Section 2308 of the 2006 International Building Code (IBC) covers conventional construction provisions for light wood-frame buildings. Section 2308.1.1 states, that portions of a building of otherwise conventional construction that exceed the limits of conventional construction must be designed in accordance with the engineering provisions of the code. Section 2308.4 permits combining engineered elements with conventionally specified elements subject to the limitations prescribed in Sections 2308.4.1 and 2308.4.2.

Answers to FAQs:

Q: It is my understanding that wood-frame buildings that conform to all of the requirements of Section 2308 do not require engineering but are deemed to comply with the engineering requirements covered elsewhere in the code. Does the code offer any guidance on how the engineer should combine prescriptive conventional and engineered systems in one building?

A: The intent of Sections 2308.1.1 and 2308.4 is to not require complete engineering for wood-frame buildings that are generally conventional, but may have portions or structural elements that do not meet the requirements prescribed in Section 2308. The term "portions" refers to parts of buildings containing volume and area, such as a room or a series of rooms. In addition, Section 2308.4 permits combining engineered and conventionally specified elements or systems subject to the limits prescribed in Sections 2308.4.1 for elements that exceed the limitations for conventional construction, and Section 2308.4.2 for structural elements or systems that are not specifically covered in the conventional construction provisions of the code. This means wood-frame buildings can be partly conventional and partly engineered. The code offers some guidance in Section 2308.4.2, which states that the design of structural elements or systems must be compatible with the performance of the conventionally framed system.

In practice, this can present challenges to the engineer who is legally responsible for the engineered portions of the building. Some engineers may simply opt to provide complete engineering rather than do limited engineering for portions of a building. This decision is up to the engineer, and it is up to the client whether or not to have the engineer provide complete engineering. In cases where the engineer agrees to provide partial engineering, careful consideration must be given to the compatibility and effects the engineering system has on the conventionally framed portions of the building. The engineer should clearly define the scope of work so the client understands that the engineer is providing partial engineering. The engineer must decide what portions or elements require engineering and what portions comply with conventional provisions. Although the engineer is only required to provide engineering for those portions and elements that are not in conformance with Section 2308, he or she should be thoroughly familiar with the entire structural system and understand how the building will function as a whole. To do this requires a reasonably thorough understanding of both the wood-frame engineering provisions as well as the prescriptive conventional construction provisions outlined in Section 2308.

Plans for a partially engineered wood-frame building should clearly identify what portions and elements are engineered and what parts are conventionally framed, so that the contractor and building inspector can readily distinguish between the two. The level of shear transfer and continuous load path detailing required for the engineered portions is likely to be more extensive. Resolving discrepancies in the engineered systems will always involve the engineer, whereas resolving discrepancies in the conventional portions of the building may or may not. The specific construction documents that are required to be stamped or signed by the engineer are generally regulated by the applicable laws of the jurisdiction. See Sections 106 and 1603 for the general code requirements for construction documents.

Although a partially engineered structure may be more challenging than a completely engineered structure, sound judgment and a thorough understanding of the principles of behavior and performance of wood-frame structures can be used to design wood-frame buildings that combine prescriptive and engineering systems into one building and still maintain the level of structural safety intended by the code.

It should also be noted that Section 2308.1 permits the use of the American Forest & Paper Association’s Wood Frame Construction Manual for One- and Two-family Dwellings as an alternate to Section 2308, subject to the limitations therein and the limitations of the code.

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.