Fire Engineering Versus Prescribed Fire Protection
When I began practicing as a fire protection engineer, the codes that were used in the US were referred to as performance codes. This was primarily due to the fact that the codes did not specify a particular wall assembly as being 200 mm (8 inches) of masonry block; but instead, referred to such a wall as being noncombustible with a fire resistance rating of at least two hours.
The codes prescribed the fire protection measures that were to be implemented to achieve the level of safety intended by the code. Unfortunately that level of safety was often undefined by any stated objectives or metrics.
As the practice of fire protection engineering has become more quantitative, so has the development of true performance codes. A true performance code, and therefore a true performance-based design, does not start with any prescribed solutions or requirements. Instead, the stakeholders on the project determine the scope of the project and define the level of safety with goals, objectives, functional statements, and performance requirements. As the design progresses, quantitative methods are used to determine if the stated level of safety is provided by the design.
Prescriptive Code Approach
Before looking at performance-based design, let’s review the prescriptive code approach. In many parts of the world, especially where the base code is a performance code, the prescriptive approach is commonly referred to as a “deemed to satisfy” document or solution. When designing a building using the prescriptive approach, one starts by determining the occupancy classification(s). Once the occupancy classification is determined, the prescriptive code identifies the requirements for the project. At times the code may offer more than one approach such as an approach that includes automatic sprinkler protection and one that does not. Such codes also typically allow for equivalencies or alternative methods to achieve the desired level of safety. There are many benefits to the prescriptive approach in that the design is often faster and less expensive. From a code enforcement standpoint, ongoing enforcement is easier in that most buildings in the same occupancy classification will have the same features. However, the prescriptive nature of the code can also inhibit innovation and new technology especially, if the design team or regulatory authorities do not accept equivalencies and alternative methods.
When the primary code is a prescriptive code, such as those in the USA, the development and maintenance of the code is both time consuming and laborious. In April 2015, the International Code Council held over 160 hours of committee hearings on just a portion of their family of codes. Those hearings are just the first step of a two-step process which will be completed later in the year and then the remainder of the codes will be processed using a similar schedule in 2016. However, it should be noted that there were four public proposals to modify the ICC Performance Code during the development cycle for the 2015 Edition and no public proposals for changes during the 2012 Edition cycle.
The reason so many resources are devoted to the prescriptive code is that interested parties want the code to recognize their products and approaches since that is the primary way buildings are designed. If a product or approach is not recognized in the prescriptive code, competitors have a distinct advantage. In other words, many design decisions are actually made in the code development process and are not project specific.
Many years ago automatic sprinkler systems were designed using the pipe schedule system. Based upon the hazard being protected, sprinkler system piping was sized based upon the number of sprinklers to which the pipe supplied water. Today, automatic sprinkler systems are designed to provide a certain density over an anticipated area of sprinkler operation. The design densities have been verified by fire tests as being capable for controlling, or in some cases suppressing, the anticipated fire. While not identical to the difference between prescriptive and performance based fire protection engineering, the evolution is similar.
Continuing with the sprinkler analogy, the goal is to control or suppress the fire. The objective is to design a system that will deliver a density that has been determined to provide either fire control or fire suppression. How one sizes the pipe, selects the pipe to be used, and locates sprinklers is determined by the designer of the system, within some limitations provided by the design standard, in this case NFPA 13, Standard for the Installation of Sprinkler Systems.
Over the years, structural engineering has experienced the same evolution. What started as load tables has evolved to a design process giving the design engineer far more latitude provided the design is capable of supporting the design load. Similar advances have been made in seismic design as our knowledge and ability to measure critical factors of performance have improved. It should also be noted that structural engineering has developed a classification system different from the traditional occupancy classification, one that assigns an importance factor to different uses. This same “importance factor” has been incorporated in the ICC Performance Code for all aspects of the design.
While the benefits of the prescriptive approach have been identified above, one of the benefits of the performance approach is that fire protection is designed for the specific project. The level of protection provided is consistent with the goals and objectives identified by the stakeholders. While the initial design may be more costly and take longer, it is anticipated that the fire protection provided will be done in a more cost-effective manner. While the prescriptive approach does not preclude the design team from communicating with other interested stakeholders, the performance approach requires that the stakeholders communicate throughout the design process. As such, the owner and occupants should have a better understanding of the level of safety provided and what might be expected should a fire occur.
Tall Timber Buildings
To better understand the difference between prescriptive and performance based designs, let’s use an issue that is confronting many throughout the world. With a sustainability goal in mind, many are interested in building taller timber buildings, typically of cross-laminated timber (CLT). Such buildings have the structural capability to carry the anticipated load and it has been reported that construction time can be decreased. However, many are concerned about the fire protection aspects of building tall timber buildings.
While the typical height of many of the recently constructed CLT buildings is 9-10 stories, some concepts and designs as high as 40 stories have been documented. Using the prescriptive approach, the height of such buildings is typically restricted to 4-6 stories depending on the occupancy classification for the proposed use of the building. Therefore, in areas where the primary code is a prescriptive code, the equivalency or alternative method options must be used for these taller buildings.
Whereas most prescriptive codes do not contain well defined goal and objective statements, a common method to determine equivalency is by a comparison to the prescriptive code requirements. Unfortunately, the existing allowable height and area tables in prescriptive codes generally lack a sound technical basis. Given those limitations, how does one determine equivalency for a taller timber building when the table prescribes a specific height, number of stories, or both?
With a performance-based design there would be no preconceived maximum height or number of stories for tall timber buildings. Instead, there would be performance statements agreed to by the stakeholders that might address life safety, property protection, and continuity of operation goals. Using the information that is available to the design team, a design can be developed using CLT for the proposed use of the building. Quantitative methods (fire test data, computer fire modeling, etc.) can be used to demonstrate that the performance statements have been met. At the same time, the project can meet other stated performance requirements such as sustainability, structural design, and shortened construction times.
During a presentation on the subject of tall timber buildings at Fire Safety Asia Conference in Singapore (2014), one of the participants discussed a tall timber project designed using a performance code in Australia. One of the challenges for that particular project was the review and approval of the design. While it is easy to say that this should not be a challenge since the regulatory agencies should be included as a stakeholder from the beginning, it must also be recognized that the verification of the performance-based design includes quantitative methods and the regulatory agency personnel may not be familiar with such methods. As such, and as was done for the project in Australia, a third party agency was utilized to review the project and present recommendations to the regulatory agency.
Performance-based design has been increasing in use and popularity for the last 20 years. The use of performance design is more prevalent in areas where a performance based code is the primary code. Where the primary code is a prescriptive code, many owners and design professionals are likely to rely on the prescriptive code, possibly with the use of equivalencies or alternative methods.
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Top image: Fire Curtain photo courtesy of Coopers Fire Ltd.