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Characterizing perpendicular-to-grain compression in wood construction applications Public Deposited

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  • Current compression perpendicular-to-grain (c-perp) design values for wood members are based on mean stress using the ASTM D143 specimen. Base design value, as determined from 0.04-in deformation in the ASTM specimen, is applied to all c-perp applications. While the standard ASTM test was presumably believed to adequately reflect relevant c-perp applications at the time it was developed (likely railroad cross-ties, wall plates and similar cross-sections), the specimen has limited applicability to many of today's c-perp bearing applications. Previous work has shown that wood-on-wood c-perp bearing is a more severe case as opposed to metal-on-wood bearing. End bearing conditions have been shown to represent a more severe c-perp loading scenario as opposed to load applied over central area. Research has shown that c-perp behavior of wood members is dependant on angle of applied load to annual ring orientation and that the most severe loading case is usually at an angle between 30 and 60 degrees to direction of applied load. Past studies also suggest that depth of member affects c-perp MOE and that higher aspect ratio may lead to instability of the member. As the c-perp behavior observed in the ASTM testing procedure is that of continually increasing stress with increasing deflection and densification, c-perp is generally believed to be serviceability rather than a life safety issue. However, in engineered wood applications, c-perp bearing may occur at areas where structural cohesiveness of a member is necessary to transmit forces through fasteners, such as truss plates. In such scenarios, c-perp has the potential to be a life safety issue. A study was designed to evaluate c-perp behavior of typical as-constructed assemblies in which members experience c-perp stresses near their longitudinal end through wood-on-wood contact. The study included both finite element analysis and experimental testing. Two as-constructed assemblies were evaluated in the study. These included assembly of the bottom chord of a truss bearing on the top plate of a wall (BC assembly) and assembly of the compression chord of a shear wall bearing on the bottom plate (BP assembly) of a wall. Finite element analysis modeled wood material as a composite with alternating earlywood and latewood layers with infinite radius of curvature. BC, BP and ASTM configurations were modeled both with load applied perpendicular and parallel to annual rings. Three BC assemblies were tested. These included BC-2X4, BC-2X8, and BC- 2X12, which had nominal 2X4, 2X8, and 2X12 members as bottom chord members, respectively. Therefore, within BC test assemblies aspect ratio of the bottom chord members varied greatly. The three BC geometries were each tested with both Douglas-fir and Spruce-Pine-Fir top plate material. BP configuration was also tested with both Douglas-fir and Spruce-Pine-Fir bottom plate material. For each test assembly, paired ASTM tests of the main member (bottom chord member in BC tests, and bottom plate member in BP tests) were conducted. Results were analyzed utilizing a variety of statistical methods. Finite element analysis revealed that strain was more uniform throughout depth of the bottom chord member when loaded perpendicular to annual rings than when loaded parallel to annual rings. In BC tests majority of deflection was found to occur in the bottom chord member. In BP test, the majority of deflection was found to occur within the bottom plate with only minimal deflection occurring in the longitudinally loaded compression chord. Due to varying assembly depths, 0.04-in, deflection was found to be a poor criterion for determining c-perp stress values. In order to account for assembly depth, stress values were based on system strain. As 0.04-in, deflection corresponds to two percent strain in the 2-in, deep ASTM specimen, it was determined that stress determination be based on 2-percent system strain. Due to large settlement effects observed in the tested wood-onwood assemblies, an offset strain was adopted as the method for determining and comparing stress values across differing assemblies and configurations tested. Within BC tests, the species of top plate material was not found to significantly affect assembly performance. This was due to the overwhelming influence of bottom chord behavior on system behavior. Within BP tests there was suggestive but inconclusive evidence of a significant difference between Douglas-fir and Spruce-Pine-Fir tests. It was recommended that further BP tests be conducted with larger sample sizes in order to determine the influence of wood species on c-perp behavior of bottom plate members. Mean stress values of BC and BP assembly tests were found to be significantly lower than that of corresponding ASTM tests of the main member. This finding lends justification to the Canadian 2/3 reduction factor for these scenarios as well as to the design procedures suggested by German researchers. It was determined that the ASTM c-perp test does not adequately represent these bearing scenarios. Adjustment factors are recommended for wood-on-wood bearing and opposite side end bearing. Aspect ratio was found to affect c-perp failure mode and led to high potential for sudden and catastrophic failure of members. Fifteen percent of 2X12 members tested failed prior to NDS design stresses due to premature failure of the nominal 2X12 bottom chord member. It was estimated that the odds of a 2X1 2 failing catastrophically are at least 16 times that of a 2X8 failing catastrophically. It was determined that as aspect ratio increases c-perp becomes a life safety as well as a serviceability issue and an adjustment factor for aspect ratio is recommended.
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