The 2006 International Building Code (IBC) permits three methods for the design of wood-frame structures. The three methods are allowable stress design under Section 2306, load and resistance factor design under Section 2307, and conventional light-frame construction under Section 2308. The terms "allowable stress design" and "load and resistance factor design" are defined in Section 1602. In addition, there are four other code sections that apply to wood-frame structures irrespective of the design method used: Section 2301 General, Section 2302 Definitions, Section 2303 Minimum Standards and Quality, and Section 2304 General Construction Requirements. Section 2305 General Design Requirements for Lateral-Force-Resisting Systems applies to engineered lateral-force resisting systems used in wood-frame structures. There are two alternate referenced standards that are permitted to be used in lieu of the requirements in Section 2305 General Design Requirements for Lateral-Force-Resisting Systems and Section 2308 Conventional Light-Frame Construction that are discussed below.

Allowable stress design (ASD)—The 2006 IBC references the 2005 edition of the National Design Specification (NDS) for Wood Construction with 2005 Supplement for ASD of wood structures. Section 1605 of the IBC and Chapters 2 and 12 of the standard, Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-05) include ASD load combinations to be used for the design of structural elements and connections.

Load and resistance factor design (LRFD)—The 2006 IBC references the 2005 edition of the NDS for LRFD of wood structures. The 2005 NDS is a dual-format standard that permits both ASD and LRFD procedures to be used. Appendix N of the NDS contains provisions for converting the reference design values in the NDS to the LRFD format. Section 1605 of the IBC and Chapters 2 and 12 of the ASCE/SEI 7-05 standard include LRFD load combinations to be used for the design of structural elements and connections.

Conventional light-frame construction—The 2006 IBC also contains prescriptive conventional construction provisions for wood light-frame structures. The word "prescription" means "the action of laying down authoritative rules or directions;" the word "prescriptive" means "acquired by, founded on, or determined by prescription or by long-standing custom." Together these definitions describe the nature of conventional wood-frame construction provisions in the IBC: they are a set of rules based on long-standing custom. IBC Section 2308.1 states, "The requirements of this section are intended for conventional light-frame construction. Other methods are permitted to be used provided a satisfactory design is submitted showing compliance with other provisions of this code." The conventional construction provisions of the IBC are intended to permit wood-frame structures to be built without requiring a registered design professional (architect or engineer) provided all restrictions, limitations, and requirements (rules) are strictly adhered to. The limitations for conventional construction are prescribed in Section 2308.2 and are generally related to building height, loads, spans, material weights, wind speed, occupancy category, and seismic design category. Section 2308 specifically allows for "partially engineered structures" under Section 2308.1.1 Portions Exceeding Limitations of Conventional Construction and Section 2308.4 Design of Elements. It is also important to note that the 2006 IBC references the 2001 edition of the Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings published by the American Forest & Paper Association (AF&PA) as an alternate to the conventional wood frame construction provisions of Section 2308.*

General design requirements for lateral-force-resisting systems—Section 2305 contains requirements for engineered design of lateral-force-resisting systems used in wood-frame structures. The provisions cover specific requirements for design of diaphragms, shear walls, chords, collectors, and anchorages of wood-frame structural systems resisting lateral load effects from wind pressure and seismic ground motion. It is also important to note that the 2006 IBC references the 2005 edition of the standard, Special Design Provisions for Wind and Seismic, published by AF&PA as an alternate to requirements for design of wood-frame lateral-force-resisting systems covered by Section 2305.

Log structures—In addition to design methods for light wood-frame construction, the 2007 IBC Supplement contains provisions for design and construction of log structures. Section 2303.1.10 of the 2006 IBC contains provisions for stress-grading of structural log members of nonrectangular shape that are typically used in log buildings. The section references ASTM D3957 Standard Practices for Establishing Stress Grades for Structural Members Used in Log Buildings, and requires structural log members to be identified by the grade mark of an approved lumber grading or inspection agency, or a certificate of inspection as to species and grade. Section 2301.2 of the 2007 IBC Supplement references ICC-400 Standard on Design and Construction of Log Structures. Click here to see flow chart for 2006 IBC wood frame building design methods.

* IBC Section 1609.1.1 Determination of Wind Loads also references AF&PA’s Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings for determination of wind loads on residential structures.

Disclaimer: The building official has the final authority in rendering interpretations of the code as prescribed in IBC Section 104.1. Opinions by International Code Council staff are based solely on the published model code and do not consider federal, state, or local amendments, ordinances, or policies.

John R. Henry, P.E., is the principal staff engineer at the International Code Council’s Los Angeles office. He can be reached at jhenry@iccsafe.org.