New standardization for structural fire protection variances
Structural fire protection addresses the low probability and high-consequence event of uncontrolled fire exposure within the built environment (Figure 1). The effects of fire in engineered buildings are mostly controlled by fire sprinkler systems. However, they may not be effective at controlling extraordinary fires, such as those resulting from arson, terrorism or other rare events.[1] Hence, structural fire protection serves as an integral safety consideration that becomes critical when and if active systems are rendered inoperative or are insufficient for a specific fire hazard. This is despite the fact U.S. building codes often incentivize the installation of such systems by reducing structural fire protection requirements as a tradeoff.
Section 703 of the 2018 edition of the International Building Code (IBC) permits the design of structural fire protection in accordance with long-standing prescriptive provisions or “alternative methods.”[2] The latter is commonly referenced when a structural fire protection variance is requested in practice. While prescriptive design provisions are clearly defined and simply applied, alternative methods of compliance are undefined. Recent efforts by the American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) have culminated in an industry consensus on the matter that is slated to be addressed in the coming round of code changes.
Limits of prescriptive design
Structural fire protection is most commonly specified using the long-standing prescriptive design method in which the thermal resistance of structural components is qualified through standard fire testing with a specific heating exposure (e.g., ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, time-temperature curve) and acceptance criteria (Figure 2). This testing excludes consideration of structural connections, structural system response (e.g., realistic load redistribution), and actual fire exposure (e.g., representation of fuel burnout). Also, the size limitation of a test furnace restricts structural component spans to typically no greater than 5 meters (17 feet), which further trivializes its representation of actual structural systems (Figure 3). Due to these limitations, it has been recognized by designers that there is no correlation between structural component performance in a standard furnace test and the associated in-situ structural system performance during an actual uncontrolled fire.[3]
The primary qualification metric of the prescriptive design method is fire resistance, expressed in hours. This metric is an artifact of standard fire testing as described above. The standardized time-temperature history does not equate to any specific real fire and the acceptance criteria are predominantly expressed in thermal, rather than structural, terms. Even in the cases where structural terms are used to define a failure condition in this context (e.g., deflection), they have a restricted relation to actual structural system limit states. For instance, in an actual structural system response to a fire, deflection can be beneficial as it can relieve thermal strains in members. Hence, the prescriptive design method is an empirical indexing system promoting construction that is thermally resistant to fire exposure. However, the actual anticipated structural system performance under fire exposure is not contemplated, confirmed or rationally optimized, meaning that true fire resistance is unknown.
The prescriptive design method is practiced by many types of designers and often by professionals with little understanding of fire dynamics or structural behavior. Typically, the design professional selects a qualified listing providing the required fire-resistance rating per the applicable building code. In this case, no actual engineering is conducted or expected. This direct specification of qualified listings is pursuant to IBC Sections 703.3(1) and 703.3(6), which permit the use of listings from approved sources (most commonly the Underwriters Laboratories [UL] Fire Resistance Directory) or via certification by approved agencies, respectively. For generic assemblies (e.g., concrete walls), the IBC provides empirical prescriptions (per IBC Section 721) and calculation methods (IBC Section 722) pursuant to IBC Sections 703.3(2) and 703.3(3), respectively. Such prescriptions and calculation methods are based on the interpolation of furnace test data and were developed for practical convenience. Pursuant to IBC Section 703.3(4), a fire-resistance “equivalency” approach may be utilized in some cases if the designer is able to demonstrate a proposed protection scheme would achieve equivalent or better performance during a hypothetical standard fire test when compared to a similarly qualified assembly. This prescriptive design approach is usually conducted by a fire protection engineer at the request of an architect or contractor, often to overcome a construction hardship. Since this type of analysis is predominately thermal-based, it is well-suited to the skills and competency of a fire protection engineer.
In cases where an alternative to the prescriptive design method is sought, the U.S. has lacked an industry consensus on what is permitted and satisfactory until recently. Specifically, IBC Section 703.3(5) says that an alternative approach may be used. However, no specific requirements, guidance or bounds are provided in the code. Consequently, structural fire protection variances tend to exhibit a wide variation in engineering rigor and conservatism, with most tending toward a less rigor/conservatism.[4] Also, some design professionals incorrectly reference IBC Section 703.3(4) as justification for the use of non-prescriptive approaches. For instance, a fire protection engineer may justify the removal of fire protection from steel structures based solely on thermal response comparisons between a standard furnace test condition and anticipated in-situ fire conditions without any appreciable structural analysis. Unfortunately, possible inherent defects or system vulnerabilities associated with this type of poor practice would not be readily uncovered since uncontrolled fire exposure rarely occurs and tests the true adequacy of structural fire protection.[5]
Industry need for a legitimatized alternative
The prescriptive design method has not changed in the last century, and there is little to no synergy between structural design and applied fire protection in this respect. Indeed, structural engineers are often completely absent from the structural fire protection design process, even though they are the only engineering discipline capable of confirming adequate structural fire safety is provided. Overall, the prescriptive design method endeavors to reduce the heating of all structural components to a similar extent with the intent of mitigating the risk of structural system failure under fire exposure. Accordingly, the vulnerability of buildings to structural failure from fire is variable across different jurisdictions, which have varying structural design requirements for ambient loads.[6]
Although somewhat uncommon, severe structural damage and collapses of engineered buildings due to fire (e.g., One Meridian Plaza; World Trade Center [WTC] 1, 2, 5 and 7; Windsor Tower; Delft Technical University building; and others) have highlighted the need for structural engineering participation in evaluating fire effects on structural systems (Figure 4). Moreover, the failure modes exhibited by these events could not have been anticipated by examining standard furnace testing results (e.g., connection failures).[7] Fortunately, the structural engineering community is embracing performance-based design as a reliable means of protecting structures from natural hazards including uncontrolled fire exposure.[8]
The term structural fire engineering (SFE) defines performance-based structural engineering design for uncontrolled fire exposure. Structural fire engineering approaches structural fire protection from both the demand (heating) and capacity (structural response) sides of the equation instead of solely focusing on the demand side, as is traditionally done. Within this framework, the thermal demand on a fire-exposed structural system can be reduced by means of rationally allocated structural insulation (i.e., membrane protection or direct application methods) or controlling fuel loads. This is consistent with the method by which structural engineers design modern buildings for gravity and lateral loads. Also, the capacity of a structural system to endure fire effects can be increased by means of reinforcement strengthening, placement, detailing, and/or continuity, increasing slab thickness and/or concrete cover, connection enhancements, increasing member size, geometric layout modifications, and/or other measures to enhance structural robustness with respect to explicit performance objectives.[9]
Accordingly, SFE allows the designer to influence more design variables as compared to the prescriptive design method, where the designer is only able to influence the level/characteristics of structural insulation. Hence, SFE provides opportunities to develop designs optimized for project economics, carbon footprint, aesthetics, quality control, site conditions, and life-cycle maintenance without compromising fire safety. An example would be increasing the reinforcement mesh density in the concrete slabs of a building to facilitate stabilized two-way compressive/tensile membrane action at high deflections due to elevated temperature using a targeted insulation approach. Such measures, which are only possible using SFE, can raise the reliability and safety of a structure to endure fire as compared to simply insulating the members.
Unlike the prescriptive design method, SFE cannot be executed by any type of design professional. Rather, SFE requires the participation (or more ideally the responsible charge) of a structural engineer in all cases.[10] Skills and competencies of other design professionals, such as fire protection engineers, are certainly applicable to SFE, but the need for structural engineering expertise cannot be avoided or circumvented as it has been the practice in the past. Unlike the prescriptive design method which relies on the metric of fire resistance, evaluation of relevant structural limit states and/or simulation of structural system performance is required for SFE. Also, SFE requires explicit consideration of performance objectives. For instance, as the vertical remoteness of occupants from the point of discharge to a public way (e.g., a public street) increases, the time required to evacuate the building will also rise (Figure 5).[11] Unlike the prescriptive design method, SFE explicitly contemplates the consequences of increased occupant evacuation times and the reliance on building refuge areas to meet other code requirements.
Filling the void with new standardization
ASCE/SEI 7, Minimum Design Loads For Buildings and Other Structures, is the standard for structural loads for the IBC, and now contains provisions pertaining to structural fire protection variances. Section 1.3.7 of ASCE/SEI 7 (2016) stipulates structural fire protection design shall be conducted in accordance with the prescriptive requirements of the applicable building code (i.e. per IBC Sections 703.3[1], 703.3[2], 703.3[3], 703.3[4] or 703.3[6] as described above), or in accordance with the performance-based design provisions of ASCE/SEI 7 Appendix E (pursuant IBC Section 703.3[5] and at the discretion of the building authority). Supporting the seminal provisions of Appendix E, the new and first-of-its-kind ASCE/SEI Manual of Practice (MOP) No. 138 (Structural Fire Engineering) is also available to the design community, which provides specific guidance for conducting SFE analyses (e.g., temperature-dependent material strength properties).
ASCE/SEI 7 Appendix E and ASCE/SEI MOP No. 138 both reinforce the fact there is no correlation between assembly performance in a furnace test and in-situ structural system performance under actual fire exposure. Accordingly, the standard says it is improper to intermingle aspects of the prescriptive design method with the performance-based method to justify structural fire protection variances. Since the prescriptive design method 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 SFE design to the one provided by a prescriptive design. Hence, it is unreasonable to require SFE design be “equivalent” to a prescriptive design. Rather, SFE conducted in accordance with the new industry-consensus standards provides a legitimate alternative approach by its own merit in cases where a structural fire protection variance is needed and cannot be addressed by furnace test “equivalency” pursuant to IBC Section 703.3(4) as described above.
Elevating the standard of care
ASCE/SEI 7 Appendix E and ASCE/SEI MOP No. 138 effectively compartmentalize the two sanctioned design methods for structural fire protection. Within the prescriptive design method, justification of code variances for structural fire protection must be conducted within the context of the standard furnace test and its acceptance criteria, and not with respect to postulations of in-situ thermal and/or structural performance. Hence, if a structural protection variance relies on an analysis of realistic fire exposures (i.e., anything other than the standard furnace exposure) and excludes structural system analyses, the variance is invariably deficient with respect to the new industry standards. The removal of fire protection from structural systems should not be taken lightly and building authorities should feel empowered to expect more from designer professionals when such is proposed as a variance. As a result, the quality and consistency of proposed structural fire protection variances should dramatically elevate moving forward.
It is envisioned the recently published industry standards described herein will legitimize the practice of SFE and ease the reluctance of stakeholders to adopt this approach for building projects. If the limitations and restrictions of the prescriptive design method inhibit the achievement of stakeholder design objectives, the only industry-endorsed alternative is the performance-based design approach as constituted in ASCE/SEI 7 Appendix E, which requires explicit consideration of structural system response under fire conditions per required performance objectives.
References
1 J. Hall’s “U.S. Experience with Sprinklers and Other Automatic Fire Extinguishing Equipment.”
2 The International Building Code (IBC), International Code Council (ICC), Washington, DC, 2018.
3 M. Law’s “Designing Fire Safety for Steel – Recent Work” and O. Pettersson’s “The Connection Between a Real Fire Exposure and the Heated Conditions According to Standard Fire Resistance Tests – with Special Application to Steel Structures.”
4 J. Jonsson’s “Are You A Competent Practitioner.”
5 J. Jonsson’s “SFPE Core Competencies: Why Do We Need Them.”
6 K.J. LaMalva’s “The Time is Right for Structural Engineers to Embrace Structural Fire Protection.”
7 K.J. LaMalva’s “Developments in Structural Fire Protection Design – A U.S. Perspective.”
8 D. Cocke’s “SEI Annual Report: Performance-Based Design.”
9 K.J. LaMalva’s “Structural Fire Protection’s Shifting Paradigm.”
10 Consult the Society of Fire Protection Engineers’ (SFPE’s) recommended minimum technical core competencies for the practice of fire protection engineering.
11 Refer SFPE/ICC Engineering Guide: Fire Safety for Very Tall Buildings, Society of Fire Protection Engineers, International Code Council, 2013.
