to the fire modeling, it was necessary to determine the time necessary to evacuate the building during a fire. This was determined through the use of a computer-based egress model. This evacuation modeling is used in conjunction with the fire modeling to determine if the paths of egress within the building will be maintained free of untenable levels of smoke and hot gases from a fire. The quantity of smoke and hot gases are tracked by the fire model and compared with predetermined performance criteria, such as the maximum permitted temperature from the fire, the maximum smoke concentration, and the maximum concentration of carbon monoxide.
The results demonstrated that at no point during the time necessary to evacuate the building were the threshold criteria for carbon monoxide concentration, smoke concentration, and temperature reached or surpassed. This result was obtained using conservative ranges for the design fires (i.e., worst-case scenarios), occupant loading and egress characteristics, and performance criteria.
The results of this analysis permitted the continued use of the interior means of egress—features originally determined to not meet the prescriptive code requirements. However, by taking advantage of the openness of the building and providing additional design features such as excess building compartmentation, quick response sprinklers, complete smoke detection, and four new interior stairs, it was determined that the level of safety within the building met the level intended by the code. The result was the preservation of the critical historic architectural features of one of this nation’s most unique museums.
ANSWERS OWNERS WANT
The performance-based design approach is a technique employed by today’s FPEs to help building owners and design professionals develop or renovate facilities that more closely meet their initial expectations for design and functionality. Best of all, the approach can be used in various forms for virtually every type of occupancy and is instrumental in achieving the balance between design freedom, cost, and fire and life safety.
ANDREW BOWMAN, P.E., is a senior fire protection engineer with Gage-Babcock & Associates and a member of the Society of Fire Protection Engineers.