Metal Stud Fire Walls

Why do we allow a wall to be classified as firewalls when it is significantly different from the tested configuration?

Typical, Metal Stud Assembly Description

"1-1/4 in. wide by min 3-5/8 in. deep fabricated from min 0.020 in. (25-gauge) thick galv steel, attached to floor and ceiling with fasteners spaced 24 in. OC max." (With 5/8 Type X gypsum wallboard both sides etc.)

 

At the 1800-degree temperature attained during ASTM-E119, Thermal Expansion causes metal stud to expand 1/4 inch per foot. That three inches of expansion, on a twelve foot stud causes the stud to bow laterally twelve inches.

How can they say that a wall constructed with 3-5/8", 16-gauge CSJ (1-5/8 inch) studs placed 16 inches on center will respond the same in ASTM E119 fire test, Actually they don’t.

 

The following was copied from UL description of ASTM-E119

 

ASTM E119

Significance and Use
These test methods are intended to evaluate the duration for which the types of building elements noted in 1.1 contain a fire, retain their structural integrity, or exhibit both properties during a predetermined test exposure.

The test exposes a test specimen to a standard fire controlled to achieve specified temperatures throughout a specified time period. When required, the fire exposure is followed by the application of a specified standard fire hose stream applied in accordance with Practice E2226. The test provides a relative measure of the fire-test-response of comparable building elements under these fire exposure conditions. The exposure is not representative of all fire conditions because conditions vary with changes in the amount, nature and distribution of fire loading, ventilation, compartment size and configuration, and heat sink characteristics of the compartment.
Variation from the test conditions or test specimen construction, such as size, materials, method of assembly, also affects the fire-test-response. For these reasons, evaluation of the variation is required for application to construction in the field.

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The following was copied from an article in, Nov/Dec 2013, Commercial Building Products.


Passive Fire Protection Safeguards Building;

by Gregg Stahl, Clark Dietrich Building Systems


Fire-rated-wall requirements

"Walls (load bearing or not), floors, and ceilings can serve as fire barriers so long as they have a fire rating. Fire barriers are tested to the requirements of American Society of Testing and Materials (ASTM). West Conshohocken, PA, E119 (ANSI/UL263) and rated to resist the spread of fire for a designated amount of time, usually in hours.

There are several important things to remember about fire-rated wall assemblies and the role of steel studs in these systems. A fire rated partition must adhere to the way the actual tested assembly was constructed, without any variation."


Is it possible the added mass would be detrimental to the fire test?

A few points:

25 gage studs are typically used for interior non-bearing conditions and generally should be installed with a gap between the end of the studs and the track.  Neither the studs or the gypsum board should be fastened to the top track (this is permitted in fire rated walls by the Gypsum Association).  This will provide for expansion as well as movement by the surrounding construction such as deflection.  This will mitigate some of the expansion.

I could not find reliable data on the temperature of the studs but I'd suspect that they will only achieve a percentage of the 1800 degrees in the furnace based on the insulating effect of the gypsum layer closer to the fire.  I believe most partitions fail based on heat transfer to the unexposed face.  Since this is a maximum of 325 deg. F above ambient, I'd find it hard to believe the studs and by connection, the fasteners that are near the unexposed side are near 1800 degrees.

The Gypsum Association in their publication GA-600 allows increases in stud depth and gage in assemblies listed in their publication without impacting the rating.  Note that you are not allowed to reduce the gage of the studs.

As a rule of thumb, it is generally accepted that constructing a fire resistant rated assembly with materials that are heavier, or providing the members at a smaller spacing than that specified in the assembly description will meet or exceed the assembly rating.

As described, the change from 25-guage to 16-guage studs, spaced at 16 inches on center instead of 24 inches on center would definitely  give the assembly more mass. The added mass would then equate to a lower rate of heat transfer, thereby allowing the assembly to maintain it's integrity for a longer time than would the assembly with the lighter weight studs spaced further apart.

Hi Pinigis

Are you sure the thermal expansion of steel at 1800 degrees is 1%?


In the spectrum of size, gauges and spacing of metal studs, the manufacturers have established the lightest configuration that will pass ASTM-E119. They should be required to establish the configuration with the heaviest gauge stud with the shortest stud spacing.
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