Cost optimized Timber Machine Strength Grading

Authors

Markus Sandomeer, René Steiger and Jochen Köhler

Abstract

These days there are many discussions about applicability and efficiency of the existing grading procedures for machine strength grading of structural timber given in the European Standard EN 14081. Different alternative grading strategies have been proposed, all of them dealing with statistical frameworks to control timber grading procedures in a way that the structural material properties of the graded timber fulfil the strength class requirements of EN 338. However, efficient machine strength grading of structural timber also has to take into account economic aspects.
This paper shows how an optimal (in terms of monetary benefit) set of timber grades can be identified by solving an optimisation problem. The objective function of the optimization problem is defined based on the outcomes of a probabilistic grading strategy which was already proposed by the authors at earlier meetings of COST E53 (Sandomeer et al. 2008). The identified timber grades can be described by means of the probabilistic characteristics of the relevant material properties (bending strength and MOE as well as density). The constraints to the optimization problem, in terms of the requirements for timber strength classes according to EN 338 are put directly into the objective function. In order to be able to solve the optimization problem, the cost of the control and the benefit of fulfilling the required characteristic values
belonging to different grades have to be specified first. It will be demonstrated how an optimal set of grading acceptance criteria for a specific grading procedure can be found by applying a predefined optimization function. A number of different grades according to EN 338 are selected as possible grades and a reject domain is defined for timber, which does not have to fulfil any requirements. Results are shown together with example values for the monetary benefit and the requirements in terms of material properties. In practice the benefit associated with timber of a particular grade would depend on several factors such as the size of the individual timber member, the total amount of available timber for a given grade, the production capacity of a given sawmill, the available grading machines and not least the market price for timber of the different strength classes. The implementation of the proposed approach in practice would have to incorporate these and other factors into the formulation of the benefit function. Different ways of how this could be done are discussed in the paper.

Published in/by

Proceedings, COST E53 conference "Economical and Technical Aspects of Quality Control of Wood and Wood Products", October 22-23, 2009, Lisbon, Portugal