Grading timber and glued structural members

Abstract

Because of its biological nature, which is influenced by many factors as discussed in Chapter 6, the quality of timber is enormously variable. Therefore, some sort of arrangement or classification must be undertaken prior to its use, in order to get the most out of this valuable resource. It is obvious that not all lumber can be used for the same purposes because not all of it has the same properties. It is the intension of this chapter to discuss the various classification or grading techniques that are applied to timber and glued structural members. A grading rule is a set of definitions of timber characteristics together with methods for measuring them. Grading rules determine, in an orderly manner, the way in which a given piece of timber from a certain species or group of species will be designated and located into any of a number of groups or categories. Grading of timber has evolved into two major categories: material graded for structural use and material graded for appearance. Strength and stiffness of timber are primary considerations for structural use, whereas appearance grades are dictated by the size of the largest clear cutting. Different grading rules apply to these two situations. Both categories will be discussed, but the majority of this chapter will be concerned with the grading rules appropriate for timber as a material for structural uses. Also, in this chapter a distinction is made between the properties of wood, that is, clear, defect-free material, and timber (called lumber in the U.S.) with all its natural defects of knots, splits, cross-grain, and distortion. The grading of timber should be viewed as part of a marketing strategy, designed to ensure that timber buyers obtain the quality of timber appropriate for their needs and timber sellers receive an optimal price for their product. Unfortunately, grading suffers from conflicting objectives and can be best described as an attempt to bring some order out of what would otherwise be a chaotic situation. Timber, being a natural material, is very variable in strength and appearance. This is compounded by the enormous number of commercial species and by the multiplicity of grading rules that evolved in isolation to take account of the vagaries of each species. There is strong historical justification for such practice. Heart rot and brittleheart may be particular problems with certain over-mature tropical hardwoods, while corewood is encountered in softwood plantations. In theory, rationalization of grading rules ought to be simple. However, rationalization will never be easy as timber grading can be a powerful tool in non-tariff protection of local interests. Naturally local grading rules are written with local timber in mind. It would be unrealistic to disadvantage home-grown material. In recent years, more objective and rational procedures for the grading and property assignment of timber have been developed, driven by increased international trade. Unfortunately, there has been only limited success to date in adopting these procedures.