{"id":4204,"date":"2014-06-24T12:31:29","date_gmt":"2014-06-24T16:31:29","guid":{"rendered":"http:\/\/www.constructionspecifier.com\/?p=4204"},"modified":"2017-01-13T15:27:39","modified_gmt":"2017-01-13T20:27:39","slug":"design-of-fire-resistive-exposed-wood-members","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/design-of-fire-resistive-exposed-wood-members\/","title":{"rendered":"Design of fire-resistive exposed wood members"},"content":{"rendered":"
\"Photo<\/a>
Photo courtesy Structurlam<\/em><\/figcaption><\/figure>\n

by Bradford Douglas, PE, and Jason Smart, PE
\n<\/strong>Designing for life safety is complex and multifaceted, and fire-related issues comprise a large portion of model codes. For nearly 15 years, a mechanics-based design method has been used in the United States to estimate the capacity of exposed wood members using basic wood engineering mechanics.<\/p>\n

This mechanics-based design method first appeared in the 1999 version of Technical Report (TR) 10, Calculating the Fire Resistance of Exposed Wood Members <\/em>(TR 10). This method was then incorporated into new design procedures of the 2001 National Design Specification<\/em> (NDS<\/em>) for Wood Construction<\/em>. These procedures were later adopted into the model building codes through reference to the NDS<\/em> for calculating fire resistance of wood members. The procedures have been used extensively for design of large, exposed wood members, but are now also beginning to be employed for estimating structural fire resistance of smaller exposed wood members.<\/p>\n

\"Excerpt<\/a>
Excerpt from Design for Code Acceptance (DCA) 2, Design of Fire-resistive Exposed Wood Members<\/em>, Table 1 (one-hr). This shows tabulated one-hour design load ratios, Rs, for flexural members exposed on three sides.<\/span> Data courtesy American Wood Council<\/em><\/figcaption><\/figure>Wood buildings can be designed to meet rigorous standards for performance, which is why the International Building Code<\/em> (IBC<\/em>) allows the material\u2019s use in a wide range of building types\u2014including structures that are taller and have more area than some designers realize. Table 601 of the IBC<\/em> shows the required fire resistance of building elements (i.e.<\/em> structural frame, walls, floors, and roofs) for each construction type. Ratings are given in hours. The exception is Type IV, where the wood structural elements are assumed to have inherent fire resistance due to their required minimum dimensions\u2014in other words, no fire resistance rating is necessary except for exterior walls.<\/p>\n

Fire resistance is a measure of the length of time an assembly or structural element can sustain a given load when subjected to a standardized fire exposure condition. Fire resistance of wood members and assemblies may be established by any one of five means listed in IBC<\/em> Section 703.3, \u201cAlternative methods for determining fire resistance.\u201d The most common methods are tested assemblies and calculated fire resistance.<\/p>\n

\"Excerpt<\/a>
Excerpt from DCA 2, Table 2 (two-hr), showing tabulated two-hour design load ratios, Rs<\/em>, for compression members exposed on four sides.<\/span><\/figcaption><\/figure>\n

Fire design of exposed wood members<\/strong>
\nThe fire resistance of exposed wood members, including lumber, glued-laminated (glulam) timber, and structural composite lumber (SCL), may be calculated using provisions of NDS<\/em>. This allowable stress design approach is referenced in IBC<\/em> Section 722, \u201cCalculated Fire Resistance.\u201d The design procedure allows calculation of the capacity of exposed wood members using basic engineering mechanics.<\/p>\n

Actual mechanical and physical properties of the wood are used, with member capacity directly calculated for a given period\u2014up to two hours. Section properties are computed assuming an effective char rate (i.e.<\/em> ?eff<\/sub><\/em>) at a given time (i.e.<\/em> t)<\/em>. Reductions of strength and stiffness of wood directly adjacent to the char layer are addressed by accelerating the char rate by 20 percent. Average member strength properties are approximated from existing accepted procedures used to calculate design properties. Finally, wood members are designed using accepted engineering procedures found in NDS<\/em> for allowable stress design. (The design procedures presented in NDS<\/em> Chapter 16 are not intended to be used for the design and retrofit of structures after a fire.)<\/p>\n

For sawn lumber, glulam, and SCL, the nominal char rate (i.e.<\/em> ?n<\/sub>)<\/em> is typically assumed to be 38 mm\/hour (1.5 in.\/hour). For a given time in hours, the effective char rate is then:<\/p>\n

?_(eff=) 1.8\/t^0.187 in.\/hour <\/span><\/p>\n

To calculate section properties of wood members, the effective char layer thickness (i.e.<\/em> aeff<\/sub>)<\/em>, for structural calculations is computed as:<\/p>\n

a_(eff=) 1.8t^0.813 in. <\/span><\/p>\n

\"This<\/a>
This photo showcases glued-laminated lumber (glulam). <\/span>Photos courtesy Structurlam<\/em><\/figcaption><\/figure>\n

The 2012 and earlier versions of IBC <\/em>have also contained an empirical calculation method for estimating the structural fire resistance of wood beams and columns exposed to a standard fire exposure for up to one hour. However, this empirical method has been removed from the 2015 IBC<\/em> in favor of provisions contained within NDS<\/em> Chapter 16 that are much broader in application and leave less room for design error.<\/p>\n

Basis for NDS<\/em> chapter 16 approach<\/strong>
\nAWC\u2019s TR 10 contains full details of the NDS<\/em> method as well as design examples.1<\/sup> TR 10 was recently revised to incorporate a new section that supports the use of the design method with smaller dimension sizes associated with lumber joist floor assemblies. It also has revised design examples to match the 2012 NDS<\/em>.<\/p>\n

The revised design tables in Appendix A allow more accurate calculation of fire resistance of columns with any slenderness ratio. They also eliminate tabulation of special cases that can be misapplied, such as:<\/p>\n