Ying Hei Chui

Simultaneous evaluation of bending and shear moduli of wood and the influence of knots on these parameters

This paper presents a method of simultaneous evaluation of bending and shear moduli of wooden beams. The method, which is based upon the measurement of two natural frequencies of a beam, is simple to perform. Existing techniques for determining shear moduli of wood based materials are either too cumbersome to carry out or prone to measurement errors. Using this method tests were conducted on matched groups of clear and knotty specimens. It was found that for clear specimens a value of 20 can be assumed for the E to G ratio. For specimens which contain knots the variation was so large that no corresponding figure can be suggested with confidence. But the ratio for knotty materials is though to be higher than 20.SummaryThis paper presents a method of simultaneous evaluation of bending and shear moduli of wooden beams. The method, which is based upon the measurement of two natural frequencies of a beam, is simple to perform. Existing techniques for determining shear moduli of wood based materials are either too cumbersome to carry out or prone to measurement errors. Using this method tests were conducted on matched groups of clear and knotty specimens. It was found that for clear specimens a value of 20 can be assumed for the E to G ratio. For specimens which contain knots the variation was so large that no corresponding figure can be suggested with confidence. But the ratio for knotty materials is though to be higher than 20.

Factors affecting mode I fracture energy of plantation-grown red pine

Mode I fracture energy of premature plantation-grown red pine is discussed, for crack growth in the longitudinal direction. It is demonstrated that fracture energy is influenced by moisture content at test and the direction that stress is applied in the radial-tangential plane. Secondary influences of moisture conditioning and density on fracture energy were observed, with the severity related to the moisture content of the material at test. Discrepancies with findings in the literature are identified and discussed. It is likely that results of this study apply to other conifer species with low extractive contents.SummaryMode I fracture energy of premature plantation-grown red pine is discussed, for crack growth in the longitudinal direction. It is demonstrated that fracture energy is influenced by moisture content at test and the direction that stress is applied in the radial-tangential plane. Secondary influences of moisture conditioning and density on fracture energy were observed, with the severity related to the moisture content of the material at test. Discrepancies with findings in the literature are identified and discussed. It is likely that results of this study apply to other conifer species with low extractive contents.

Influence of rotatory inertia, shear deformation and support condition on natural frequencies of wooden beams.

SummaryThe degree of influence of rotatory inertia and shear deformation on natural frequency determinations is discussed. It is found that for material such as wood, which has a relatively high ratio of bending modulus to shear modulus, these effects need to be taken into account even for supposedly ‘slender’ beams. The discussion covers the four most common types of support condition, namely simply supported, free-free, clamped-free and clamped-clamped ends. As a perfectly rigid clamped condition can never be achieved in practice, especially for relatively ‘soft’ material like wood, consideration is extended to also include partially-clamped beams.The degree of influence of rotatory inertia and shear deformation on natural frequency determinations is discussed. It is found that for material such as wood, which has a relatively high ratio of bending modulus to shear modulus, these effects need to be taken into account even for supposedly ‘slender’ beams. The discussion covers the four most common types of support condition, namely simply supported, free-free, clamped-free and clamped-clamped ends. As a perfectly rigid clamped condition can never be achieved in practice, especially for relatively ‘soft’ material like wood, consideration is extended to also include partially-clamped beams.

Measurement of wood shrinkage in jack pine using three dimensional digital image correlation (DIC)

Abstract Dimensional stability is one of the most important properties of lumber. Shrinkage is especially critical in this regard. In this study, longitudinal, radial and tangential shrinkages in jack pine (Pinus banksiana) have been found to be 0.4, 3.3 and 5.7%, respectively, by means of three dimensional digital image correlation (3D DIC). Full field shrinkage data were extracted from two types of specimen. It was demonstrated that the 3D DIC method is a suitable method of providing localized shrinkage values. The shrinkages in the three anatomical dimensions exhibit different variation patterns from pith to bark. Detailed shrinkage measurements provided data for a better understanding and modeling of shrinkage within a tree stem, which could be useful for lumber distortion simulation.

Comparison of NDE techniques for assessing mechanical properties of unjointed and finger-jointed lumber

Abstract Non-destructive evaluation (NDE) methods are common for grading structural lumber with static bending as the traditional NDE method for strength. More recently, longitudinal and transverse vibration techniques have also been proposed for grading lumber. In this study, unjointed and finger-jointed sawn lumber has been evaluated by these traditional and relatively new NDE methods. In total, 188 pieces of 38 mm×89 mm black spruce lumber were tested. Of these, 40 were unjointed, 47 had 2–3 finger joints, and 101 had 5–7 finger joints. The main objective was to evaluate the reliability of the various NDE techniques in predicting the bending stiffness and tensile strength of finger-jointed lumber with different number of finger joints. Results show that all NDE methods provide stiffness values of unjointed and finger-jointed lumber that correlate well with laboratory measured static bending stiffness with R2 values ranging from 0.76 to 0.97. Moreover, lumber with finger joints has lower bending stiffness than unjointed lumber. Based on the correlation coefficients, there is no evidence that finger joints affect the precision of the strength prediction by NDE methods.

Simultaneous measurement of elastic constants of full-size engineered wood-based panels by modal testing

Abstract Engineered wood-based panels are widely used in structural applications. Accurate measurement of their elastic properties is of great importance for predicting their mechanical behavior during structural design. In this study, an efficient non-destructive test method for measurement of effective elastic constants of orthotropic wood-based panels is proposed based on a modal testing technique. An algorithm was developed based on an improved approximate frequency equation of transverse vibration of orthotropic plates under the boundary condition, in which two opposite sides are simply supported and the other two are free (SFSF). The method is able to predict the frequency ranges and mode indices as well as corresponding normalized sensitivity to elastic constants based on initial estimates of orthotropic ratios with uncertainties and measured fundamental natural frequency. Full-size engineered wood-based panels including cross laminated timber (CLT), oriented strand board (OSB), and medium density fiberboard (MDF) were tested with the proposed method. In general, the measured elastic constants of the three types of panel based on modal test agreed well with those corresponding values measured by static tests. More tests are needed with a range of panel sizes and types for further validation of the proposed test method.

Compression control and its significance in the manufacture and effects on properties of poplar LVL

SummaryIn order to control the compression effectively, the main processing parameters for hot-pressing of poplar LVL were investigated in this study. Results from an orthogonal experiment show qualitatively that compression of poplar LVL is influenced by pressing pressure and moisture content of the veneers. High press pressure and veneer moisture content lead to high compression during hot pressing. It is shown that compression has significant effects on modulus of elasticity, modulus of rupture, specific gravity and thickness swelling of poplar LVL. Modulus of elasticity, modulus of rupture and specific gravity appear to be directly proportional to compression within the compression range of 5% to 20%. Horizontal shear strength results indicate that, due to inadequate contact, proper glue bond may not be achieved between veneers of LVL with low compression. Thickness swelling appears not sensitive to compression between the compression range of 4% to 10%.

Seismic Response of Midrise Light Wood-Frame Buildings with Portal Frames

AbstractThe seismic response of midrise light wood-frame buildings incorporating portal frames and shear walls were investigated via numerical simulation. Because portal frames and shear walls exhibit different ductility characteristics, of specific interest in this paper is the seismic force-modification factor for the combined building system. An approach to estimate the ductility-related force-modification factor, Rd, for the hybrid light wood-frame building containing two types of lateral load-resisting systems with different Rd was proposed. One-, four-, and six-story buildings with layouts that rely on shear walls only or a combination of shear walls and portal frames were designed using the Rd estimated from the proposed approach. Frequency and nonlinear time history analyses were performed on these multistory hybrid buildings using a modified macro element model to represent the lateral-resistant element in finite-element program, ABAQUS. The results show that the use of a Rd value higher than the...

Contribution of Type-X Gypsum Wall Board to the Racking Performance of Light-Frame Wood Shear Walls

AbstractThis paper presents an experimental investigation of the racking performance of light-frame wood shear walls to evaluate the effect of Type X gypsum wall board (GWB). Twelve shear walls sheathed with oriented strand board (OSB) alone, GWB alone, or in combination were tested under static monotonic or reversed cyclic lateral load. The racking performance of these wall specimens in terms of stiffness, strength, ultimate displacement, ductility, energy dissipation, and failure modes was investigated. Specifically the effects of panel orientation, GWB panel joint taping, and double-layer GWB were examined. It was found that shear walls sheathed with GWB provide ductility ratios similar to or higher than those of OSB sheathed walls. The direct superposition rule can be applied to estimate the overall strength of a shear wall sheathed with single-layer OSB and GWB on opposite faces, provided there is no panel joint taping and the panel orientation is the same for both materials. The superposition rule p...

Investigation of Shrinkage in Softwood Using Digital Image Correlation Method

Distortion in lumber, such as twist, cup, spring and bow, can cause serious problems for its use in service. Lumber distortion is largely a result of differential shrinkage in wood in different directions and the presence of different types of wood (i.e. juvenile and mature wood) in a single piece. Shrinkage varies within tree because of different types of wood. In this paper, the digital image correlation (DIC) method was used to investigate shrinkage variation within tree. The DIC method is an image-based, non-contact and full-field displacement and strain measurement method. Two softwood species grown in Eastern Canada, jack pine and white spruce, were used in this study. In this paper, average full-field shrinkage over each growth ring was measured, and the growth ring position in relation to pith of the tree was recorded. The shrinkage variations in the radial, tangential and longitudinal directions in a tree stem are presented. The work described in this paper is part of a larger study to develop a modeling technique to predict distortion of lumber based on its position in a tree stem.