Even so, it should be noted that this information is the product of decades of research in the development of LRFD methods for wood.Īs an example, consider a simple (single) 2×8 purlin. However, NDS 2012 is still structured in a manner reflecting ASD theory the listed design values (e.g., F b, F c, F v, etc.) all reflect allowable stresses. NDS 2012 lists the LRFD adjustment factors (K F), resistance factors ( ) and time effect factors ( ) in both the body of the code and the appendices. At nearly double the size of earlier editions, this text contains side-by-side instruction, examples, and theory of both ASD and LRFD methods for wood design. The most current (sixth) edition has been re-titled Design of Wood Structures ASD/LRFD. Breyer’s Design of Wood Structures has been a premier text for wood design for many years. Times are changing, and little by little the LRFD approach is becoming more mainstream for wood design. On the other hand, NDS 2012 has made a major LRFD leap by placing the LRFD adjustment factors in Chapter 2 (page 12). Aside from this, NDS 2005 does little to embrace the LRFD methodology.

Elsewhere, references to LRFD can be found in the “Applicability of Adjustment Factors…” within the main body of the code (e.g., Table 4.3.1). Interestingly enough, the LRFD adjustment factors are listed as the last tables in the last appendix of NDS 2005. The 2005 version of the National Design Specification ® (NDS ®) for Wood Construction lists adjustment factors and other values enabling the use of LRFD with the strength design load combinations of ASCE 7. The adoption of LRFD methods for wood has a history not unlike its concrete, masonry and steel counterparts. However, provisions for LRFD design in wood have gradually become more predominant, and it may be only a matter of time before ASD methods are relegated to appendices while LRFD becomes the primary basis of design. Textbooks, codes and even the design values listed in technical catalogs of proprietary fasteners and hardware reflect the ASD approach. ASD has been the basis for engineering wood systems for decades. While allowable stress design (ASD) methods are still acceptable, the perception is that the majority of engineers use LRFD for concrete, masonry and steel systems. In recent years, the push to LRFD has been embraced by many for the design of steel and masonry systems.

For many, working stress design for concrete is a totally foreign concept, while LRFD ‘strength’ design is what you probably learned in school and practice to this day. If your office and its culture are consistent with most structural design firms, you probably embraced the load and resistance factor design (LRFD) approach for reinforced concrete years or even decades ago.

Lrfd beam design