Detailing for Fire Resistance

An important yet little known piece of information for many designers is that there are many sources for tested assemblies that meet 1-hour and 2-hour fire-resistance ratings required for wood buildings—not just UL.

In addition to UL’s Fire Resistance Directory, assemblies can be found in publications such as:

• Intertek Testing Services’ Directory of Listed Products
• Gypsum Association’s Fire Resistance Design Manual

They can also be selected from one of the prescriptive assemblies provided in IBC Section 721, which are based on ASTM E 119 or UL 263 test results, by calculating an assembly’s fire resistance using IBC Section 722, or by other methods indicated in Section 703.3 of the code.

Assemblies tested by the wood industry are also available. AWC’s Design for Code Acceptance 3: Fire-Rated Wood Floor and Wall Assemblies contains fire ratings of wood-frame wall and floor/ceiling/roof assemblies. Other sources include APA – The Engineered Wood Association’s Fire-Rated Systems (Form W305), and Wood Truss Council of America’s Metal Plate Connected Wood Truss Handbook. Some manufacturer websites and catalogues also reference tested assemblies that include their products.

Designers also have the option of integrating exposed, fire resistance-rated heavy or mass timber structural members into their designs, adding warmth to interior spaces. Because these products are thick and solid, they char on the outside at a slow and predictable rate, while retaining strength, slowing combustion, and allowing time to evacuate the building. The char protects the wood from further degradation, helping to maintain the building’s structural integrity and reducing its fuel contribution to the fire.

Per IBC Section 722, the fire resistance of exposed wood members may be calculated using the provisions of Chapter 16 of the NDS. AWC’s Technical Report No. 10: Calculating the Fire Resistance of Exposed Wood Members, contains full details of the NDS method as well as design examples.

Photo: Costea; courtesy of LPA, Inc.

SOUTH TAHOE HIGH SCHOOL
Location: South Lake Tahoe, California
Architect: LPA, Inc.
Structural Engineer: LPA, Inc.
Size: 67,500 square feet

Fire resistance of exposed wood members can be calculated using the provisions of Chapter 16 of the NDS.

Acoustics

With spaces that vary from gyms to libraries (and every noise level in between), acoustic consideration is an obvious priority for school design. The IBC divides sound into two categories. Airborne sound is measured with sound transmission class (STC) ratings and is relevant both to wall and floor/ceiling assemblies. Structure-borne sound is measured through impact insulation class (IIC) ratings and only relates to floor/ceiling assemblies.

While the IBC requires STC and IIC ratings of 50 for assemblies in apartment buildings and hotels, it has no such requirements for educational facilities. However, many school districts have established their own minimum ratings, often with similar STC and IIC baselines.

Tested wood-frame assemblies are available to meet a wide variety of acoustic performance levels. This is illustrated in Figure 6, which shows the progression from single-stud through staggered stud and double-stud construction. Double-stud walls can achieve a rating of approximately STC 63 when insulated with batt insulation and covered with two layers of gypsum wallboard on the outside faces of the studs.

Source: Acoustical Considerations for Mixed-Use Wood-Frame Buildings, WoodWorks

Beyond gypsum wallboard and insulation, options for improving performance include (among others) resilient channels in walls and floors, and concrete topping (or other similar material) on floor assemblies. For a more in-depth discussion of acoustic detailing, the WoodWorks paper, Acoustical Considerations for Mixed-Use Wood-Frame Buildings, is also relevant to the design of wood-frame schools.⁵

For information on the acoustical design of mass timber floor and wall assemblies, as well as a list of tested assemblies, see the WoodWorks paper Acoustics and Mass Timber: Room-to-Room Noise Control and its accompanying Inventory of Acoustically-Tested Mass Timber Assemblies.

Durability

There is a misperception that wood buildings require greater levels of maintenance than those made from other materials or don’t last as long, and architects have cited this perceived limitation as a particular issue for schools. However, with proper design and detailing, wood schools can match the durability performance of schools made from any other material.

In the context of durability, there are two main concerns: areas of high traffic and high moisture.

In high-traffic areas, the structural material doesn’t tend to be at risk unless the structure is also the finish material, as it often is with a CLT or other mass timber school. Common options for avoiding damage include high-durability finishes, such as hard tile, medium-density fiberboard, impact-resistant gypsum, and vinyl wall coverings. To make these finishes cost-effective, they are often added just to the lower portion of the wall (e.g., the bottom 6 feet) where the most wear and tear can be expected.

In high-moisture areas such as bathrooms and labs, it is useful to both use durable finish materials and elevate the wall structure and finishes off the floor by installing a curb below the walls. See the WoodWorks publication Wood-Frame Schools: Durability Techniques for Interior High Traffic and Moisture Areas https://www.woodworks.org/wp-content/uploads/Wood-Frame-Schools-Durability-Techniques-WoodWorks.pdf.

For information on durability detailing related to the building envelope, including moisture, fungi, and termite control, the Architectural Record CEU, “Designing for Durability,” is available at www.ThinkWood.com.