What 2x10 Equivalency Means, and Why It's Important

AWC created the notion of 2x10 equivalency (go here for some background on the politics of AWC and NAHB). Equivalency is the only way for alternative products to compete with the following 2x10 exclusion as adopted into law by the 2012 IRC R501.3 (new section in the 2015 IRC R302.13).

Fire protection of floors. Floor assemblies, not required elsewhere in this code to be fire resistance rated, shall be provided with a 1/2-inch gypsum wallboard membrane, 5/8-inch wood structural panel membrane, or equivalent on the underside of the floor framing member.


4. Wood floor assemblies using dimension lumber or structural composite lumber equal to or greater than 2-inch by 10-inch nominal dimension, or other approved floor assemblies demonstrating equivalent fire performance.  


What Does Equivalency Mean?

IBC Section 722.6.2, that provides counsel with respect to wallboard membrane equivalency, defines “Time Assigned to Wallboard Membranes.”  In this section, a 1/2-inch gypsum wallboard membrane is assigned a 15-minute membrane rating. The actual performance of the floor system is the 15-minute membrane rating plus the endurance time of the structural member attached to it. This seems to be intended to fit into the fire service notion of the “20-minute rule.”

SBCA believes the following:

  • The only standard comparative benchmark testing that exists, and is referenced by the building code (IBC Chapter 35 and IRC Chapter 44), is the ASTM Standard E119—08a Test Methods for Fire Tests of Building Construction and Materials.*
  • Equivalent performance under R501.3 expects a floor will perform to a 15-minute level of fire endurance performance at a minimum as defined in IBC Table 721.6.2 (1):

These assumptions mean that 2x10s under R501.3, given they are exempted from the requirement of a 1/2-inch gypsum wallboard membrane applied to the underside of the floor framing member, must perform under the ASTM E119 standard conditions to a level equal to or greater than 15 minutes (and given that the expected single membrane performance is closer to 20 minutes, probably 20 minutes should be the standard). The same is true for any structural member, such as the following, that could providing an equivalent level of performance:

Historical and current testing suggest achieving a 15-minute fire endurance performance rating for any unprotected system without using a gypsum wallboard membrane is extremely challenging. The questions this situation raises include:

  1. If any of the systems listed above that purport to provide the fire service with greater than 15-minute membrane performance under ASTM E119 fire endurance conditions do not, what happens when the first firefighters unexpectedly fall through a floor because they believed in this expected performance? What will be the impact of any data in the public domain that suggests that the system did not provide this expected level of performance?
  2. How does this meet the legal requirement of the IRC where the intent of the code is stated as follows?

R101.3 Intent. The purpose of this code is to establish minimum requirements to safeguard the public safety, health and general welfare through affordability, structural strength, means of egress facilities, stability, sanitation, light and ventilation, energy conservation and safety to life and property from fire and other hazards attributed to the built environment and to provide safety to fire fighters and emergency responders during emergency operations.

  1. Could public domain data, in existence since at least 1992, cause this rise to the level of gross negligence for anyone who is party to making R501.3 part of the law?

These are tough questions to address, and only time will tell whether answers will be provided.

Full-design load or design stress standardized tests* show the fire performance of all floors is less than 10 minutes. 1/2-inch regular gypsum wallboard protection increases endurance up to about 20 minutes for all floor types. 2x10s tested at full-design stress levels today would have performance similar to, if not less than, the USDA Forest Product Laboratory (FPL) tests performed in the 1980s (the FPL report was published in March 1983), in which the 2x10s were tested at precisely full-design stress levels and yielded 5 to 6 minutes of unprotected performance. 

The SBCRI-SBCA goal is to provide a set of facts that have test data behind them, provide all appropriate references and undertake common-sense analysis. From there, and based on the merits, the reader can decide. We appreciate any comments, data and new analysis that will add to our knowledge.

Ultimately, SBCRI hopes that this information will lead to action, as appropriate, by building code officials and the fire service community. We know that even bad laws can become entrenched and can get in the way of thoughtful and valuable safety advancements. The foregoing information highlights a bad law and bad equivalency concepts. The pathway to true equivalency is to correct IRC R501.3 as follows:


4. Wood floor assemblies using dimension lumber or structural composite lumber equal to or greater than 2-inch by 10-inch nominal dimension, or other approved floor assemblies demonstrating equivalent fire performance.

*100% design load is the same as full-design stress level tests. This testing yields much different results than non-standardized 50% design load test, or an applied 40 psf floor load, or a 250 lb. dummy standing test as some have suggested is allowed for public policy decision making. ICC-ES allows both non-standard testing and some kind of char rate calculations to prove equivalency as well.

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