By Jason Smart, PE, Bradford Douglas, PE, and John “Buddy” Showalter, PE
Click here to view the article and images in the June 2024 digital edition of STRUCTURE magazine.
The 2024 edition of the ANSI/AWC Fire Design Specification (FDS) for Wood Construction provides a new design resource where fire design of wood members, assemblies, and their connections is required by applicable building codes. The 2024 FDS (Figure 1) was approved as an American National Standard in September 2023 and applies to fire design of wood construction covered under the 2024 ANSI/AWC National Design Specification (NDS) for Wood Construction.
Since 2001, fire design provisions in Chapter 16 of the NDS have provided designers with a calculation-based means of demonstrating compliance with code-required structural fire-resistance ratings for exposed wood members. More recently, additional calculation procedures have been developed to address the added fire resistance and thermal benefits of protection using additional wood cover, gypsum panel products, and some types of insulation.
Shortly after the release of the 2021 FDS pre-standard, AWC’s consensus committee formed a new task committee to review the pre-standard. This consensus development process culminated in the first version of the FDS being approved as an ANSI American National Standard in September 2022. In the 2024 FDS, these calculation procedures for fire design of protected wood members have been combined with design provisions consistent with those of the 2024 NDS Chapter 16 for design of exposed wood members.
In developing the provisions of the FDS, the most reliable data available from laboratory tests and experience with structures in service have been carefully analyzed and evaluated for the purpose of providing, in convenient form, a national standard of practice. But before delving into the contents of this new design resource and how it can be used in the fire design of wood construction, a brief review is in order for how fire safety has historically been addressed in wood construction and how modern code provisions related to fire safety were developed.
History of Development
Long before any models were developed to quantify char depth in wood as a function of fire exposure time, the general fire performance behavior of wood was recognized in traditional heavy timber construction, which was sometimes referred to as mill construction.
In traditional heavy timber construction, the exterior walls are constructed of noncombustible materials and the interior structural elements are large-cross-section wood members with sufficient size to afford acceptable performance during a fire. Such interior structural elements, including floor assemblies and interior partitions, were typically constructed without concealed spaces, which could otherwise obscure a fire in its incipient stages and conduct it to other portions of the building. Traditional heavy timber construction is still recognized in U.S. model building codes, under its current designation as Type IV-HT construction in the International Building Code (IBC) and Type IV in the NFPA 5000 Building Construction Safety Code.
Most of the building elements in IBC Type IV-HT and NFPA 5000 Type IV construction are not required to have specified minimum fire-resistance ratings because they are recognized as having some qualitative level of inherent fire resistance owing to their large cross-sectional size. But for other construction types, building codes typically require specific building elements to meet minimum fire-resistance ratings. These fire-resistance ratings can be established either through standardized testing, calculation, or a number of other code-recognized methods. While charring of the wood causes section loss of an exposed wood member, it has been long understood that the layer of char which develops on the wood surface provides the tradeoff benefit of a certain degree of thermal protection to the remaining uncharred section beneath the char layer. This protective char layer thickens as the char front penetrates deeper into the wood member, thereby providing an increasing amount of insulation to the uncharred wood below and slowing the rate of char progression through the cross-section of the member. The resulting nonlinear rate of char progression through the wood member as a function of fire exposure time can be accurately modeled as a means of predicting performance under specified fire exposure conditions – a fact that is explicitly included in calculation procedures recognized in U.S. model building codes.
Overview of FDS Contents
The main body of the FDS contains three chapters, plus additional design information provided in supplements. As with most design standards, Chapter 1 provides the necessary scoping information, notation and definitions to terminology used in the standard, referenced material standards and general fire design requirements. Chapter 2 specifies ignition and flammability requirements for wood products and provides referenced test standards to be used to show compliance with these requirements. Chapter 3 establishes fire-resistance design provisions that apply to all wood structural members, assemblies, and connections designed using the NDS. The supplements, which are referenced in the FDS, provide information on tested flame-spread performance of wood-based interior finish materials and fire-resistance ratings of tested wall and floor/ceiling assemblies.
General Fire Design Requirements (Chapter 1)
Among the general fire design requirements in Chapter 1 are provisions governing minimum cross-sectional dimensions of heavy timber members. These minimum dimensions are consolidated into three tables, plus an additional table providing requirements for toppings on heavy timber floor decks.
Chapter 1 also lays out the general design requirements for fire-resistance-rated wood members and assemblies. Specifically, Section 1.7 addresses the critical design concepts of maintaining continuity of fire-resistance ratings between adjacent rated assemblies, and ensuring minimum required fire resistance of construction that supports gravity loads from a fire-resistance-rated building element or assembly.
Also in Section 1.7 are general provisions requiring protection of connections between wood members or assemblies that are required to have a fire-resistance rating. This protection must be designed in accordance with FDS Section 3.10 to protect and insulate every part of the connection for the full required fire-resistance-rating time.
Portions of Section 1.7 that were revised in the 2024 FDS are the newly enhanced provisions for detailing at joints and intersections between fire-resistance-rated wood assemblies. The FDS Commentary on these provisions includes diagrams, such as the example in Figure 2, to illustrate how intersections between fire-resistance-rated assemblies can be detailed to prevent premature burn-through.
Ignition and Flammability (Chapter 2)
The ignition and flammability requirements of Chapter 2 have been coordinated with requirements of model building codes but have been expanded to more thoroughly address requirements specific to wood materials and assemblies. Section 2.2 clarifies requirements for ignition-resistance of wood products used on exterior surfaces. While wood products are exempt from the code requirement to perform testing in accordance with NFPA 268, Standard Test Method for Determining Ignitability of Exterior Wall Assemblies Using a Radiant Heat Energy Source, because ignition of wood usually occurs at or above the heat flux limit of 12.5 kW/m2, reference to the model code requirements is provided.
Flame spread performance requirements of wood products used as interior finish materials are addressed in Section 2.3. This is where material classifications are provided based on flame-spread index and smoke-developed index, as determined through testing in accordance with ASTM E84. Reference is made to FDS Supplement A, in which flame spread indices and smoke-developed indices are reported for various wood and wood-based materials.
Also in Section 2.3 are provisions addressing qualification of fire-retardant-treated wood (FRTW) and wood-based products with fire-retardant coatings. For both product types, the effects of the fire-retardant treatment or fire-retardant coating on strength and stiffness are required to be evaluated in the approval process and accounted for in design.
Section 2.4 contains provisions addressing vertical and lateral flame propagation on exterior wall coverings and building projections. In addition to ignition resistance and flame spread requirements, combustible exterior wall coverings and building projections have limitations based on coverage area, coverage height and fire separation distance. Under certain conditions, testing of exterior wall covering assemblies in accordance with NFPA 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components, is required.
Minimum performance requirements for firestopping, fireblocking, and draftstopping are provided in Section 2.5, along with a list of materials that are prescriptively recognized for use as fireblocking and draftstopping. The FDS Commentary provides technical justification and values to quantify the relevant parameter of fire performance (i.e., burn-through time) for each prescriptively recognized fireblocking material listed in this section.
Fire Resistance (Chapter 3)
The scope of 2024 FDS Chapter 3 is aligned with that of 2024 NDS Chapter 16, in that it contains design provisions for determining the fire resistance of any wood construction that is covered under the scope of the NDS. Section 3.1 sets general requirements for establishing fire-resistance ratings. This includes defining the standard exposure under which fire-resistance ratings are to be established and loading requirements for both allowable stress design and load and resistance factor design of loadbearing fire-resistance-rated assemblies. Section 3.8 complements the provisions of Section 3.1 by referencing FDS Supplement B, which provides listings and descriptions of tested 1- and 2-hour fire-resistance-rated wall assemblies, floor/ceiling assemblies and roof/ceiling assemblies.
Provisions for evaluating the charring of wood as a function of standard fire exposure time are given in Section 3.2. Guidance is given in this section for calculation of char depth on surfaces of exposed wood members, as well as for calculation of char penetration at intersections and abutting edges between wood members. Various possible configurations of intersections and abutting edges are addressed, each associated with unique design guidance. The FDS Commentary provides further clarification, with example details corresponding to each of these possible configurations—one of which is shown in Figure 3.
Section 3.3 is where designers will find provisions for calculation of structural fire resistance of exposed wood members. While these structural fire resistance provisions should be familiar to those who have designed in accordance with previous editions of NDS Chapter 16, they also provide additional guidance, clarification, and design options for fire design of structural glued-laminated timber beams, lumber decking, and CLT. New design guidance is also provided in Section 3.3 for determining effective char depth at bearing interfaces with one or more members bearing perpendicular-to-grain.
Section 3.4 lays out provisions that can be used to quantify protection time provided by various materials, such as additional wood cover, fire-rated gypsum panels and certain types of insulation, when used as protection for the purpose of increasing the fire-resistance rating of a wood member or assembly. As stated in Section 3.5, such protection delays the onset of charring on each protected surface by an amount of time equivalent to the calculated protection time. In this way, calculated fire-resistance ratings exceeding 2 hours are possible for protected wood members and assemblies, even though the calculated contribution of the wood members to the structural fire resistance is not permitted to exceed 2 hours.
The procedures for calculating the thermal separation time and burn-through time provided by an assembly are described in Sections 3.6 and 3.7, respectively. The concepts of thermal separation and burn-through time are used in the design of fire-resistance-rated assemblies that are intended to separate one fire area from another. In most cases, thermal separation and burn-through resistance are provided by adding wood cover, fire-rated gypsum panels, and/or insulation to the assembly. Another application where the concept of thermal separation time is used is in the case of structural connections between members or assemblies that are required to be fire-resistance-rated. As specified in Section 3.10, such structural connections must be covered with protection that is designed to provide a thermal separation time no less than the fire-resistance rating required of the connected members.
Besides the calculation procedures laid out in Sections 3.2 through 3.7, one other procedure for calculating the fire-resistance ratings of assemblies is addressed in Section 3.9. This procedure, referred to as the component additive method, may be used for calculating the fire-resistance ratings of wall assemblies, floor/ceiling assemblies and roof/ceiling assemblies having components as described in Section 3.9. Assigned contribution times are provided for various components toward the overall fire-resistance rating of the assembly. These assigned contribution times are consistent with protection times and component fire-resistance ratings that would be calculated in accordance with the provisions of Sections 3.2 through 3.5. They are summed, within the constraints of Section 3.9.1, to determine the structural fire resistance time for the assembly. The thermal separation time must also be determined, in accordance with Section 3.9.2, and the fire-resistance rating of the assembly is taken as the lesser of the structural fire resistance time and the thermal separation time. Because this component additive method has been validated for calculation of fire-resistance ratings of assemblies rated for up to two hours, its applicability is not limited to one-hour fire-resistance ratings like previous component additive methods.
Conclusion
The 2024 edition of the ANSI/AWC Fire Design Specification for (FDS) Wood Construction, approved as an American National Standard in September 2023, applies to wood construction covered under the ANSI/AWC National Design Specification (NDS) for Wood Construction and provides a new design resource for use where fire design is required by applicable building codes. In addition to containing provisions consistent with those provided in NDS Chapter 16 for fire design of exposed wood members, the 2024 FDS provides new calculation procedures for fire design of protected wood members and assemblies. Because the FDS represents a more comprehensive design standard for fire design of wood construction, it is referenced in NDS Chapter 16 for design considerations that are not fully addressed in NDS Chapter 16. The 2024 FDS is accessible for free on the AWC website at
bit.ly/3UJ29RV. The 2024 NDS is accessible for free on the AWC website at awc.org/publications/2024-nds/. ■