Length effects in 38 mm spruce–pine–fir dimension lumber

This paper deals with some of the problems arising when the strength of a structural member varies along its length such as occurs in timber members where knots and other natural growth characteristics create cross sections with varying strengths along the length of the member. Failure may then take place wherever a weak cross section happens to be subjected to high stresses. A different concept of strength must be introduced into the design codes in order to reflect the probability of such an occurrence. Strength becomes a function of length, depth, and load configuration, all of which must be considered in the design process if both economical and safe structures are to emerge. In this paper, which was directed toward establishing the length effects for 38 mm thick spruce–pine–fir in tension and compression as well as bending, the results of a testing program are described. It was, for instance, found that when the length of tension members is doubled from 3.0 to 6.0 m, the strength reduces to 86% of the previous strength, or if the length is quadrupled (as may occur in the bottom chords of trusses), the strength could reduce to 73% of the original strength. A review of the available research information is presented and evaluated in order to assess the adequacy of the available size effect information for design purposes. The present size effect requirements incorporated in some existing timber codes are also reviewed. Key words: timber, strength, size effects, length effect, weakest link theory, design method, load configuration, bending, compression, tension.