A STUDY ON BUCKLING STRENGTH OF TIMBER MATERIALS (PART 1): EVALUATION OF ELASTIC BUCKLING STRENGTH

Abstract

 Buckling is a critical problem for column material. The Japanese Building Standards Law prescribes the upper limit of the slenderness ratio for wood materials at 150. This upper limit is based on the result of lumber evaluations; therefore, it might not be suitable for engineered wood, such as glulam or LVL. Recently, large or medium-sized wooden buildings have attracted much attention, and larger wooden buildings have been built in various regions of the world. The buildings with long columns whose slenderness ratio is over 150 are attractive in Japanese building market.

 In previous studies, one of the authors suggested a method to evaluate the buckling strength based on experimental data using LVL for brace in structural walls and plywood. The cross-sectional shape was rectangular and limited.

 In this study, we evaluated the buckling strength using glulam and LVL for columns. The cross-section was square, i.e., 120 × 120 mm and 60 × 60 mm. The slenderness ratio was set between 30 and 200, and the laminated direction for deflection was also set as a parameter. These specimens were evaluated using compression tests, where both ends were pinned-support. The bending Young's modulus was measured by a non-distractive vibration test before conducting the compression tests. We conducted monotonic loading tests with short columns to define the yield strain for the evaluation of buckling strength. Regarding the short columns, the yield strain of each material was defined as 1300–1400 μ for the glulam specimen and 1400–1500 μ for the LVL. The limitation slenderness ratio was calculated using these values. From the vibration tests, Young’s modulus of glulam was 10.31–10.39 kN/mm², and that of LVL was 13.75–13.82 kN/mm².

 Three specific values were used to evaluate the buckling strength of the specimens, namely maximum stress (σ₁), stress level evaluated by the Southwell method (σ₂), and plastic buckling strength (σ₃). The values of σ₁ and σ₂ were similar; however, the conclusion was that σ₁ was more appropriate for the evaluation of safety. By comparing σ₃ and Euler's buckling strength formula, and comparing σ₁ and σ₃, we determined that the critical slenderness ratio of glulam and LVL was 100. This value corresponds with Standard for Structural Design of Timber Structures, and the basis of the standard value can be shown more theoretically. Furthermore, the comparison of the Euler buckling strength formula and test results proved that the buckling strength can be evaluated safely for materials with a slenderness ratio of up to 200. Regarding the suitability of Young's modulus for Euler’s formula, we found that we could use a 5% lower limit value of the material tests or those in JAS for glulam and LVL. The test results and calculated values correlated well.