How Solid Wood Walls Respond to Fire
What is the fire rating for a solid wood wall? Fire ratings are determined by either an actual test applied to a specific construction or by calculations derived from such tests. Because of the diverse nature of the log building industry, it is difficult to physically test every specific construction especially if one considers that each test cost tens of thousands of dollars to prepare, perform, and analyze. Therefore, the Log Homes Council holds the position that evidence from existing research provides an acceptable basis for determining the fire rating of the wall.
Until 2007, when the ICC published ICC 400, Standard on the Design & Construction of Log Structures, one of the biggest problems for the log home industry was the lack of recognition of log construction in building codes. To identify the potential performance of a log home in a fire, one had to reference the ICC International Building Code (IBC) for the definition of heavy timber construction. The most similar type of construction in the codes continues to be heavy timber, using massive beams, posts, rafters, and other structural members. The IBC lists requirements for Type IV construction in section 602.4:
- Beams & Girders: Not less than nominal 6x10 (inches, width x depth; 152x254 mm)
- Columns: Not less than 8 inches (203 mm) nominal when supporting floor loads; nominal 6x8 (inches, width x depth) or greater for roof and ceiling loads only.
- Rafters, purlins, timber trusses: Not less than nominal 4x6 (inches, width x depth; 102x152 mm)
- Decking: Floors and roofs shall be without concealed spaces. “Wood floors shall be of sawn or glued-laminated planks, splined or tongue-and-groove, of not less than 3 inches (76 mm) nominal in thickness covered with 1-inch (25 mm) nominal dimension tongue-and-groove flooring, laid crosswise or diagonally…” For wood roof decks, the dimension changes to 2 inch nominal (51mm).
An interesting component of the IBC Type IV Construction is the allowance for exterior structural members in section 602.4.7: “Where a horizontal separation of 20 feet (6096 mm) or more is provided, wood columns and arches conforming to heavy timber sizes shall be permitted to be used externally.” This is a relevant note because the typical exterior wall of Type IV construction is masonry or other material that provides a 2-hour fire-resistance-rated construction. Further in 602.4.7, “Partitions shall be of solid wood construction formed by not less than two layers of 1-inch (25 mm) matched boards or laminated construction 4 inches (102 mm) thick, or of 1-hour fire-resistance-rated construction.” It would appear that most log walls would satisfy the partition component of Type IV construction.
The importance of being recognized as Type IV construction goes beyond just the size of the structural elements. The expected use, or occupancy group, of the building will impact the required fire-resistance rating for the exterior walls as determined by the fire separation distance between structures and the height of the wall. For residential construction where neighboring structures are 30-feet or more apart, the exterior wall is not required to be fire-resistance rated, but within 5 feet of another structure a 1-hour rating from both sides is required.
The comparison of log buildings to Type IV construction is relevant because many log structures likewise employ structural timbers as joists, rafters, beams, and posts. The deviation is that the timbers also act as the structural component of the walls. As wall-logs, the wall members have structural capacity and are fully supported along their length. However, the stacking of logs to form a solid wall produces a different dynamic in fire, more like that of a glued laminated beam.
The key section in the code is really IBC 704.6: “The wall shall extend to the height required by Section 704.11 and shall have sufficient structural stability such that it will remain in place for the duration of time indicated by the required fire-resistance rating.” So, what is required to satisfy that requirement when the exterior wall is solid wood construction?
What Happens to Wood in a Fire?
The only elements we can have some influence over are the presence of heat, oxygen, and the availability of the fuel source: the wood itself. Wood is a cellular material that makes up the bulk of the tree. Water, tannins, waxes, gums, starches, alkaloids and oils occupy the cell cavities, contributing to the color, odor, taste, decay resistance, and flammability of the wood. It is like a honeycomb composed mainly of dead, hollow, tubular cells. This cellular structure is what gives wood it’s amazing strength, insulating value and allows it to hold water, oxygen, and nutrients.
Insulating Effect of Char
As an organic material, wood is combustible. Yet its insulating and charring characteristics produce an astounding response to fire. While wood begins to char at 300oF, commonly exceeded in the first five minutes of an accidental fire, the wood beneath the char remains structurally sound. Compare this unique response to that of structural steel which loses 50% of its strength at 1000oF. The charring effect of wood results in a protective coating over the surface of the uncharred material. This protective char coat is very similar to the effect created by some intumescing chemicals used to protect materials and assemblies.
Chapter 17 of the Wood Handbook (Wood Handbook: Wood as an Engineering Material, FPL-GTR-113) includes a discussion on charring, explaining that the “The temperature profile within the remaining wood cross-section can be used with other data to estimate the remaining load-carrying capacity of the uncharred wood during a fire and the residual capacity after a fire.” This is essentially what the American Forest & Paper Association (AF&PA) demonstrates in their Technical Report 10, Calculating the Fire Resistance of Exposed Wood Members and summarized in their Design for Code Acceptance Series, DCA 2 – Design of Fire-Resistive Exposed Wood Members. These documents are relevant to the discussion as they are the original research that effectively became Chapter 16 of ANSI/AF&PA NDS-2005, the National Design Specification® for Wood Construction (NDS®). The NDS is referenced in the model building codes for wood design and in the ICC400 Log Standard specifically for calculating fire-resistance of log walls.
The NDS notes that a nominal char rate of 1.5 inches per hour in the direction perpendicular to the surfaces of exposure to fire is commonly demonstrated and thus assumed for solid sawn and glued laminated timbers. This assumption is supported by information in the Wood Handbook that notes that the studies and tests performed contained many variables that could not be controlled in a manner sufficient to derive a distinct char rate per wood species.
The wood that remains within the estimated charring is analyzed in accordance with the NDS to determine its remaining strength after a fire. Essentially, the initial size of the timber is specified to provide the required section properties plus the estimated char layer per surface that is exposed. For beams (3-sided exposure) and columns (4-sided exposure), this is illustrated below from AF&PA’s Technical Report 10.
While the effect of charring is to protect the structural wood fiber beneath it, charring does remove the structural capabilities of the affected area. This is critical to the connection of timbers to each other. When timber connections are made with hangers and/or fasteners, the connection must take fire-resistance into consideration. For example, if the connection is intended to survive a one-hour fire event, it is likely that the fastener length will need to be a longer length so that it is connected to structurally sound wood. Its diameter may also be a factor to resist the heat flux of the fire. APA-The Engineered Wood Association Technical Note #Y245 also presents the approach taken by AF&PA, but this paper goes further to describe and illustrate how connections can be protected for fire-resistive construction. Concealed by 5/8” Type “X” gypsum or 1-1/2” of wood, the fasteners are less likely to conduct heat through the connection.
Code Comparison: Solid Wood Walls vs. Heavy Timber Construction Type
Heavy timber construction is considered fire-resistive if the structure can maintain its integrity for a specific amount of time during a fire. The structure can consist of timber framing to provide the entire support; curtain walls or load bearing walls must be fire-resistive construction. The intent of the codes is to provide a barrier to movement of fire through containment with minimal impact on structural integrity. Containment is measured by temperature rise on the wall surface opposite of the fire exposure while the construction continues to support the design loads.
Load Transfer Comparison
In heavy timber construction, the structural loads placed on the assembly are transferred from spanning members (beams, rafters, joists) to specific bearing areas (post, column, mullion, bearing wall). In log buildings, the exterior bearing wall is a continuously supported solid wood member supporting the same heavy timber structural frame members.
- Rather than substantial concentrated loads on a few vertical members supporting the entire framework, log wall construction spreads the loads throughout the entire structure.
- Secondly, the log wall assembly is likely to be only exposed to fire on one side while the timber column will have three or all four surfaces exposed.
These are important considerations since the log building is likely to be less prone to collapse in any one area under a fire condition.
Fire Exposure Comparison
Since log walls are a solid assembly extending from subfloor to roof, there is only opportunity for exposure on two sides of the assembly. The concept that the unobstructed height of the log wall is to be used in determining fire resistance is supported by Rule 1 from T.Z. Harmathy’s Ten Rules of Fire Endurance Rating (published in 1965 in Fire Technology). This rule states:
The “thermal” fire endurance of a construction consisting of a number of parallel layers is greater than the sum of the “thermal” fire endurance characteristics of the individual layers when exposed separately to fire.
This rule, as substantiated by testing on glu-lam beams, supports the fact that the fire rating of a log wall would be based on the total height of the assembly as defined by contiguous courses (layers) and protected openings. The effective height would be determined to the bottom of an adjoining horizontal interface (floor or roof assembly).This philosophy was adopted by the committee that developed the IWUIC, resulting in the definition below:
LOG WALL CONSTRUCTION. A type of construction in which exterior walls are constructed of solid wood members and where the smallest horizontal dimension of each solid wood member is at least 6 inches (152 mm). This definition is directly correlated to the requirement for Heavy Timber Construction that calls for a minimum 6” dimension to structural members. In the absence of supporting research and/or testing, it is impractical to include log wall technologies that have narrower dimensions. It is also important to note that ICC400 does not consider sealants or other materials between logs, so that the fire-resistance is considered only on the basis of the width of the log wall. However, sealants (e.g., chinking systems) can have a dramatic effect on the wall’s performance, but ASTM E-119 testing of the specific materials and assembly would be required to acknowledge a rating.
Paths for Code Compliance
By the time the ICC IS-LOG Committee was developing the ICC400 Standard on the Design & Construction of Log Structures, AF&PA had fine-tuned its research and modified the analysis to a mechanics-based design method. With the precedent established in the UWUIC that defined log wall construction and acknowledged a minimum 6” width (at the narrowest point) as a one-hour fire-resistive construction, Section 303 of ICC400 was able to establish three distinct paths for establishing the fire resistance ratings of log walls.
1.Prescriptive Path: This path adopted the IWUIC precedent. If the criteria in this path is satisfied, the code is satisfied.
2.Calculated Path: The path adopted NDS Chapter 16 as the method for calculating fire resistance. The principle applied is that, “The induced stress shall not exceed the resisting strength which has been adjusted for fire exposure”. The adjustment is the removal of wood due to char rate and thickness of the char layer. It is important to note that this provision sets a maximum of a 2-hour rating on any analysis.
3.Tested Path: It is critical to understand that if neither the Prescriptive or Calculated Paths can be applied, that the reported results of full scale testing of a specific wall assembly in accordance with ASTM E119 by an accredited fire testing laboratory can be used to establish the fire resistance rating.
Calculating Fire Resistance Rating
The desire to establish a reasonable fire rating for solid wood walls included a search for existing methodology that could be logically applied. Initial research for this paper uncovered formulas for determining fire resistance for beams and columns published by the American Institute of Timber Construction, American Forest & Paper Association, and adopted into each of the model building codes (BOCA, ICBO, SBCCI). Additional research on glued laminated timbers demonstrated that layered wood members perform in a similar fashion, This has been considered in ICC400 and, as noted in the previous discussion of the Insulating Effect of Char, these calculations have been updated and incorporated into the building codes through the NDS.
In DCA 2 - Design of Fire-Resistive Exposed Wood Members, guidelines are given for calculating the effect of fire on columns (exposed on four sides) and beams (exposed on three sides). This simplified approach based on empirical testing provides the calculation of fire resistance rating for a given timber beam size, in minutes, equal to:
Minutes of Fire Resistance Rating = 2.54 Zb [4-(b/d)].
Where:
b = the breadth (width) of a beam or larger side of a column before exposure to fire, inches. By definition of this section, the minimum breadth is 6-in. nominal (5.5-in. actual per the NDS).
d = the depth of a beam or smaller side of a column before exposure to fire, inches. It is assumed that each horizontally laid wall-log (as defined by ASTM D-3957) acts as a beam to support roof/floor loading.
Z = the load factor is typically established at 1.3 for a beam
Using these criteria, the ratings for a nominal 4”, 6” and 8” thick log wall were calculated as follows. The first three calculations maintain the premise assumed above. The last provides a more conservative approach that coincides with the definition of heavy timber construction.
a) If b>=3.5", d=3.5" , and Z=1.3; (2.54*1.3*3.5*[4-(3.5/3.5)]) = 34.67 minutes
b) If b>=5.5", d=5.5", and Z=1.3; (2.54*1.3*5.5*[4-(5.5/5.5)]) = 54.48minutes
c) If b>=7.25", d=7.25" , and Z=1.3; (2.54*1.3*7.25*[4-(7.25/7.25)]) = 71.82 minutes
d) If rating = 60 minutes, b=d, and Z=1.3; (60/(2.54*1.3*[4-(4/4)])) = 6.057 inches.
Applying NDS Principles to a Log Wall
While it has been established that the log wall is capable of acting as a beam (a structural member that is designed to resist bending), the nature of the wall assembly is similar to that of decking when exposed to fire. In the discussion of fire design in Chapter 16 of the NDS, there is a provision for Timber Decks (section 16.2.5) that can be applied to log walls. This section can be applied in that the log wall should be “designed as an assembly of wood beams fully exposed on one face” if a tongue and groove exists between logs.
Section 16.2.5 also makes a distinction between tongue-and-groove (T&G) decking as opposed to pieces that are butted to one-another. In a log wall, like in a plank floor or roof, these butted joints can be at the ends of the piece and/or along the sides. A conservative approach to designing a log wall with butt joints to resist a 1-hour fire event would be to include the required effective char layer to the exposed face and to be sure that the edges at the butt joints are reduced by 33% of the effective char rate. For 1-hour fire endurance, the horizontal dimension of the log section would be reduced by a 1.8” char layer, and the vertical dimension would be reduced by 0.6” along the top and bottom edges.
Whether horizontal or vertical, it is clear that the design of the joint between logs and the sealants used to protect them have as much of an effect on fire performance as they do on air infiltration. The reported performance of planks, tongue and groove decking, and glued laminated beams substantiates this concept.
When designing the fastening schedule for the log wall, the fire-resistance rating of the wall should be considered in addition to edge and end distance or the need for lead holes. For a one-hour rated assembly, the fasteners should be placed a minimum of 1.8” from either edge of the log profile, referencing the narrowest width of the log wall. When sealants or coatings approved for the required endurance time are used, this distance to the edge of the log can be reduced (e.g., when covered by a rated chinking product).
The following table from AF&PA TR10 demonstrates the design load rations for exposed butt-joint timber decks. It is relevant to log walls in that the tabulated values (resulting from principles of engineering mechanics presented in the report and included in NDS Chapter 16) show that a wall assembly consisting of nominal 6x6’s would remain at 100% of the structural capacity of the original cross-section after a one-hour fire event. This would support an argument that the prescriptive code be changed from an actual 6” minimum dimension to 5-1/2”.
| Rating | 1-Hour | 1.5-Hour | 2-Hour | ||||||
| Beam Width | 1.5 | 2.5 | 3.5 | 5.50 | 2.5 | 3.5 | 5.5 | 2.5 | 5.5 |
| Beam Depth | Design Load Ratio, R_s | ||||||||
| 2.5 | 0.05 | 0.12 | 0.15 | 0.18 | ---- | ---- | ---- | ---- | ---- |
| 3 | 0.09 | 0.24 | 0.30 | 0.36 | 0.03 | 0.04 | 0.05 | ---- | ---- |
| 3.5 | 0.14 | 0.35 | 0.44 | 0.53 | 0.08 | 0.12 | 0.16 | ---- | ---- |
| 4 | 0.15 | 0.45 | 0.57 | 0.68 | 0.14 | 0.21 | 0.28 | 0.02 | 0.08 |
| 4.5 | 0.21 | 0.54 | 0.68 | 0.80 | 0.19 | 0.30 | 0.39 | 0.04 | 0.16 |
| 5 | 0.24 | 0.61 | 0.77 | 0.92 | 0.24 | 0.38 | 0.50 | 0.06 | 0.24 |
| 5.5 | 0.27 | 0.68 | 0.85 | 1.00 | 0.29 | 0.45 | 0.59 | 0.09 | 0.32 |
At openings in the log wall, the common practice of installing a wood frame (buck) in which to install windows or doors is beneficial to the fire performance of the opening. When the buck is largely self-supporting and consists of nominal 2x lumber or larger pieces, the buck protects the ends of the logs and the bottom of the header because it will be sacrificed to the fire rather than the log. Therefore, the header log(s) can be assumed to experience the same charring as the remainder of the log wall.
