斜め格子パネルを用いた木質Ｉ形梁の長期性能と設計法

<p> BACK GROUND AND PURPOSE</p><p> The authors have developed I-shaped beam (I-beam) using Diagonal Lattice Panel (DLP) for the web and Laminated Veneer Lumber for the flange to promote the utilization of Japanese domestic timber. DLP, which consists of two ply of diagonal laminas arranged at 45 degree with some grid interval is able to compose a beam with high efficiency in cross section. The previous paper described short-term mechanical properties of the I-beam including strength, stiffness, stress distribution in bending and shear. That research results revealed that this I-beam has length effect in constant shear stress and design method was established.</p><p> This report focuses on its properties in long-term loading test in bending and shear. Another essential purpose of this paper is to confirm whether length effect exists in long-term loading like in short-term loading.</p><p> RESEARCH SUMMARY</p><p> 1. Long-term tests to acquire DOL factors and creep property were conducted in a facility without air conditioning. The span of bending test was 3990 mm and was loaded at two points of equally divided into three sections. In shear test, concentrated load was applied at the center of the span of 1500 mm.</p><p> 2. DOL tests were carried out at three stress levels in bending and five stress levels in shear. The strength in fifty years later was estimated by liner regression equation between load duration in common logarithmic scale and stress level. DOL factors were defined as the ratio of the strength in fifty years later to the average strength of short-term tests, they were approximately 0.52 in both bending and shear.</p><p> 3. Climograph between monthly average temperature and humidity shows seasonal variation. Strong correlation was observed between time to failure and temperature during loading.</p><p> 4. The previous research of the authors found length effects in short-term shear strength that is a relation of the length ratio to the power of -0.252. In the test results in this paper, length effect in long-term loading was also found like in short-term tests. This results suggest that composite lattice structure generally has length effect like this.</p><p> 5. In the creep tests, loads were applied for 35 days or more, for up to 4 months. Relative creep for fifty years is about 1.5 in both bending and shear according to Log time law. This value is considered appropriately in practical because of including mechano-sorptive creep, or MS creep. Composite beam like this I-beam is considered to have creep behavior like ordinary timber, because each parts resist with its axial direction that is parallel to grain.</p><p> 6. MS creep was observed in time-deflection history. Creep behavior was affected by high humidity due to typhoon and rainy weather several time. Particularly large creep observed with approaching of typhoon 21. It was consistent with findings in other previous research. MS creep in shear tends to be relatively larger than bending, because the web of I-beam is thin.</p><p> 7. Creep limit of this I-beam was estimated by creep function method that was proposed by Sugiyama. In this method, different levels of loads are applied at the same and creep limit is defined as load value at which creep function is dramatically changed. Based on the method, the creep limit of the I-beam in shear was estimated from 0.52 to 0.58 of stress level.</p><p> 8. Shear creep properties were compared between each web material (DLP, plywood and OSB) in I-beam and small specimen. Shear creep is thought to depend on the creep characteristic of the web material in the 45° direction, and the shear creep of the I-beam using DLP is smaller than that using OSB or plywood.</p>