STUDY ON STRENGTH OF GLUED LAMINATED TIMBER BEAMS WITH ROUND HOLES

Estimation of splitting strength of glued laminated timber composed of heterogeneous grade by finite element analysis

Abstract

 To economically design a building, the beams used may contain holes for placing piping. For steel structures and reinforced concrete structures, methods for evaluating the strength of beams with round holes have been established. However, for timber structures in Japan, a method for evaluating the strengths of beams with round holes has not yet been established.

 Several strength evaluation methods have been proposed for the standards of overseas structural design in Japanese research papers. These methods calculate the beam strengths when a hole is split by a tensile force that is perpendicular to the grain. It is possible to determine the strengths of beams with round holes using the proposed strength evaluation method. However, the proposed strength evaluation method has only been verified for homogeneous timber and it is not clear whether this method can be applied to glulams that are glued and laminated timbers composed of heterogeneous grade materials and mainly used for beams in Japan.

 The purpose of this study is to experimentally verify that the proposed strength evaluation method can be applied to glulams. An additional objective is to verify that the strengths of these glued and laminated timbers can be estimated by finite element analysis (FEA).

 Chapter 1 introduces the proposed strength evaluation method. Chapter 2 introduces the shear bending experiments performed on glulams with round holes. Chapter 3 introduces the element experiments for examining shear and tension strength parallel to the grain, tension strength perpendicular to the grain, and mode I fracture energy. Chapter 4 presents the results of verification that the proposed capacity evaluation method is not applicable to glulams. Chapter 5 proposes a method for estimating the strength of glulams with round holes by FEA and shows the verification results.

 The results of this study are as follows.

 1) It was demonstrated that the strength evaluation method proposed for glulams cannot be applied as it currently exists.

 2) The stress distributions around the round holes were obtained by FEA and the method of estimating the splitting strengths using the mean stresses acting across a potential fracture area shown in Reference 11 was verified. As a result, it was found that the splitting strengths were accurately estimated when the round holes were small or large.

 3) By using Equation (5) for correcting ams and Equation (6) for considering the size effect in the mean stress method, it was shown that the splitting strengths can be accurately estimated even when the round holes are small or large.