The role of sprinklers in performance-based design

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

This 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 requirements. Examples of prescriptive requirements are maximum exit travel distances, minimum fire resistance ratings for building construction, and pipe schedule sprinkler layouts. The advantage of using the prescriptive approach from the design standpoint is that it is usually simpler and less expensive. Compliance with prescriptive requirements is also easier for building and fire officials to evaluate. The disadvantage with the prescriptive code approach is the one-size-fits-all approach to code compliance. This tends to tend to limit design innovation in solving design challenges. Also, with prescriptive codes, though the overall goal of the requirement may be understood (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 unnecessary 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 scenarios; and (3) quantitative assessment of design alternatives against the fire safety goals and objectives using accepted engineering tools, methodologies, and performance criteria.”

The structural engineering design community has been doing performance-based design for many years. Internationally, several countries have adopted performance based building codes and have been practicing performance-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 innovative 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 stakeholders is critical to the PBD process. Furthermore, it is difficult, particularly for code officials, to evaluate compliance 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 projects which may have difficulty complying with prescriptive 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 building 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 temperature, blood carboxyhemoglobin (COHb), or a combination of factors. Stated in quantitative terms these become the performance criteria. Examples of performance criteria would be maintain visibility of a least 25 feet, a smoke layer 8 feet above the walking surface, limit occupant exposure to temperatures not exceeding 120 degrees F, or a COHb concentration in the blood less than 12%.

Once performance criteria is established design fire scenarios are developed. A typical design scenario would be an arson fire set in stacked-stair storage in an exhibition room. All reasonable design fires and design scenarios must be considered. Examples of general design scenarios can be found in Chapter 5 of NFPA 101, The Life Safety Code, 2009 edition.

Many in the sprinkler industry are a little suspicious about performance-based design. This was probably due to the experience with PBD in other countries where the PBD effort was often directed at figuring out how to avoid providing sprinkler protection. This view is not widely held among U.S. fire protection engineers who in my experience consider sprinklers an essential part in the PBD process.

A major advantage of sprinklers is that by reducing or controlling the size of a potential design fire the rate of heat release is reduced resulting in lower temperatures and less smoke production. It is common practice in PBD to take a conservative approach to the affect of sprinkler protection on the fire. At the moment of sprinkler operation the growth of the fire is stopped and the fire output (heat release rate, smoke production) is held constant for the remainder of the analysis.

In the sprinklered condition fire size can be further reduced by decreasing the time to sprinkler activation. Sprinkler activation times can be calculated using the techniques contained in Annex B Engineering Guide for Automatic Fire Detector Spacing of NFPA 72, National Fire Alarm Code, 2010 edition. For a given design fire size or growth rate sprinkler operation times can be decreased by reducing spacing, using sprinklers with a lower temperature rating or RTI, or lowering room ceiling height.

Returning to our RSET/ASET example, sprinklers can used to justify increases in the available safe egress time resulting from reduced fire output.

Sprinklers are also considered critical in the PBD of smoke management systems. Design fires without sprinkler protection can grow to be very large producing quantities of smoke which cannot adequately be “managed” by a smoke management system.

Sprinkler protection can also be used in PBD to supplement building fire resistance. Increased sprinkler densities can be used to limit impact of fire on structural elements, with the result of reducing or eliminating the requirement for passive fire protection measures. It has long been recognized by building codes that “closely spaced” sprinklers used in conjunction with fixed glass partitions can be considered an alternative to one hour fire resistance rated wall in atrium separation. It has also long been a provision in NFPA 13 for rack storage that sprinklers can be used to provide steel columns the required one hour fire resistance rating in lieu of passive protection.

Sprinklers can play a large role in the PBD for existing building renovations. It is often said sprinkler protection “cures a lot of ills” as they are frequently used to compensate for various code deficiencies. NFPA® 101A Guide on Alternative Approaches to Life Safety, 2010 edition is a document that contains “Fire Safety Evaluation Systems” (FSES) for several occupancies. The FSES’s are indexing methods used to perform qualitative risk assessment of a given occupancy to aid in determining equivalency with NFPA 101. In each of the indexes the presence of complete building sprinkler protection is given the highest point score. For example in the FSES for a health care occupancy the presence of total system smoke detection coverage is given a point value of 4 while total building sprinkler protection using quick response sprinklers is assigned a value of 10.

Sprinkler protection can also be factored into PBD when assessing firefighting forces in addressing required response times and manpower needs.

Now we must be cautioned that sprinklers do not “cure all ills”. A common criticism from certain quarters of the industry is that too much reliance is placed on sprinklers at the expense of other proven fire protection measures. This criticism applies to both performance-based design and to use of sprinkler trade-offs in prescriptive design. Though these complaints are often voiced by those with a vested interest in passive fire protection products, they should not be completely ignored. Fire protection engineers are trained to consider the use of multiple safeguards so that the failure or ineffective performance of one fire protection feature does not result in failure of system. The concept of multiple safeguards must be part of any PBD. This brings us to an important issue in PBD with respect to sprinklers, which is, their effectiveness and reliability. That will be the subject of next month’s article.
For those interested in more information about PBD the following publications can be found at www.sfpe.org

SFPE Engineering Guide to Performance Based Fire Protection, 2nd edition, SFPE and NFPA
SFPE Code Official’s Guide to Performance-Based Design Review, published by SFPE and ICC, 2004
Introduction to Performance-Based Fire Safety, Richard Custer and Brian Meacham, SFPE and NFPA, 1997

Have a fire safe New Year.

Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer and past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates, Inc. He can be reached via email at SDannaway@ssdafire.com.