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Advertising supplement provided by
National Gypsum Company

Hourly Rating

In Volume No. 1 everything is categorized by hourly rating—beam assemblies, floor assemblies, and floor-ceiling assemblies.

Building codes are based on hourly ratings.

There are two test standards that are used. One of them is ASTM E119, written by the American Society for Testing Material, and it outlines the entire test process: “What are the requirements? How big a piece of assembly? How soon before failure? What’s the temperature rise?” This is not established by Underwriters Laboratories. UL does have a similar test: UL 263.

“If we look at the pictures of an assembly after testing,” Piccolo says (Figure 1), “we say ‘Oh, my goodness, it must have failed. Everything fell off.’ But what we need to realize is that it is a post-test photo, and the assembly has been subjected, after the fire, to a hose-stream of certain pressure that will ensure that the structure is still viable.”

“This is a two-sided assembly. If you look, there is a second layer of drywall on the other side of the assembly, which has not failed during the course of the test. It would be acceptable for the drywall to fall off on one side, but still maintain the structural integrity on the other side. It’s very possible that this assembly did pass, even though we had some burn through the first layer. But (the fire) didn’t burn all the way through the assembly.”

Tests, by various labs, are routinely conducted to measure many of the characteristics of gypsum board, such as strength, durability, and sound resistance. But the most important tests are for fire-resistance. In a special furnace, typical wall assemblies fitted with thermocouples are exposed to varying temperatures—some in excess of 2100 degrees. Then, the reaction of the assembly is carefully observed. If the wall assembly successfully withstands the high temperatures, an appropriate hourly rating is assigned—one, two, three or four hours.

ASTM E119 follows a Standard Time Temperature Curve when conducting a fire test (see Figure 2). We see in the graph that at 5 minutes the temperature reached 1000 degrees and at 1 hour the temperature is at 1700 degrees. After one hour the temperature continues to climb but does not climb as drastically. At the end of four hours, the furnace is at approximately 2000 degrees.

This is the standard, and no matter whose product is subjected to it, it’s always the same. There are variations with the hose stream and other things, depending on the type of assembly, but in standard testing, everything should be tested the same way, so we have a basis of comparison.

It is important to keep in mind that the test results are achieved in a controlled, laboratory environment. In addition, certain UL fire ratings for many system designs are achieved as a result of engineering studies conducted by UL as opposed to actual fire tests in the laboratory. Performance of any fire-rated system in an actual field installation may vary from the published rating, due to the variability of system components, installation techniques which might be used and actual fire conditions.

In testing an assembly, one of the things we are concerned about is heat transmission through the device. When the assembly is in the test furnace, thermocouples are strategically placed over the inside of the wall. Thermocouples measure temperature of the assembly during the test. The average of all the thermocouples cannot exceed 250 degrees over ambient, or normal, room temperature. This is to prevent igniting something on the other side of the wall. In most cases, things will not spontaneously combust at 250 degrees over room temperature. That is one of the test criteria.

Another test criteria is that at any one particular point on the assembly structure, no single thermocouple can report a temperature of 325 degrees over ambient in any one spot. If a single thermocouple rises above 325 over ambient, the test is “failed.” The last point, and this is a fairly important point, is that during the course of the test we do not get a failure—the structure cannot collapse, or show any evidence of burn-through to the other side of the tested assembly. During the course of the test, the assembly has to maintain its structural integrity. If it’s a one-hour assembly, it’s a one-hour test—not 59 minutes. A variance of 30 seconds is acceptable. A two-hour assembly is tested for 120 minutes, a three-hour assembly is a three-hour test.

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