Douglas Fir - Larch, Allowable Wood Stresses

The material properties shown below are extracted from the National Design Specification (NDS) Supplement - Design Values For Wood Construction, published by the American Forest & Paper Association, Inc.  Similar spreadsheets can be created for the 25 other species of lumber using the Base Design Values beginning on page 31.  You can obtain a copy of the NDS by following this link: to the American Wood Council's website.  Look for the Allowable Stress Design (ASD) Manual For Engineered Wood Construction.

For those of you newly interested in wood design, you'll note two patterns in the following spreadsheet:

  • The grades of wood are listed in descending order. Select structural wood is stronger than #1 and Better wood, etc.

  •  Member unit strength decreases as member depth increases.

For now, assume that smaller members are likely to be free of knots and other wood grain defects whereas larger members will incorporate some number of wood grain defects.  It is reasonable therefore to reduce the allowable unit stresses in larger members to account for the presence of such defects.  Please refer to the American Institute of Timber Construction's Timber Construction Manual for a discussion of lumber grading.  A link to their site is provided below (look under "Publications", then "Other Publications"):

This spreadsheet provides allowable working stresses for lumber used in dry conditions.  No increase has been included for repetitive member use.  All members are oriented with their wide faces parallel with the direction of imposed loads (not used as planks).  As a design reference, printed pages like these are usually inserted before the calculations related to roof and/or floor framing.

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