Charred wood and fire resistance
This refers to “Construction Concerns: Charred Wood vs. Increased Fire Resistance” by Gregory Havel (fireengineering.com, July 11, 2016).
Although it is possible that the fire service has been taught that mass timber structures behave like “fire-resistive” materials during a fire, this is certainly not an assumption supported by the American Wood Council (AWC). Today, model building codes use the term “fire resistance” to describe the ability of a material or assembly to resist or retard the passage of heat from fire. Research and fire service experience support the argument that, as char forms during pyrolysis, the rate of charring decreases. The author suggests that char is nothing more than black residue that has little to do with the fire performance of wood products; however, char acts as an insulator that protects the wood beneath and slows the rate of burning of the protected wood. Charring of wood products serves an important role in the fire performance of wood products, and this role is well understood, but how we use this information to make safer mass timber buildings is just starting to evolve. In fact, the U.S. Bureau of Mines has written about precharring of timber structural members as a novel fire-retardancy technique.
Model building codes reference the National Design Specification® for Wood Construction, which provides for design of exposed fire-resistance rated wood products subjected to the ASTM E119 standard fire test exposure. How wood products behave within a fire-rated assembly has been demonstrated in hundreds of tests. With these data, AWC engineers are developing a method to calculate the contribution of membrane protection, including gypsum wallboard, when it is used to protect wood products. Additionally, the U.S. Forest Products Laboratory will be performing char rate tests at various design fire exposures to develop char rate models for wood products exposed to nonstandard time/temperature curves.
Fire resistance ratings based on the ASTM E119 fire exposure can now be demonstrated for wood used as an exposed structural member or as part of a fire-rated assembly. For exposed wood members, all wood initially chars at about the same rate until a char layer is formed. After a brief period of fire exposure, the rate of char formation becomes relatively constant, despite a gradually increasing furnace temperature. Burning of large wood members creates a protective insulating char layer on the exposed surfaces, protecting the inner core, which continues to maintain its nominal strength and stiffness properties under near-ambient temperatures over long-duration fire exposures. For wood protected by a membrane, such as gypsum wallboard, charring of the wood will begin at the initial charring rate when the membrane fails, but it will similarly slow as the char layer is created.
Havel is correct that char has no structural integrity. In the models, the heated wood fiber immediately beyond the char front is assumed to have a 20 percent strength and stiffness reduction since the fibers are at an elevated temperature. This assumption is conservative since the elevated temperature zone ranges from the char layer temperature (approximately 550°F) to the inner layers at ambient temperature (<150°F) over a very small region.
Experience has demonstrated that ignition and sustained combustion of single or even multiple wood building elements are dependent on many factors. At one extreme is the image after a wildfire has passed through a forest and all that remains are solid trunks of scorched trees. In this instance, the tree trunks remain since their outer charring has denied the fire a further fuel source. Similarly, fire modelers are working to identify how charring of combustible room surfaces retards further fire development once room contents are consumed.
The AWC has long been committed to providing the fire service with information to ensure wood buildings are safe from fire.
Kenneth E. Bland, P.E.
Vice President, Codes & Regulations American Wood Council
Gregory Havel responds: Although the fire service has not been formally taught that mass timber structures behave like fire resistive materials during a fire, this has been stated or implied in much literature and advertising during the past 130 or more years, from the time of the original “mill” and “heavy timber” construction to the present. This has resulted in periodic statements to the contrary by the National Board of Fire Underwriters in 1880; in publications like Fire Engineering and its predecessors; and in the works of noted fire service authors like Emanuel Fried, Francis Brannigan, and Vincent Dunn.
Some of this information has been included in fire service textbooks:
“Type IV structures are extremely stable and more resistant to collapse due to the effects of fire than other construction types that are not protected by a fire suppression system [bold text is mine]. When involved in a fire, the heavy timber structural elements form an insulating effect derived from the timber’s own char that reduces heat penetration to the inside of the beam.” (Fire Inspection and Code Enforcement, Seventh Edition, IFSTA/Fire Protection Publications, Oklahoma State University, Stillwater OK, 2009; page 128.)
The Cross Laminated Timber Handbook, US Edition, 2013, is distributed by the American Wood Council, the Wood Products Council, and several other organizations and government agencies promoting the use of wood and mass timbers in construction. In the opening paragraph of Chapter 8 “Fire Performance of Cross Laminated Timber Assemblies [CLT],” Section 2.2 “Fire Performance Attributes of CLT,” it states
“CLT panels provide excellent fire resistance. This is due to the inherent nature of thick timber members to char slowly at a predictable rate, allowing massive wood systems to maintain significant structural capacity for extended durations when exposed to fire.”
Yet, the following paragraph describes a full-scale compartment fire test that was conducted in a test building of CLT with the walls protected with ½-inch fire-rated gypsum board and ½-inch standard gypsum board and while the ceilings were protected with one-inch mineral wool insulation and ½-inch fire-rated gypsum board.
“In an attempt to replicate a similar fire severity, such as those encountered in typical residential dwellings, a design fire load of 69,600 BTU/ft2 (790 MJ/m2) was used and burned for a duration of slightly over 1 hour. It is reported that flashover occurred after about 40 minutes. The fire severity started to decline after 55 minutes and was extinguished, as planned, after an hour-long duration. Furthermore, the measured charred depth on the gypsum-protected CLT compartment elements was very low, ranging from approximately 3⁄16 inch to 3⁄8 inch (5 mm to 10 mm). No elevated temperatures were measured, and no smoke was observed in the room above the fire room. From this full-scale design fire test, one can conclude that CLT buildings can be designed to limit fire spread beyond the point of origin, even when massive timber construction is used.”
I am familiar with the ASTM E119 test. I am also familiar with the recent testing conducted by the Firefighter Safety Research Institute at Underwriters Laboratories (UL FSRI) and the National Institute of Standards and Technology (NIST), which have resulted in flashover with legacy (cellulose-based) room furnishings in 20 to 30 minutes and with modern (petroleum-based plastics and synthetic fabrics) room furnishings in five to seven minutes. I wonder about the realism of the test described, whether the walls were load-bearing and whether the floor above carried a live load. I do not wonder about the fire or heat penetrating the drywall board to char the mass timber behind it during the one-hour duration of the test, since this never would have happened if the test room had been equipped with an automatic fire sprinkler system.
In the article, I do not suggest that “char is nothing more than black residue that has little to do with the fire performance of wood products.” I compared the char to the charcoal that we use for outdoor cooking and for drawing in art classes: a material that is lightweight; brittle; and with little tensile, compressive, or shear strength. “Soot” is the word usually used to describe the black residue, which has little to do with fire performance and which is usually the product of the carbon contained in smoke condensing on cool surfaces such as the interior of a chimney or flue or the walls of a room before they have been preheated.
The resistance of any material to the passage of heat (its insulating value) is a function of the relationship between the ambient temperature on one side of the tested wall or floor-ceiling assembly and the heated space on the other side. The greater the temperature difference between the two sides of the assembly, the more quickly will the heat be transferred through the insulator. From the Cross Laminated Timber Handbook, it appears that the test was conducted with the standard time-temperature curve stated in ASTM E119. Yet, in the tests conducted by UL FSRI and NIST, the temperatures in a compartment fire with modern furnishings exceed those in the standard time-temperature curve by several hundred degrees Fahrenheit; are achieved more quickly; and result in a more rapid rate of heat release.
Fire science textbooks usually state that the temperatures in fully involved room fires are in the range of 1,000°F to 1,100°F and probably higher in a fire in which the room is furnished with modern plastics and synthetics. Yet, Bland indicates that the temperature at the surface of the char layer in the test room is approximately 550°F (near the temperature at which wood will begin to char) and less than 150°F at the uncharred wood. How long does the surface of wood or its char layer need to be exposed to a temperature of 1,000°F before its temperature will begin to rise toward that 1,000°F? And, how much more quickly and deeply will wood char at 1,000°F than at 550°F?
I believe that my original advice to firefighters is still accurate: When operating at fires in buildings of heavy timber, mill, or mass timber, keep in mind the following:
- Since 1880, the heavy timber and mill buildings have been required to include NFPA 13-compliant automatic fire sprinklers. When buildings like these survive fires, it is more likely because of the operation of the fire sprinklers than the formation of a layer of char on the timbers.
- Heavy timber, mass timber, and mill buildings without automatic fire sprinklers are unlikely to survive the fire; and they contain so much burning fuel post-flashover that the fire eventually dies down for lack of fuel to a level that the fire department is able to extinguish.
- The dimensions of the timber, manufactured timbers, or mass timber components were chosen to carry specified loads with a safety factor. When the surface chars, the wood turns from load-bearing to a lightweight combustible insulation (charcoal) with no load-bearing capacity. When enough of the surface chars deeply enough, the fire will literally have burned through the safety factor, and the structural members will collapse. Although we may have access to the design calculations on a new building, the load-bearing capacity changes with age and varying live loads, especially if the building is a victim of “deferred maintenance.” We will not know during a structure fire the exact time and place of an impending structural collapse caused by the charring of mass timber components.
- We must not risk the lives of our firefighters and our own lives on the assumption that a layer of char that formed on exposure to fire makes the timber product fire resistive or slows the rate of heat transfer to the interior of the timber, especially when it is not backed up with an automatic fire sprinkler system.
Let’s make our legacy a healthier fire service
Yet again, Chief Bobby Halton inspires me. “Behavior Is the Truth” (Editor’s Opinion, June 2016) was not only spot on but even a bit emotional. I copied it, scanned it, and sent it out to my department. I’ve been talking for two years about the fact that it’s our generation’s responsibility to kick the cancer thing (and cardiac and mental issues) as best as we can for the next generation. My vision is for a kid to open his Firefighter I manual in 25 years, look at the history chapter and see the Firefighter Cancer Support Network, and ask the instructor, “They had a cancer support network back then? Really?”
Wilton (CT) Fire Department
Note: In fireEMS in Fire Engineering, August 2016, page 39, the first sentence in the last paragraph in column 1 should read as follows:
As a result of the insights garnered through this fire rescue call, the Reno Fire Department has drafted an advanced life support smoke inhalation protocol that includes the authorization to administer a cyanide antidote kit not only to patients in cardiac arrest or with profound Altered Level of Consciousness and hypotension but also to those exhibiting signs and symptoms consistent with cyanide toxicity.
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