Collapse Time for Different Assemblies
Need-to-know info from Underwriters Laboratories study
By James M. Dalton, Peter Van Dorpe, Robert G. Backstrom and Steve Kerber

Editor’s note: In the February issue of FireRescue magazine, authors James M. Dalton, Peter Van Dorpe, Robert G. Backstrom and Steve Kerber share some information and tactical tips for fighting fires in buildings made of lightweight construction. What follows here is additional information from their article not published in the print issue. For the complete article, click here.


Underwriters Laboratories (UL), in partnership with the with the Chicago Fire Department, the International Association of Fire Chiefs and Michigan State University, was awarded funding from the Department of Homeland Security/ Federal Emergency Management Agency’s, Assistance to Firefighters Grant program to subject a representative group of lightweight and engineered assemblies to ASTM E119, Fire Tests of Building and Construction Materials, the industry standard test.

In addition to the standard data collection of ASTM E119, video, thermal imaging, sound recordings and deflection data were also collected. This expanded data set enabled the project participants to present the findings in a format more easily accessible and understood by the fire service and the general public alike.

An interactive training program available at www.ul.com/fire/structural.html details the motivation, methodology, testing and lessons learned from this project. This project will allow fire service professionals to better understand the hazards and assess the safety risks that building occupants and firefighters are subjected to when encountering lightweight engineered lumber assemblies in fire conditions.

The Tests
Twelve furnace fire tests were conducted on assemblies representative of both traditional dimensional lumber (“legacy”) construction, and lightweight engineered (“modern”) construction methods. Six different structural elements, three ceiling finishes and four floor or roof finishes were used in various combinations to make the assemblies listed in Figure 1.

The usual floor load of 40 lbs./ft.2, evenly distributed across the assembly, was modified to more accurately reflect conditions encountered on the fireground. The floor was loaded on two adjacent sides only, with two 300-lb. mannequins placed at the center of the assembly. Note that this load is less than that normally used in the ASTM E119 test. Also note that the 14'. span of the test furnace is well below the capabilities of the engineered assemblies tested. Fully loaded assemblies built to their maximum allowable spans would likely fail earlier than indicated by these tests.

The ASTM E119 fire endurance test is designed to express fire resistance ratings in terms of hours, (i.e. ½ -hour, 1-hour, 2-hour, 3-hour, or 4-hour rated assemblies). The test is designed to provide a comparison of fire performance between test samples within the laboratory environment, in much the same way that “miles per gallon” ratings are used to compare the fuel efficiency of various automobiles. These hourly time ratings are not intended to convey the actual time a specific component or assembly will withstand a real fire event. It’s critically important that everyone looking at this data understands that the “collapse time” numbers are not an indication of operational time inside lightweight constructed buildings on the fireground.





The Results
Figure 1. Collapse Time for Assemblies

A Final Word
Fires in today’s “modern” residential buildings pose greater risks than their “legacy” predecessors. These structures are subject to rapid fire spread through areas of unprotected wood construction, the collapse of unprotected dimensional lumber, and the collapse of lightweight engineered wood components. Understanding the testing methods employed and the results of this study, even on a basic level, will assist firefighters in conducting a safer fireground operation the next time the alarm bell sounds. A working knowledge of these results is also a critical step for all members of the fire service who are actively engaged in the growing movement to enhance firefighter safety by modifying the current code requirements for residential construction.

James Dalton is the coordinator of research and development for the Chicago (Ill.) Fire Department. Peter Van Dorpe is a battalion chief for the Chicago (Ill.) Fire Department. Bob Backstrom is a senior staff engineer with Underwriters Laboratories. Steve Kerber is a research engineer with Underwriters Laboratories and has 12 years of firefighting experience.


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