Fire-Rated Wood ConstructionSep 21 2016 · 0 comments · NISHKIAN DEAN, Wood Frame ·0
Calculated Fire Resistance
Edwin T. Dean, PE, SE
Wood frame is an economical construction type and if properly detailed durable and fire-safe. The level of fire resistance required of a building is established by the building code and is a function of the size, use and occupancy of the building. The fire rating is driven by the need to provide ample time for occupants to exit the facility, retain structural stability long enough for fire-fighting personnel to combat the fire and for the protection of the contents of the building and adjacent structures.
There are several recognized methods of providing fire resistance using wood frame construction. First of all, timber members can be protected from fire exposure by enclosing them within a fire-resistant assembly. There are many types of wall, floor, floor-ceiling and roof-ceiling assemblies available for this method. Secondly, fire resistance can be provided by the use of fire-retardant-treated (FRT) wood in wall and roof assemblies. And finally by using the inherent fire durability of large exposed wood members. There are two code-approved methods of achieving a level of fire protection for timber structures with exposed members. Heavy Timber Construction, the traditional method, is included in the codes based on a long history of satisfactory performance; however, is limited in applicability to roof and column construction. The newer method is based on calculating the capacity of timber members exposed to fire and can be applied to all members of the structural frame. The use of calculated fire resistance for wood construction has gained prominence in recent years due to the increasing interest in mass-timber construction where the wood is left exposed as part of the architectural character of a building, yet falls outside of the criteria for the prescriptive Heavy Timber Construction.
Calculated Fire Resistance
Calculated fire resistance as a design method first appeared in the 1999 version of Technical Report No. 10, Calculating the Fire Resistance of Exposed Wood Members (TR 10). This method was then incorporated into new design procedures of the 2001 National Design Specification (NDS) for Wood Construction. These procedures were later adopted into the model building codes through reference to the NDS for calculating fire resistance of wood members. The fire resistance of exposed wood members, including lumber, glued-laminated (glulam) timber, and structural composite lumber (SCL), may be calculated using provisions of NDS. This allowable stress design approach is referenced in IBC Section 722, “Calculated Fire Resistance.” The design procedure allows calculation of the capacity of exposed wood members using basic engineering principals, validated by testing for fire resistance of up to 2 hours.
The inherent fire resistance of wood comes from the charring that occurs on the wood that then acts as an insulator to slow the wood from further deterioration. The basic char rate is 1.5” per hour of resistance. The capacity of the member remaining, after accounting for the charring, is used to establish its acceptability to support the required design loads during and after the fire. For example, a 6 ¾” wide x 15” deep glulam floor beam trying to achieve 1 hour fire rating would have to be designed as 3 ¾” wide x 13 ½” deep glulam to account for 1 ½” reduction due to charring on each exposed side (assuming two sides and the bottom are exposed). Also, for glulam beams to meet the 1 hour fire rating, the bottom of the beam needs to include an additional tension lamination (since the standard tension lamination is 1 ½” thick). Member connections need to be similarly covered by wood or other fire proofing, to protect them for fire exposure and provide the required fire resistance.
Exposed wood frame or timber construction has a particular appeal and is particularly popular in the development of “creative work space” that is popular now. Architects, engineers and building and fire officials throughout the country recognize the fire resistance demonstrated by structural wood beams and columns in actual fires. Structural fire design provisions have been incorporated in Chapter 16 of the NDS, which is referenced in Section 722.1 of the 2012 IBC as a method of calculating fire resistance of exposed wood members. Contact the Nishkian firms to discuss the applicability of fire-rated wood construction.
Edwin T. Dean, PE, SE is Vice President and Managing Principal of Nishkian Dean a structural engineering consulting firm in Portland, Oregon.
- Technical Report No. 10, Calculating the Fire Resistance of Exposed Wood Members, American Wood Council, 2016, is available at www.awc.org/pdf/TR10.pdf.
- 2012 International Building, International Code Council, 10th printing October 2015, is available at http://codes.iccsafe.org/app/book/toc/2012/2012%20Houston/IBC/index.html
- ANSI/AWC NDS-2012, National Design Specification (NDS) for Wood Construction (Chapter 16, Fire Design of Wood Members), 3rd printing February 2015, is available online at, http://www.awc.org/codes-standards/publications/nds-2012