The frequency of uncontrolled fire exposure in engineered buildings is low due primarily to the effectiveness of fire sprinkler systems. However, certain events and/or circumstances may result in uncontrolled fire exposure that may affect a building’s structural system. During such extraordinary events, heating of structural systems from fire exposure causes thermal load effects that are not contemplated in conventional structural engineering design, such as reduced material strength and loads induced by restrained thermal expansion. Under these conditions, it is critical that structural systems remain stable to protect occupant life safety, and perhaps satisfy other performance objectives.
Two Design Philosophies
In structural engineering, the fundamental philosophy used to design structures can be simply expressed as: Capacity > Demand. The demand refers to loads that are imposed on a structural system including its self-weight. The capacity refers to the global and local ability of a structural system to carry the imposed demand. The design of a structural system evaluates the demand and capacity with respect to specific performance objectives including strength, stability, and stiffness.
Standard fire resistance design follows the long-standing provisions in building codes for structural fire protection. This approach primarily involves the selection of qualified assemblies from available listings to meet prescribed levels of fire resistance, and often does not require substantial engineering participation. Standard fire testing serves as the basis of standard fire resistance design. However, this testing does not include member connections, structural system response, or natural fire exposure. Consequently, this approach does not necessarily credit nor discount the level of fire resistance based on the capacity of the structural system itself to endure fire effects, nor does it properly evaluate all aspects of the demand (e.g., thermally-induced forces). Also, the consideration of demand due to heating is not with respect to specific performance objectives, but rather superficial failure criteria enforced during standard fire testing.
As an alternative approach, structural fire engineering explicitly evaluates the demand and capacity of structural systems under fire loading in a similar manner as other design loads are treated in structural engineering practice. Within this framework, the demand on a structural system under fire loading can be reduced by means of rationally-allocated structural insulation, control of fuel loads, and/or other fire exposure mitigation techniques. Also, the capacity of a structural system to endure fire effects can be increased by means of specific member sizing, connection detailing, and/or other measures to enhance structural robustness with respect to explicit performance objectives.
New Industry Standard
ASCE/SEI 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) serves as the parent standard for structural engineering in U.S. building codes. This standard is published on a six-year revision cycle and was last released in 2010. The next edition of ASCE/SEI 7 is scheduled to be released at the end of 2016, and will commence a new industry-consensus standard of care for structural fire protection.
Per the upcoming edition of ASCE/SEI 7, the default option is for the designer to strictly adhere to the requirements and restrictions of standard fire resistance design without exception. As mentioned, this approach is based on an empirical indexing system that excludes consideration of realistic thermal demands and structural system response. As an alternative, the designer may adopt a structural fire engineering approach as constituted in the new ASCE/SEI 7 Appendix E (Performance-Based Design Procedures for Fire Effects on Structures). Notably, the prospective inclusion of Appendix E in ASCE/SEI 7 will mark the first time that fire effects are considered as an explicit design load in a U.S. structural engineering standard.
ASCE/SEI 7 Appendix E provides requirements for structural fire engineering design, and does not pertain to and should not be used for standard fire resistance design. The appendix is also limited to analysis of structural systems that are not significantly damaged by other hazard events, such as an earthquake or an explosive blast. Although comparatively less comprehensive at the current time, Appendix E will provide a level of guidance for fire effects that is modeled after well-established seismic provisions in ASCE/SEI 7.
Appendix E is organized into six primary sections with associated commentary sections. Section E.1 to E.3 presents introductory material on scope, definitions, and general requirements. Section E.4 specifies mandatory (occupant life safety) and discretionary (e.g., building resiliency) performance objectives. Section E.5 presents analysis methods for determination of thermal response of structures to fire exposure with reference to applicable standards from the SFPE and NFPA. Section E.6 presents methods for determining the response of structural systems to thermal load effects from fire exposure, including development of temperature histories for structural components; material properties at elevated temperatures; and structural analysis techniques.
Impact on Practice
Since standard fire resistance design does not contemplate structural system performance or explicit performance objectives, there exists no practical method for a designer to quantitatively compare the level of safety provided by a structural fire engineering design to that provided by a standard fire resistance design. Hence, the industry-consensus embodied in the upcoming edition of ASCE/SEI 7 is critical, and should serve both designers and building authorities alike.
Until now, designers have had to decide on their own what constitutes a satisfactory structural fire engineering design. Consequently, there has been significant inconsistency in the industry when deviations from standard code provisions are sought. For instance, there may be justification for the removal of protective insulation from steel structures based solely on temperature field information. The upcoming edition of ASCE/SEI 7 will prohibit this practice, obligating the designer to analyze the structural response due to the thermal demand without exception. Furthermore, the selective adoption of provisions from both standard fire resistance design and structural fire engineering for a given building project will be prohibited.
Structural fire engineering aims to provide an acceptable level of performance, but does not necessarily define specific requirements for design or construction. Hence, this approach can provide enhanced design freedom in situations where standard fire resistance requirements are found to be overly restrictive and/or unsuitable for the application. For instance, structural fire engineering may be necessary as part of building code variances in order to demonstrate the adequacy of innovative and/or nonconventional architecture.
Since structural fire engineering evaluates both demand and capacity, this approach allows the designer to influence more design variables as compared to standard fire resistance design, in which case the designer usually can only influence the level of fire resistance. This added flexibility provides opportunities to develop alternative designs that are optimized for aesthetics, functionality, and/or costs without compromising fire safety. Additionally, this approach allows for efficient analyses of nonconforming existing building construction (e.g., historic preservation) prior to undertaking costly rehabilitation.
The application of structural fire engineering for building projects has merit and enormous potential, but remains relatively limited. This can be attributed to the fact that designers and building authorities generally lack comprehensive, industry-consensus guidance for practicing and evaluating structural fire engineering. However, this emerging field is fast approaching a “renaissance period” with the release of ASCE/SEI 7 Appendix E. Furthermore, the ASCE/SEI Fire Protection Committee is currently developing a companion design guideline entitled ASCE/SEI Guideline: Structural Fire Engineering. This guideline will provide recommendations for analysis techniques, input parameters, structural acceptance criteria, and design examples to further support and supplement the content of Appendix E.
In the future, it is envisioned that U.S. building codes will begin to incentivize the adoption of a structural fire engineering approach. For now, the adoption of a structural fire engineering approach is elective per the discretion of project stakeholders and building authorities. At a minimum, structural fire engineering represents an emerging market opportunity for qualified specialists that can provide stakeholders enhanced design freedom and explicitly-designed structural fire safety. At its paramount, building authorities may be empowered to require a structural fire engineering approach to better protect the public in certain instances, such as for buildings that have a high consequence of structural failure and/or specific potential threats.
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