Reexamination of “Restrained vs. Unrestrained”
By Kevin J. LaMalva, P.E.
The vast majority of structural fire protection designs are conducted using standard fire resistance design (i.e., the prescriptive approach). This approach allows designers to select fire resistant assemblies from available listings that are qualified through furnace testing. For certain listings, the designer must judge whether a “restrained” or “unrestrained” classification is appropriate for the application. Often times, architects task fire protection engineers or structural engineers to make such judgments. Notably, many listings permit less applied fire protection to achieve a certain fire resistance rating if a “restrained” classification is adopted, as compared to an “unrestrained” classification.
What is Restraint? (Furnace Test)
ASTM first introduced the “restrained vs. unrestrained” concept in the 1970s,1 based on the observation that thermal restraint provided by the furnace enclosure generally enhances the performance of assemblies during furnace testing. An assembly is considered “restrained” if it bears directly against the edges of the furnace at the outset of the test.2 Accordingly, the UL Fire Resistance Directory specifies that the furnace enclosure boundaries provide approximately 850,000 kip-in. of flexural stiffness, 3 which is significantly higher than that provided by most structural systems.
What is Restraint? (Actual Construction)
In actual building construction, restraint of structural assemblies occurs when the surrounding structural system resists their thermal expansion when exposed to heating. The effect of thermal restraint must be carefully evaluated since it may dominate the behavior of a structural system under fire exposure.4 Complicating matters, it is known that a multitude of factors influence restraint conditions (e.g., varying spring stiffness as illustrated in Figure 1), and these factors may increase or decrease structural system endurance under fire exposure. For instance, thermal restraint may generate forces sufficient to cause yielding or fracture of connections, perhaps precipitating structural collapse. Alternatively, thermal restraint may limit the deflection of structural members and provide added stability.
Figure 1: Thermal Restraint (Furnace Test vs. Actual Construction)
Why Reexamine Now?
The practice of structural fire protection is at a critical inflection point due to improved technology and increased ability to apply engineering principles at the intersection of the fire safety and structural engineering disciplines. Notably, the next edition of the ASCE/SEI 7 standard (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) commences a new industry-consensus standard of care for structural fire protection.5 Per a new Chapter 1 section (Fire Resistance), standard fire resistance design is specified as the default option, in which the designer is required to strictly follow the long-standing fire resistance provisions of the applicable building code without exception, extrapolation, or variance. Accordingly, standard fire resistance design is properly characterized as an empirical indexing system that helps to mitigate the risk of structural failure under fire exposure.
The alternative to standard fire resistance design is structural fire engineering, as constituted in the new ASCE/SEI 7-16 Appendix E (Performance-Based Design Procedures for Fire Effects on Structures).6-7 Structural fire engineering aims to control the heating of a structural system and provide adequate structural endurance in order to satisfy explicit performance objectives under fire exposure (e.g., full occupant evacuation to a public way). To support the provisions of Appendix E, the ASCE/SEI Fire Protection Committee has developed a new ASCE/SEI Manual of Practice (Structural Fire Engineering), which is currently under review with a tentative release later this year.
The new standard of care for structural fire protection will impact how designers consider thermal restraint. For instance, ASCE/SEI 7-16 Section E.2 states that thermal restraint is entirely dependent on adjacent structural framing and connection details, which are not contemplated in standard fire resistance design. Accordingly, Section CE.2 states that furnace testing does not provide the information needed to predict the actual performance of a structural system under fire exposure, since furnace testing qualifies assemblies in isolation without interconnectivity or interaction with the surrounding structural system. In light of these new developments, the industry has begun to reexamine the “restrained vs. unrestrained” concept in order to better serve designers going forward.
An industry workshop on the “restrained vs. unrestrained” concept was hosted at the headquarters of Simpson Gumpertz & Heger Inc. in Waltham, MA on 17 July 2017 (Figure 2).
Figure 2: Industry Workshop (17 July 2017)
The wide industry representation at this workshop provided valuable insights and perspectives on the “restrained vs. unrestrained” concept and its application to structural fire protection. Salient points expressed during the workshop include the following:
- Standard fire resistance design has served the industry well over the past century, and should continue to be the default option for structural fire protection design.
- Changing market demands over the past century have led to cases in which the practice of standard fire resistance design is “stretched and distorted” beyond its original intent and capability
- Structural fire engineering represents an emerging opportunity for designers, but certain knowledge gaps must be appreciated and appropriately accounted for if deemed pertinent
- ASCE/SEI 7-16 crystallizes what designers have known for decades: there is no correlation between assembly performance in a furnace test and in-situ structural system performance under fire exposure.
- Furnace testing standards only provide examples/guidance for restraint classifications, so the designer is ultimately responsible for such judgments.
- Restraint conditions of a furnace test differ from those of the in-situ structural system.
- Although consideration of restraint is a common task in the industry, designers are concerned about the liability associated with making uncertain judgments.
The Path Forward
The general consensus of the workshop was that clarification/reform of the “restrained vs. unrestrained” concept could benefit the industry at large. It is envisioned that such clarification/reform would materialize within fire testing standards, and be adopted by building codes. Also, it was expressed that any clarification/reform should ideally relieve designers of the obligation to make judgments concerning restraint when employing standard fire resistance design. Such judgments are better reserved when employing structural fire engineering, where the future is bright for collaboration between fire protection and structural engineers.
Kevin J. LaMalva, P.E. is with Simpson Gumpertz & Heger, Inc.
1ASTM E 119: Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM International, 1971.
2ASTM E 119: Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM International, 2016.
3UL Fire Resistance Directory, Underwriters Laboratories, 2015.
4Bailey, C.; Lennon, T.; Moore, D., “The Behaviour of Full-Scale Steel-Framed Buildings Subjected to Compartment Fires,” The Structural Engineer, Vol. 77, No. 8, 1999.
5ASCE/SEI 7-16: Minimum Design Loads and Associated Criteria for Buildings and Other Structures, American Society of Civil Engineers: Structural Engineering Institute, 2016.
6Post, N.M., “Guidance for Structural Fire Engineering Making Its Debut,” Engineering News Record, February 2017.
7LaMalva, K.J., “Structural Fire Protection’s Shifting Paradigm,” Fire Protection Engineering, Issue #74, Society of Fire Protection Engineers, Q2 2017.