FPE Corner The role of sprinklers in performance-based design


Samuel S. Dannaway, PE, President, S.S. Dannaway Associates, Inc., Honolulu


T


his article will look at the role sprinklers play in performance-based fire safety engineering. Let’s begin with a brief primer on performance-based


design. In general U.S. building codes and fire safety standards


are prescriptive in nature, i.e., compliance with the code is achieved by following a set of cookbook style require- ments. Examples of prescriptive requirements are maxi- mum exit travel distances, minimum fire resistance ratings for building construction, and pipe schedule sprinkler lay- outs. The advantage of using the prescriptive approach from the design standpoint is that it is usually simpler and less expensive. Compliance with prescriptive require- ments is also easier for building and fire officials to eval- uate. The disadvantage with the prescriptive code approach is the one-size-fits-all approach to code compli- ance. This tends to tend to limit design innovation in solv- ing design challenges. Also, with prescriptive codes, though the overall goal of the requirement may be under- stood (life safety, property protection) it is not often clear how the requirement actually accomplishes that goal. In many cases prescriptive code compliance tends to an ultra-conservative approach to fire safety design with many overlapping requirements resulting in an unneces- sary use of resources and increased construction costs. The alternative to prescriptive design is performance-


based design. The SFPE Engineering Guide to Performance Based Fire Protection, 2nd edition, jointly published by SFPE and NFPA defines performance-based design (PBD) as: “An engineering approach to fire protection design


based on: (1) agreed on fire safety goals and objectives; (2) deterministic and or probabilistic analysis of fire sce- narios; and (3) quantitative assessment of design alterna- tives against the fire safety goals and objectives using accepted engineering tools, methodologies, and perfor- mance criteria.” The structural engineering design community has been


doing performance-based design for many years. Internationally, several countries have adopted perfor- mance based building codes and have been practicing per- formance-based fire safety engineering for some time. In the U.S., the fire safety engineering design community has been slowly moving towards performance-based design. The advantages of PBD is it allows one to design to


achieve specific goals and objectives, enables use of inno- vative design methods, and can result in more efficient use of design and construction resources. The PBD approach is not without its difficulties. First, it is difficult for the various stakeholders in the process (owner, user, architect, engineer, building and fire officials, etc.) to agree on the stated goals and objectives. Agreement between all stake- holders is critical to the PBD process. Furthermore, it is


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difficult, particularly for code officials, to evaluate com- pliance with a stated goal and objective. There are also gaps in our knowledge base in terms of data and tools to apply to the approach. The SFPE Engineering Guide to Performance Based


Fire Protection establishes an excellent framework which can be used to approach performance-based design. The basic steps in the framework are:


Define Project Scope Identify Goals Define Stakeholder and Design objectives Develop Performance Criteria Develop Design Fire Scenarios Develop Trial Designs Evaluate Trial Designs Select Final Design Prepare Design Documents


When a given trial design is evaluated and does not


meet the performance criteria one enters a feedback loop to either develop a new trial design or redefine design objectives. PBD is particularly well suited for large complex pro-


jects which may have difficulty complying with prescrip- tive code measures. Compliance may not be possible as it would affect the desired aesthetic or function of a building feature. For example, in a large convention center it may not be possible to have large exhibition spaces which comply with prescriptive exit travel distance requirements without designing in horizontal exits which would affect the function of large space areas or providing costly exit passageways. Using PBD a timed egress analysis could be performed which may show that longer travel distances are acceptable. A time egress analysis looks at evacuation as a function of time rather than distance. The project scope would be to design an egress system. The next step in the PBD process is to define the goal which would be to minimize the chance of injury or fatality due to fire. This goal is then translated into a stakeholder objective, such as; ensure there is no loss of life outside the room of fire origin. The objective is next stated in engineering terms as a design objective, such as; prevent flashover in the room of fire origin. The time required to exit the build- ing will be determined (RSET = Required Safe Egress Time) and compared to the time available for safe egress from the building (ASET = Available Safe Egress Time). The design is considered adequate if it can be shown that the ASET > RSET. The ASET is established by determining how long the


egress paths remain tenable. Tenability can be evaluated based on visibility, height of smoke layer, maximum tem-


Continued on page 34 January 2011


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