We recently posted a blog here that asked the question: What Do Most Engineers Use for Masonry Design: ASD or SD? We thought this same question should be asked for wood design and we asked the opinion of Philip Line, American Wood Council’s Director of Structural Engineering. Here is what he said:

During a February 2014 web seminar on the 2012 National Design Specification for Wood Construction® (NDS®), I asked the audience of design professionals the following question:

“Do you predominantly use ASD or LRFD provisions of NDS?” The response indicated 100% use of ASD as the predominant design method for wood. While this unofficial poll shows heavy use of ASD, it is known that LRFD is used, and is sometimes the only permitted option. For example, ASCE 41-13 Seismic Evaluation and Retrofit of Existing Buildings permits determination of wood default properties in accordance with the LRFD provisions of NDS, not ASD.

The NDS has been a dual format specification, addressing both ASD and LRFD, since the 2005 edition and is envisioned to remain a dual format specification for the foreseeable future. Maintaining both design formats in the NDS provides design options while also ensuring consistency of basic design criteria regardless of whether ASD or LRFD is used.

Designers considering LRFD will find that member sizes determined for common load cases are often similar between ASD and LRFD. When there are differences, the more efficient design may be associated with ASD for some cases and LRFD in other cases (i.e. LRFD will not always produce more efficient designs). This similarity in designs is due in part to LRFD provisions being based on consideration of performance established by ASD and also due in part to discrete member sizes in wood. For example, although slight differences may result between ASD and LRFD, the difference may not warrant an increase or decrease in the number of framing members or a change to the next larger or smaller member size.

Within the AWC Wood Design Standards Committee, both formats are considered necessary. ASD is the “reference” basis of design values and proprietary connectors and familiar to most practicing engineers. LRFD is what young engineers are learning as a more rational design method and in some cases will provide more efficient designs or may be the only option permitted.