A.K. AGGARWAL Associate Profcssor Department of Cívil Engineering PNG Universíty of Technology LAE, Papua New Guinea S. AGGARWAL Lecturer Department of Forestry PNG University of Technology LAE, Papua New Guinea
ABSTRACT
Papua New Guínea (PNG) has the second largest forest area in the West Pacific, and at the same time a very large number of timber species, which make their stress grading a tedious and a time consuming process. Up until now, some of the major and commonly available species have been graded into groups, depending on their strength properties, while the lesser known species are locally consumed, as their strength characteristics have not yet been determined. For structural purposes, classification of PNG timber has been done both in its natural form (round) and in its swan and dressed condition. The information on various strength groups and basic working stresses of different species has been compiled and is available in the draft PNG Timbers Structures Code - a document which is likely to be published soon by the National Institute of Standards and Industrial Technology. According to the draft Standard, PNG timber are classified into seven strength groups - S 1 to S7, when they are unseasoned (green), and into eight groups - SD 1 to SD8, when they are in dry (seasoned) condition. This classification is based on some basic mechanical tests and on air-dry density of timber measured at 12% moisture content. For structural design, the Standard recommends grading both by visual and mechanical methods. In the visual category, three grades viz. `select structure, `standard structure' and `common building' have been specified and grading for each is carried out according to the nature of defects - such as size and number of knots, stope of grain, shake and splits. The `select structure' grade has the highest stress grade rating while the `common building' grade is placed two stress grades iower on a mechanical stress grading scale. The mechanical grading system classifies sawn structural timber into 12 stress grades ranging from F2 to F34 depending on their bending, tensile, compressive and shear strength properties. A Table giving basic working stresses for different stress grades has been provided in the Standard and ü is based on the tests carried out on clear wood specimens i.e. timber which are free from all kinds of defects. For poles and logs (round timber) strength groups of unseasoned timber (S1-S7) have been equated to a stress grades and while assigning stress grades to logs, it is assumed that they are obtained from mature trees grown in forests and not in plantations.
Publication Source: Proceedings of the 12th International Symposium on Nondestructive Testing of Wood University of Western Hungary, Sopron, 13-15 September 2000, ISBN 963 7180 88 5 Publisher: University of Western Hungary, H 9400 Sopron, P.O.Box 132, FAX: +36 99 311 103
