Effect of Spiral Grain Occurrence on Strength Properties of Pinus Patula Grown in Kenya

Abstract

Spiral grain occurrence in timber is an important wood quality characteristic. High spirality in timber beyond permissible limits reduces wood strength and causes distortion. It is also time consuming during straightening and fastening the material. Spiral grain occurrence should therefore be minimized in wood to allowable specifications. It is for these reasons that this study was undertaken to determine allowable spiral grain in Pinus patula logs, its relationship to slope of grain in sawnwood and the effect of the grain angles on strength properties. Spiral grain angles were taken on standing trees from Muguga plantations in Central Kenya at Diameter at Breast Height (DBH) using a hammer, hollow metal tube, pencil and protractor. The trees were then felled and grain angles taken along their merchantable length. Using a grain detector the corresponding slopes of grain were determined after the logs were converted into sawnwood. Results showed that spiral grain angles are directly proportional to slope of grain of sawnwood. Special Structural (SS), General Structural (GS) and Reject grade of sawn timber corresponded with grain angles not exceeding 2.5o, between 2.5 – 5o and not less than 5o respectively. There were also significant variations in spiral grain angles between and within trees. Trees with grain angles between 0o and 5o were categorized as good seed sources. Spiral grain seemed to reduce bending strength and stiffness of construction timber. Bending strength was reduced by almost 40% at 1:10 (SS grade) and 32% at 1:6 (GS grade) spirality. The reduction factor due to spiral grain on strength is highest between SS grade and Reject grade. It is recommended that selection of seed sources should consider low occurrence of spiral grain to ensure compliance to timber specifications for construction purposes.