Original paper(Vol.48 No.9 pp.1011)

Estimation of bending strength of structural glued laminated timber composed of homogeneous-grade finger-jointed lumbers of Japanese larch

Takeda Takashi; Hashizume Takeo

Abstract:It is known that bending strength (MOR) of a glued laminated beam (glulam) is decreasing when the depth of the glulam is increasing. The first study on this phenomenon in which the Weibull brittle fracture theory was applied to wood was reported by Bohannan (1966). He studied clear wood beams and found that for geometrically similar beams MOR was proportional to the depth of the beam to the power 1/9 (B-model). On the other hand, Komatsu (1997) proposed the equation for predicting MOR of glulams composed arbitrary laminae without using the so-called 'size-effect factor' (K-model). We attempted to compare these two models to the experimental results of Japanese larch gulams as reported by us. The types of used glulams were horizontally laminated timbers (H-type) and vertically laminated timbers (V-type).
First, we compared two models to the experimental data of H-type. In higher grade, the estimates by K-model had good agreements with the data, but not in lower grade. Then, the estimates by using size-effect factor obtained from strength distribution of finger-jointed and without-joint laminae agreed with the data in lower grade. This differences between higher and lower grades may be caused by the quantity of defects excluding finger-joint. For V-type, the estimates by the 'average model' proposed by Hayashi (1992) agreed with the data in both grades.

Key Words:modulus of rapture, size-effect factor, horizontally laminated glulam, vertically laminated timber