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|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.
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
The suite of standards, at various stages of development, is:
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
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 5574 provides 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
Nestor Iwankiw and Craig Beyler are with Hughes Associates.
3rd Quarter 2013 – An Overview of Approaches and Resources for Building Fire Risk Assessment– Brian J. Meacham, Ph.D., P.E., FSFPE, Worcester Polytechnic Institute
Spring 2006 – How Can a Fire Risk Approach Be Applied to Develop a Balanced Fire Protection Strategy – Man-Cheung Hui, Arup
Winter 2005 – Challenges Facing Engineered Structural Fire Safety - A Code Official's Perspective – Jonathan C. Siu, P.E., S.E., Seattle Department of Planning and Development
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