New Standards for Engineering Design of Structural Fire Protection

Issue 79: New Standards for Engineering Design of Structural Fire Protection

By Nestor Iwankiw, Ph.D., P.E. & Craig Beyler, Ph.D.

The fire science to provide the underpinnings for engineered structural fire protection has been under development since the 1960s and has reached a high level of predictive capabilities over the last half century. An engineered approach to the design of structural fire protection has been available since the publication of the Swedish Design Manual: Fire Engineering Design of Steel Structures in the 1970s.¹ This effort was later used as the starting point for the development of the Eurocodes² a generation later.

While tools for engineered structural fire protection are included in numerous chapters of the SFPE Handbook of Fire Protection Engineering,³ standards for engineered structural fire protection have been much slower to emerge in the Americas. Today, a suite of standards is nearing completion that will support engineered design of structural fire protection, and SFPE is playing a central role along with the National Fire Protection Association and the American Society of Civil Engineers.

The suite of standards, at various stages of development, is:

• NFPA 557⁴ "Determination of Fire Loads for Use in Structural Fire Protection Design”
• SFPE S.01 "Engineering Standard on Calculating Fire Exposures to Structures”⁵
• SFPE S.02 "Standard on the Development and Use of Methodologies to Predict the Thermal Performance of Structural and Fire Resistive Assemblies” (to be available for public comment in 2014)
• ASCE/SEI 7, "Minimum Design Loads for Buildings and Other Structures”⁶ (under revision)

These standards form the central pathway for engineered design for structural fire resistance as a substitute to the long-standing and conventional prescriptive ratings based solely on furnace testing to the standard time-temperature curve (e.g., ASTM E119⁷ or ISO 834⁸).  While prescriptive fire ratings in the US building codes have had a satisfactory historical safety record, they embody significant limitations and assumptions.  One of these is the underlying comparative rather than predictive nature of the standardized fire test and its acceptance criteria. Another is the reliance on a furnace exposure to a laboratory size test assembly, and a third is the single standard fire test time-temperature exposure, based upon 19th century understanding of fire.

An engineered approach takes into account the anticipated fire load and compartmentation in the building, the compartment ventilation openings, and heat transfer properties of the boundary assemblies of the compartments to determine the fire exposure to the building, in lieu of the use of a single, century-old specification of the fire environment. The engineered approach then computes the thermal response of the building and the structural response to fire heating. Under the engineered approach, the actual building performance under fire conditions is understood and controlled through engineering design. This contrasts with the existing specification approach of prescriptive fire resistance ratings in which the expected building performance under fire exposure is never actually analyzed.

Engineered structural fire protection offers a design alternative for conditions that are outside the range of the building code’s prescriptive limits or where project stakeholders require that actual fire performance be understood and controlled.  This engineered approach is intended to result in a more robust design solution that is specifically customized for the given project conditions and that can realize long-term cost efficiencies. Consistent with the modern trend in reliability-based design methods, the suite is based on a fire and collapse risk model.  As such, the fire risks to the building may be understood and controlled. 

The technical benefits of engineered structural fire protection include consideration of natural (actual) fire exposures in the analyses of their thermal effects on structural integrity.  A determination can be made if structural failure is expected, whether it is localized or can propagate into a catastrophic global collapse, and what design countermeasures can be taken.  The overall goal is a safe, comprehensive and robust solution that often can also provide a more balanced and efficient use of resources.  In some cases, engineered structural fire protection can be applied to obtain more conservative designs than prescriptive ratings at a reduced overall cost.  The "alternate means and methods” clause of the building codes is currently available to enable an engineered structural fire protection submittal, subject to the approval of the authority having jurisdiction.  It is anticipated that, in the future, the engineered structural fire protection approach implemented by the suite of standards will be directly referenced by the building code.

One of the essential ingredients in this engineered approach is the design-basis fire, for which a major input variable is the design fire load.  NFPA 557⁴provides the methodology to determine the fire load in direct parallel with other structural loads, such as gravity, wind, or seismic, for which the framing must be designed to avoid building collapse.  Fire load is best represented in terms of the heat energy potential of the combustible materials. 

Consistent with this practice, NFPA 557 expresses these quantities in energy units of MJ.  Fire load density is defined as the combustible heat energy per unit floor area of the compartment (MJ/m²). The fire load calculations require the selection of a suitable risk performance limit of the probability for structural collapse (e.g., 10^-6/yr), subject to the approval of the authority having jurisdiction.   NFPA 557 quantifies the fraction of fires that are structurally significant in different types of occupancies as a function of construction type and whether or not active fire protection is provided.  From this, a cumulative probability index can then be computed.  Finally, this procedure determines the design fire load density based on an extreme value statistical distribution formula.  

SFPE S.01⁵ provides methodologies for computing thermal exposure for structural elements and boundaries exposed to fire.  It considers both local and fully-developed compartment fire exposures.  It was developed to provide the temperature or heat flux to the structural members or compartment boundaries.  Unlike the remainder of the suite, this standard is based on a worst case, deterministic approach.  The standard may be revisited to cast it in risk terms to better fit the other standards in the suite in a future revision.  Other areas for future development include traveling fires, applicable to many large area fire compartments.

The SFPE Standard on the Development and Use of Methodologies to Predict the Thermal Performance of Structural and Fire Resistive Assemblies, currently under development, addresses the transfer of heat from the fire to the structural system or fire resistive compartment boundaries.  Because of the wide range of methods required to analyze heat transfer to and through the many materials available, the standard does not provide heat transfer methodologies.  Rather, it provides the requirements for heat transfer methodologies that are proposed for use for a specific material or design.  It provides method requirements, method input data definition requirements, method verification and validation requirements, method applications requirements, and documentation requirements.  It will be used by method developers, users, and authorities having jurisdiction.

Once the fire load and its thermal effects have been determined, typically by a fire protection engineer, the final part of the engineered structural fire protection evaluation is the structural response to the temperature effects of the fire on the structure.  The fundamental building load combination to be used for this purpose is given in ASCE/SEI 7.⁶   It prescribes the amount of nominal dead and live gravity load that is to be superimposed on the structure during the design fire exposure.  The structural fire effects include the degradation of the construction material properties, most importantly its strength and stiffness, at high temperatures along with thermal expansion or its restraint in members and connections.  The design strength of the structure must then be evaluated for these conditions based on the applicable material design standard.  For example, ANSI/AISC 360⁹ and its Appendix 4 would be appropriate for structural steel.  An appendix on "Fire Effects on Structures” (Appendix E) is currently under development for ASCE/SEI 7.  This appendix directly cites the other standards in the suite as elements in the determination of fire effects on structures.  The introduction of new material into ASCE 7 as appendix material is a common method of introducing new methods.  It is anticipated that, over time, portions of the new appendix will migrate into the body of the standard.

Nestor Iwankiw and Craig Beyler are with Hughes Associates.  

References

1. Petterson, O. Magnusson, S., and Thor, J. Fire Engineering of Steel Structures, Publication 50, Swedish Institute of Steel Construction, Stockholm, 1976.
2. Eurocode 1, Basis of Design and Actions on Structures - Part 1.2 Actions on Structures Exposed To Fire. BS EN 1991-1-2, British Standards Institution, London, 2007.
3. Dinenno, P. (Ed.) SFPE Handbook of Fire Protection Engineering, National Fire Protection Association, Quincy, MA, 2008.
4. NFPA 557, Standard for Determination of Fire Loads for Use in Structural Fire Protection Design, National Fire Protection Association, Quincy, MA, 2012.
5. SFPE S.01, Engineering Standard on Calculating Fire Exposures to Structures, Society of Fire Protection Engineers, Bethesda, MD, 2011.
6. ASCE 7, Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers, Reston, VA, 2010.
7. ASTM E119, "Standard Test Methods for Fire Tests of Building Construction and Materials,” American Society for Testing and Materials, West Conshohocken, PA: 2012.
8. ISO 834, Fire-resistance Tests - Elements of Building Construction, International Organization for Standardization, Geneva, 2012.
9. ANSI/AISC 360, Specification for Structural Steel Buildings, American Institute of Steel Construction, Chicago, 2010.

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