Lumber is a general term which includes boards, dimension lumber, and timber. The product is manufactured by sawing logs into rough size lumber or cants (square timbers) which are edged, resawn to final dimension and cut to length.
This section deals with lumber products used for structural framing. In the context of North American construction materials, it usually refers to wood originating from softwood species of trees. In the smaller sizes it is known as “dimension lumber,” and in the larger sizes as “timbers.”
For many years, the design values of Canadian dimension lumber were determined by testing small clear samples and applying appropriate adjustments depending on grade, size, moisture and safety factors. Although this approach has worked well in the past, there were some indications that it did not always provide an accurate reflection of how a full sized member would behave in service.
Beginning in the 1970s, new data was gathered on full-size graded lumber. In the early 1980s, the wood industries in Canada and in the United States embarked on a joint comprehensive testing program which involved testing thousands of pieces of dimension lumber to destruction to determine their in-service characteristics. It was agreed that this testing program should simulate, as closely as possible, the structural end use conditions to which the lumber would be subjected to. This is called in-grade testing.
The in-grade design values resulted in new relationships between species, sizes and grades. The change reflects the importance of wood characteristics in predicting strength properties. The presence and location of characteristics such as knots influence wood strength.
This is also shown in the size factor for lumber. There are significant differences in strength properties between 2 by 4 and 2 by 12 lumber. The difference relates to the size of characteristics permitted in for each lumber size as established by grading rules. Full size testing has resulted in size factors for each size of lumber.
Design values for different grades have changed. Based on the two major testing programs, researchers have concluded that no consistent difference could be found between No. 1 and No. 2 grades for Canadian dimension lumber. As a result the same design value is applied for both grades No.1 and No. 2. Select Structural grade, which is sometimes available, has higher design values.
On a species level, the effect of the in-grade testing raised S-P-F and Hem-Fir values while Douglas Fir-Larch values decreased slightly, when compared to design values based on previous small clear testing.
The data which resulted from the in-grade testing programs has been used to update the design values which are applied in Canada and the United States.
For more information on the Lumber Properties program please refer to the Canadian Lumber Properties book.
U.S. Design Value acceptance
Following the testing program, a new ASTM Standard D1990 (“Establishing Allowable Properties for Visually Graded Dimension Lumber from In-Grade Tests of Full-Sized Specimens”) was approved. Data for bending, tension parallel to grain, compression parallel to grain, and modulus of elasticity were analyzed in accordance with this Standard.
The Canadian lumber design values based on this data are published in the “National Design Specification for Wood Construction” (NDS) ANSI Standard and are also published in the National Lumber Grades Authority’s “NLGA Standard Grading Rules for Canadian Lumber”. These values have been approved by the American Lumber Standards (ALS) Board of Review. Design values published in the NDS are adopted in all U.S. model building codes including ICBO, BOCA and NCSBCS.
Canadian Design Value Acceptance
The Canadian Design values for wood were developed in a limit states design format. The CSA O86 Technical Committee on Engineering Design in Wood approved these design values in 1989. They were generated with a reliability-based design approach that incorporated the in-grade testing data to generate specified strengths. The procedures are described in Canadian Lumber Properties.