|The Practice of Performance-Based Design|
From the Technical Director
The Practice of Performance-Based Design
By Morgan J. Hurley, P.E. | Fire Protection Engineering
Performance-based design is typically implemented in one of two methods: (1) as a means to determine equivalency to a prescriptive code or standard, or (2) as an approach to achieve broadly defined fire safety goals and objectives. At present, performance-based design is typically practiced in the former method. However, for fire protection engineering to continue to rise in stature as a mature engineering profession, the profession must strive for the latter.
The present approach to performance based design is necessitated by the prescriptive codes that are adopted in almost every jurisdiction. A regulatory authority is required to enforce the adopted code, and they generally can't choose to enforce a code other than the adopted code.
Prescriptive codes typically contain an "equivalency" or "alternate methods and materials" clause that permits designers to use alternative approaches than those specifically recognized by the code. These clauses require the designer to demonstrate that the alternative approach results in a design that is at least as safe as that achieved by following the prescriptive requirements.
Performance-based design has been practiced through the use of equivalency clauses for decades, generally via an ad hoc manner that varied from designer to designer and from jurisdiction to jurisdiction. More recently, the development of performance-based design guidelines has created a consistent framework that can be used to demonstrate equivalency to prescriptive codes.
However, this approach does not allow fire protection engineering to reach its full potential. First, even with a consistent framework, the use of equivalency clauses is not perfect. Determining whether a proposed design provides an equivalent level of safety to prescriptive requirements frequently requires determining the "intent" of the prescriptive requirement, which can be a subjective task. Secondly, using a prescriptive code as the basis for performance can hamper the maturing of the fire protection engineering profession.
A parallel that is frequently cited as a possible course for fire protection engineering is structural engineering.1 Structural engineering codes do not specify member size and spacing as a function of building height and area. Rather, structural codes specify the loads that a building must withstand as a function of the building type and reference design methods that can be used to ensure that a structure can support the specified loads. Engineers have wide latitude in designing a building structure to meet the requirements of the code.
The structural engineering parallel for fire protection engineering is desirable for a number of reasons. First, engineers would have far greater latitude in developing fire protection strategies for buildings. This would foster innovation while ensuring that fire protection is tailored to the needs of the building and the community. Second, greater engineering rigor is brought to bear in solving fire protection problems.
The fire protection engineering profession has made significant strides in the last decade towards developing the infrastructure necessary to support performance based codes. Performance-based codes have been published in several countries, and numerous design guides have been published that provide accepted methods for designing to meet the requirements of the performance codes. However, in many cases when performance codes are used today, designers typically refer to prescriptive or deemed-to-satisfy requirements for large portions of a project.
It will be a while before the fire protection engineering profession is ready for performance-based design on all projects, without reference to prescriptive codes. However, it serves as a good goal to which the profession should strive.