Determining Fire Resistance
Determining Fire Resistance
The International Building Code (IBC) lists five acceptable methods for actively determining the fire resistance of structural materials, systems and assemblies. (A sixth method permits relying on the work of approved agencies.) However, few building designers, and even fewer component manufacturers, have extensive experience with all of these methods, largely because tested fire-resistance designs have reduced the need for using the calculative options.
What value is there, then, in becoming familiar with all of the methods for determining fire resistance that are allowed in the IBC?
All five methods are included in the code to give designers the greatest flexibility possible, and they can come in handy when limited test data is available. Component manufacturers who are familiar with these methods can be a critical resource for their customers. Instead of simply following an architect’s initial specifications, component manufacturers can make suggestions and develop efficient designs that provide effective structural and fire performance.
Method 1: Documented Fire-Resistance Designs
The first method of determining fire-resistance is probably the most familiar and commonly-used. It is straightforward because it relies on known quantities: namely, the design specifications of assemblies that have actually been tested.
Documented designs are tested according to ASTM E119 or ANSI/UL 263 standards. A summary of the tested metal plate-connected wood truss assemblies are provided in SBCA research report SRR 1509-01 “Fire Resistance Rated Truss Assemblies.” Keep in mind, though, that testing isn’t required. Evaluation by making fire-endurance performance comparisons and undertaking fire engineering calculations to expand code compliant applicability is allowed by method 4.
Method 2: Code-Prescribed Assembly Designs
The second method in IBC 703.3 refers to table 721, which offers prescribed fire performance information for specific materials. Unfortunately for component manufacturers, only one floor or roof assembly involving wood trusses is provided (table 721.1(3), item 21-1.1):
Floor or Roof Construction: Wood joists, wood I-joists, floor trusses and flat or pitched roof trusses spaced a maximum 24" o.c. with 1/2" wood structural panels with exterior glue applied at right angles to top of joist or top chord of trusses with 8d nails. The wood structural panel thickness shall not be less than nominal 1/2" nor less than required by Chapter 23.
Ceiling Construction: Base layer 5/8" Type X gypsum wallboard applied at right angles to joist or truss 24" o.c. with 1-1/4" Type S or Type W drywall screws 24" o.c. Face layer 5/8" Type X gypsum wallboard or veneer base applied at right angles to joist or truss through base layer with 1-7/8" Type S or Type W drywall screws 12" o.c. at joints and intermediate joist or truss. Face layer Type G drywall screws placed 2" back on either side of face layer end joists, 12" o.c.
Method 3: Component Additive Method (CAM)
The third method refers to Section 722, which again refers to the values in table 721. This data can be used to design an assembly with a calculated maximum fire-resistance rating of one hour. Referring specifically to wood assemblies, IBC 7126.96.36.199 states:
The fire-resistance rating of a wood frame assembly is equal to the sum of the time assigned to the membrane on the fire-exposed side, the time assigned to the framing members and the time assigned for additional contribution by other protective measures such as insulation. The membrane on the unexposed side shall not be included in determining the fire resistance of the assembly.
When using CAM, it’s important to stick closely to the dictates of the code. Given the complexity of providing an assembly that is equivalent to a code-prescribed system, it is wise to consult with someone who has relevant fire engineering calculation experience.
Method 4: Engineering Analysis Method
The fourth method allows designers to use comparable building elements, components and assemblies based on their fire-resistance ratings as determined by ASTM E119 or UL 263 testing. Essentially, this method combines a comparison of tested building element designs, fire-endurance engineering analysis, and IBC 104.11, which allows the use of alternate methods and materials that are analyzed by an approved source and approved for use by a code official. A common basis for the analysis this method calls for can be found in T. Z. Harmathy’s Ten Rules of Fire Endurance Rating, published in 1965 yet still widely used today.
Method 5: Alternative Protection
The fifth method listed in IBC 703.3 is simply to use alternative protection methods allowed by IBC 104.11. If an approved source can develop a design that complies with the intent of the code, the signed and sealed design should not be denied by the building official.
A building designer may submit a design that has been fire-resistance rated using any of the methods described here to a code jurisdiction. If it is not a listed design, the building designer should submit details showing how the design was determined and how it complies with the intent of the building code. Component designers who understand all the options available to the building designers they work with are best able to offer ideas and act as a resource when issues arise.
SBCA’s 2-Hour Rated Truss Assembly Design
SBCA has developed an assembly design that CMs can share with building designers to be used as is or as an aid in developing other ideas for assemblies more tailored to your specific project.
The SBCA example specifies the creation of a 2-hour-rated, fire-resistant membrane, the most critical feature of a fire-resistant assembly. The rationale for the example’s specifications rest on the extension of industry-accepted gypsum board fire-resistance values using Harmathy’s first and second rules.
Rule 1: The “thermal” fire endurance of a construction consisting of a number of parallel layers is greater than the sum of the “thermal” fire endurances characteristic of the individual layers when exposed separately to fire.
Rule 2: The fire endurance of a construction does not decrease with the addition of further layers.
The design is then made more conservative with the use of a “resilient channel” or air gap in accordance with Harmathy’s third rule.
Rule 3: The fire endurance of constructions containing continuous air gaps or cavities is greater than the fire endurance of similar constructions of the same weight, but containing no air gaps or cavities.
To evaluate the validity of the assumptions made using Harmathy’s rules, the design values of the specified elements in this example assembly are compared with the tested values of similar designs.
Once the overall resistance rating is thus confirmed, the fastener system is the next essential element, as fire endurance depends on the ability of the assembly to hold the elements in place as expected. The fastener schedule is calculated using the National Design Specification for Wood Construction (NDS), based on the design value of the fasteners and the calculated loads of the assembly.
Additional detail and design specifications are available in SBCA Research Reports 1509-01 and 1509-02. Keep in mind that SRR 1509-02 only summarizes the design. To use the full design, consult the UL listing cited in the report or contact SBCA for more information.