Analyzing Fire Safety
However, fire protection engineers can use these tools to evaluate a specific building's ability to withstand fire. This approach is called "performance-based" design.
The performance-based approach can be used to tailor fire- and life-safety systems for unique architectural designs. This may help reduce the costs of unnecessary fire-protection features by identifying specific potential hazards and more efficient protections against those hazards.
However, reducing the cost of construction is not always the result. In many instances, performance-based design increases the cost of fire-protection systems. But owners may choose to go to the additional expense to produce a building with special design characteristics.
This can give architects more freedom to develop creative structures, bypassing inhibitions imposed by prescriptive codes. Such design freedom can potentially lead to greater functionality within a building while maintaining the level of safety intended by the code.
It should be noted that the performance-based design approach can be used for an entire building, or it can be applied to specific areas only, while the rest of the building complies with prescriptive requirements.
Performance-based design can be applied to virtually every type of building and occupancy. It can be used in historic buildings, for example, to preserve their authenticity, or in industrial facilities that house hazardous materials or operations not covered by prescriptive codes. Monumental buildings that do not fit the norms regulated by a prescriptive approach can also benefit.
Safety Not Sacrificed
Current building codes rely on redundancy to provide safety. Certain situations present a paradox when fire protection is both too conservative and too relaxed. This can occur, for instance, when multiple fire-protection features are in place, but a unique hazard exists that these systems were not designed for.
Fortunately, better fire-protection technologies — addressable fire alarm systems, quick-response sprinklers, and computer-based fire modeling — have opened the door to a wide range of design and construction opportunities that are not available through the traditional prescriptive code approach. The typically higher cost of a performance-based design can often be offset by reducing redundancy as a result of the analysis.
Many instances of performance-based design are approved for construction through waivers or variances without rigid, quantifiable proof of their adherence to safety principles.
But often a building must be evaluated through a comprehensive, formalized performance-based design process. Simulated "design fires" and evacuation modeling help quantify whether the building can meet the stated fire- and life-safety objectives.
Evaluating the Smithsonian
My firm, Gage-Babcock & Associates, conducted a performance-based analysis of the Smithsonian Institution's Arts and Industries Building, a historic museum on the National Mall in Washington, D.C.
The project consisted of a complete renovation of the interior, preserving historic features while providing modern mechanical, electrical, and fire-protection systems. A key dilemma was that the interior configuration could not easily accommodate typical modern systems.
The 90,000-square-foot (8,400-square-meter) ground floor is almost 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.
In developing the fire- and life-safety analysis for the building, we used structured guidelines from "Engineering Guide to Performance-Based Fire Protection Analysis and Design of Buildings," published by the Society of Fire Protection Engineers.
To conduct the analysis, we used the computer-based Fire Dynamics Simulator (FDS), which models the effects of fire in complex buildings. Developed by the National Institute of Standards and Technology (NIST), FDS is a computational fluid dynamics model that describes the transport of mass, momentum, and energy from fire-induced flows across hundreds of thousands of separate volumes within a building.
We used FDS in conjunction with 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 based not only on the expected fuel loading but also on several criteria that exceeded the current building code requirements.
To determine the time needed to evacuate the building during a fire, we used an egress modeling system called Evacnet4, produced by the University of Florida. This simulation determined how long the exit routes would be maintained free of untenable levels of smoke and hot gases during a fire.
The fire model tracked the quantity of smoke and hot gases and compared them to predetermined performance criteria, such as the maximum permitted temperature from the fire, the maximum smoke concentration, or the maximum concentration of carbon monoxide.
The results demonstrated that at no point during the time needed to evacuate the building were the threshold criteria for temperature, smoke, or carbon monoxide reached or surpassed. This result was obtained using conservative ranges — "worst-case scenarios" — for the design fires, occupant loading, egress characteristics, and performance criteria.
These results permitted the continued use of the interior means of egress, which were originally determined to not meet the prescriptive code requirements. However, by taking advantage of the openness of the building and providing additional design features, we could demonstrate that the safety in the building met the level intended by code.
The new design features were quick-response sprinklers, complete smoke detection, four new interior stairs, and excess building-compartmentation — the introduction of continuous barriers to enclose spaces, and thus control any fire hazard at its origin. The result was the preservation of the critical historic architectural features of one of this nation's most beloved museums.
Such a performance-based design approach can help building owners and design professionals develop or renovate facilities that more closely meet their initial expectations for design and functionality.
The approach can be used in various forms for virtually every type of occupancy, and it is instrumental in achieving the balance between design freedom, cost, and 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, an international membership organization representing over 3,500 professionals.