Planar shear moduli of rigidity of an oriented strand board from bending and shear tests

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1 Materials and Structures / Matériaux et Constructions, Vol. 37, August-September 2004, pp Planar shear moduli of rigidity of an oriented strand board from bending and shear tests W. H. Thomas Dept. of Civil Engineering, University of Sierra Leone, Sierra Leone ABSTRACT The planar shear of rigidity has been recently utilised in the structural grading of oriented strand board (OSB). However, data on the planar shear moduli of rigidity of oriented strand board are sparse. This has been attributed to the nature of existing tests for determination of planar shear properties, which are deemed to be complex and expensive. The planar shear moduli of rigidity of an 18-mm thick oriented strand board have been determined from bending test and the planar shear test in European Standard EN 789. The ratio of the average planar shear of rigidity in the major axis from the shear test to the average planar shear of rigidity in the major axis from the bending test is The corresponding ratio of the average planar shear moduli of rigidity in the minor axis is The close comparison of the stiffnesses obtained from the shear and bending tests suggests that a bending test is a likely viable alternative to the shear test for determination of the planar shear stiffness. However, comparative tests of other types and thicknesses of OSB are required in order to establish the levels of bias in the planar shear stiffnesses obtained from the bending test. RÉSUMÉ Le module de cisaillement dans le plan a été récemment utilisé dans l évaluation structurale de panneaux de particules orientés (OSB). Cependant, les données sur les modules de cisaillement dans le plan de ces panneaux de particules sont éparses. Il existe en effet de nouvelles méthodes d essais visant à déterminer les propriétés de cisaillement dans le plan, qui sont estimées complexes et onéreuses. Les modules de cisaillement dans le plan de panneaux de particules orientés de 18 mm d épaisseur ont été déterminés à partir d essais en flexion et en cisaillement dans la norme européenne EN 789. Le rapport du module moyen de cisaillement dans le plan dans l axe principal, dans l essai en cisaillement comme dans l essai en flexion, est de Le rapport correspondant du module moyen de cisaillement dans le plan dans l axe secondaire est de La comparaison étroite des rigidités obtenues dans l essai en cisaillement et dans l essai en flexion suggère que ce dernier est une alternative viable à l essai en cisaillement pour la détermination de la rigidité planaire de cisaillement. Cependant, des essais comparatifs d autres types de panneaux de particules avec d autres épaisseurs sont nécessaires pour prévenir les différences de niveaux de rigidité planaire de cisaillement obtenus à partir de l essai en flexion. 1. INTRODUCTION The ratios of the planar shear of rigidity to the bending of elasticity of wood and wood-based sheet materials such as plywood, particleboard and oriented strand board (OSB) are much smaller than those for steel and concrete. Consequently, the planar shear of rigidity is more important in the assessment of deflection and vibration of structures consisting solely or wholly of wood and wood-based beams and panels, especially those of medium to low span to depth ratios. Tests for determination of the planar shear of rigidity of wood-based structural panels in the American Society for Testing and Materials Standard, ASTM D 2718 [1] and in the European Standard, EN 789 [2] are direct shear tests. Although the test-piece preparation and assembly are complex, the direct shear test appears to be the standard method for determination of planar shear stiffness of wood-based panels in North America and /04 RILEM 480

2 Materials and Structures / Matériaux et Constructions, Vol. 37, August-September 2004 Europe. However, in the case of solid and laminated timber beams, a bending test is an acceptable method for determination of the planar shear of rigidity [3]. In principle, a bending test can be used to determine the planar shear of rigidity of a wood-based panel. The advantages of a bending test are that it is simple and cheap to implement compared to the planar shear test in the American Society for Testing and Materials Standard [1] or the European Standard [2, 4]. Another appeal of a bending test is that it simulates the complex stress-strain state through the thickness of orthotropic panels, such as oriented strand board, when used in flooring and roofing. Extensive data have been produced for the bending, tensile and compressive strengths and stiffnesses of oriented strand board [5, 6]. However, data for the planar shear moduli of rigidity of OSB are sparse [4-6]. Bending and direct planar shear tests have been used to determine the planar shear moduli of rigidity of an oriented strand board. This is a first step in investigating the suitability of bending tests in determining the planar shear moduli of rigidity of oriented strand boards. 2. METHODS AND TESTS Planar shear is shear that is parallel to the neutral plane of a beam or plate. This shear is also referred to as rolling or interlaminar shear. A planar shear test to EN 789 and a bending test were carried out on samples of oriented strand board in order to compare the planar shear moduli of rigidity obtained from the two test methods. Eighteen-millimetre thick (nominal) unsanded (both faces) structural-grade oriented strand board was selected for this study because of its popularity in domestic floors. 2.1 Bending The planar shear of rigidity was determined by the bending test method commonly adopted for solid timber beams [1, 3]. The principle of the method is that a simply supported beam is loaded midway between the supports (Fig. 1) and readings of load versus midspan deflection are recorded. The applied load must be sufficient to provide a reliable estimate of the bending stiffness of the beam but shall not exceed the estimated proportional limit or fifty percent of the estimated load capacity in bending. The test procedure is repeated for at least four different spans. The theoretical basis of the method is that the total linear elastic deflection of a prismatic beam of span, L, under centrepoint load, P, can be expressed in terms of the planar shear, G, and the apparent and true bending moduli of elasticity, E m,app and E, respectively. The expression for the deflection is as follows [1]: PL 3 /48E m,app = PL 3 /48EI + PL/4GKA (1) where A and I are the sectional area and second moment of sectional area of the beam and K is the shear coefficient that relates the effective shear strain to average shear stress at the section. The European Standard pren 408 [4] and ASTM Standard D 198 [1] assign a value of to K for a beam of rectangular cross-section. For a prismatic beam of thickness, t, Equation (1) reduces to the following: 1/E m,app = 1/E + 1/KG(t/L) 2 (2) Fig. 1 Schematics of bending and shear tests. From Equation (2), the slope of the line connecting multiple data points of 1/E m,app versus (t/l) 2 is equal to 1/0.833G (1.2/G). Approximately equal increments of (t/l) 2 are recommended and the value of t/l shall be between 0.05 and Significant shear deflection occurs within this range of ratio of t/l. A centre-point bending test rig with an adjustable span was used for this test. The selected spans were 100, 125, 150, 200 and 350 mm (Table 1). These roughly satisfy the requirements for approximately equal increments of (t/l) 2, with t/l being between 0.05 and 0.20 or L/t being between 5 and 20. In order to exclude plate action, it is required that the width/span ratios of the test pieces shall not exceed 0.5 [1]. A uniform width of 50 mm achieved this requirement. For each span and principal direction, four test pieces were prepared, that is, a total of 40 test pieces (Table 1). The test pieces were conditioned in a 20 2 C and 65 5% relative humidity environment. Trial tests enabled the determination of suitable machine crosshead speeds [3] and approximate values of failure loads for the test pieces. Crosshead speeds of 0.27, 0.43, 0.62 and 0.75 mm/min were found to be suitable for the 100-, 125-, 150- and 200-mm span test pieces, respectively. These speeds Table 1 - Test methods and test pieces Method of test Dimensions of test piece Number of test piece Direct shear EN 789:1992 Bending Width = 225 Length = 100 Width =50 Spans = 100, 125, 150, 200 and for shear in major or minor axis 4 for each span for bending in major or minor axis 481

3 Thomas were satisfactory for the tests for bending moduli of elasticity in both major and minor axes. In the case of the 350-mm span test pieces, crosshead speeds of 1.4 and 2.2 mm/min were suitable for the determination of the bending moduli of elasticity in the major and minor axes, respectively. The load was applied from a Satec loading machine with an accuracy of 0.01 N. A Linear Variable Differential Transducer that was previously calibrated by slip gauges measured the centre deflection to the nearest 0.01 mm. The load and deflection readings were recorded in a data logger and a computer. The maximum applied loads were limited to sixty percent of the experimentally estimated failure loads. 2.2 Direct shear In the direct shear (planar) test, a rectangular test piece is bonded between steel plates bevelled at opposite ends of the test piece to provide knife-edges for loading the plates at the faces bonded to the test piece. The test piece is loaded in compression such that the line of action of the applied force would pass through the diagonally opposite corners of the test piece. A schematic of the loading arrangement is illustrated in Fig. 1. A suitable gauge measures slip between the plates and the effective shear of rigidity is calculated from the linear range of the plot of load versus slip [2]. The planar shear moduli of rigidity in the major and minor axes correspond to tests with the load parallel and perpendicular to the face grain direction on the test pieces, respectively. The test method and procedure were in accordance with EN 789 [2]. Thirty-two tests, sixteen for each principal direction, were carried out at the Centre Technique du Bois et de l Ameublement, Paris, France [5], as part a programme of experimental research on the mechanical properties of OSB that was supported by the European Committee for Standardization. The surface dimensions of the test pieces were 225 mm x 100 mm (Table 1). The test pieces were cut from OSB sheathings of the same type and grade as used for the bending test and conditioned in a 20 2 C and 65 5% relative humidity environment. Table 2 - Apparent bending of elasticity in minor axis from bending test Span Average thickness Apparent E m,app Table 3 - Apparent bending of elasticity in major axis from bending test Span Average thickness Apparent E m,app Fig. 2 - Bending test: 1/E m,app versus (t/l) 2 for minor axis parallel to span. 3. RESULTS 3.1 Bending The load-deflection curve for each test piece was typically linear. The apparent bending of elasticity, E m,app, of each test piece, is given by FL 3 /48I w, where F/ w is the slope of the central two-thirds of the load-deflection curve. As the maximum applied load was about half the estimated failure load, the central two-thirds of the load-deflection curve corresponds to the part of the curve between 0.08F max and 0.33F max, where F max is the estimated failure load. The calculated average values of the apparent bending moduli of elasticity in the minor and major axes of the test pieces are listed in Tables 2 and 3, respectively. The planar shear of rigidity in each principal direction was determined from the graph of 1/E m,app versus (t/l) 2 (Figs. 2 and 3). The slope of the line of best fit through the plotted points, k, is equal to 1.2/G. Thus, the planar shear, G is equal to 1.2/k. The calculated values of k for the planar shear moduli of rigidity in the Fig. 3 - Bending test: 1/E m,app versus (t/l) 2 for major axis parallel to span. major and minor axes of the test boards are x 10-4 and x 10-4 mm 2 /N, respectively. Thus, the planar shear moduli of rigidity in the major and minor axes, G xz and G yz, are and N/mm 2 respectively. 3.2 Direct shear Summaries of the planar shear moduli of rigidity in the minor and major axes of the test pieces are given in Tables 4 and 5, respectively [5]. Two test results for the planar shear of rigidity in the minor axis for tests pieces that failed wholly or partially in the bond between the test piece 482

4 Materials and Structures / Matériaux et Constructions, Vol. 37, August-September 2004 Table 4 - Planar shear of rigidity in minor axis from shear test and metal plates were rejected. The mean value and coefficient of variation of the planar of rigidity in the major axis are 72 N/mm 2 and 28.4 percent, respectively. The mean value and coefficient of variation of the planar of rigidity in the minor axis are 60 N/mm 2 and 22.8 percent, respectively. 4. DISCUSSSION Y Y Y Y Y Y Y Y Y Y Y Y Y Y15 - Y Y16 - Table 5 - Planar shear of rigidity in major axis from shear test X X X X X X X X X X X X X X X X This investigation seeks to compare the values of the planar shear stiffnesses obtained from direct shear and bending tests. The ratio of the average planar shear of rigidity in the major axis determined from the shear test to the average planar shear of rigidity in the major axis from the bending test is The corresponding ratio for the planar shear of rigidity in the minor axis is The average percentage difference between the shear stiffnesses obtained from the two methods is about 15 percent. Given the close agreement between the planar shear moduli of rigidity obtained from the bending and shear test methods, it appears that the bending test can be used to determine the planar shear of rigidity of the test board. The bias in the planar shear rigidity obtained from the bending test compared to the shear from the shear test or vice versa, which has been roughly calculated is also useful in design. The difference in the loading arrangements and sizes of test pieces may account for the difference in the planar shear stiffnesses obtained from the two test methods. However, the average values of planar shear of rigidity in the major and minor axes, G xz and G yz, respectively, for the OSB test panel (51 72 N/mm 2 ) are low compared to other published values of G xz and G yz that are typically in the range from 150 to 200 N/mm 2 [4, 6]. The relatively low shear stiffnesses from the bending and shear tests are related to the density distribution through the thickness, the relative layer thickness, resin distribution and degree of strand orientation in the test board. The coefficients of variation of the mechanical properties of wood-based panels such as OSB generally range from 8 to 18 percent. The coefficient of variation of the shear stiffnesses determined from the shear tests is 25 percent. Shrestha [4] obtained coefficients of variation of between 18 and 35 percent for the planar shear moduli of rigidity from a modified ASTM 2718 shear test. These relatively high coefficients of variation from shear tests may be mainly related to the loading arrangement and size of test piece [1]. In the absence of coefficients of variation from the bending test, comparison of coefficients of variation of the planar shear moduli of rigidity from the bending and shear tests cannot be made. The degree of orthotropy in shear stiffness, G xz /G yz, for the test board is 1.27 from the bending test. The ratio G xz /G yz from the shear test is The degree of orthotropy in planar shear stiffness is deemed to vary from 0.8 to 1.2, with a median value of about unity [7]. These results are consistent with the conclusions reached by Shrestha [4] that the planar shear stiffnesses in the major and minor axes are practically equal. The small difference between the planar shear stiffnesses in the major and minor axes often results in the use of the average shear stiffness or the smaller shear stiffness to characterise their magnitudes. A major advantage of the bending test is that it simulates the direction of gravity loading and the resulting stress-strain (shear) distribution in floor panels. The stress-strain distribution is parabolic through the thickness and zero at the surfaces of the test piece. In contrast, in the case of the planar shear test of EN 789, nearly uniform in-plane shear stress prevails at the surfaces and through the thickness of the test piece. According to Fridley and French [8], in the bending test the shear stress is created as in an as-built or in-use condition and not forced, as is the shear test. Evaluation and significance Stiffness tests were carried out to determine the planar shear stiffness in the major and minor axes of the OSB panels. A range of L/t from 5 to 20 has been used. This is recommended in the bending test for determination of the planar shear stiffness of a solid timber beam. For OSB panels, the stiffness ratio, E x /G xz, varies typically from 50 to 75 while the ratio E y /G yz varies from 15 to 35 [5, 7]. By comparison, the ratio E/G z for solid timber beams is about 16. The low ratio of planar shear of rigidity to bending of elasticity of OSB compared to solid wood, suggests that an increased upper limit of L/t of up to about 35 could be used in bending test for determination of planar shear stiffness of OSB. In the bending test method, a stiffness test is carried out on an individual test piece of given span. Four different spans of test pieces were utilised in the bending test. An alternative procedure is to use a long test piece with a span to thickness ratio of between 20 and 40. For each test piece, stiffness tests are carried out on three or more different spans. The selected spans shall be such that the increments of (t/l) 2 are approximately equal. An elastic constant is often a predictor of strength and other elastic constants. Numerous studies of solid wood 483

5 Thomas have shown that there is strong correlation between the bending of elasticity and planar shear of rigidity [9]. In the case of OSB, such a correlation has so far not been shown [6, 7]. However, if there is a correlation, linear or non-linear, between the planar shear of rigidity and the planar shear strength, the planar shear of rigidity determined from bending test can roughly give an indication of the planar shear strength. In the introduction, the lack of data on the planar shear stiffness of OSB has been given as one of the reasons for exploring the bending test for determination of the planar shear stiffness. The relatively simple bending test should bring about an increase in the data on planar shear stiffness. Although the planar shear properties are not used in the classification of OSB in the European Standard EN 300 [10], they are used in the rating of design rated OSB in North America [11]. For structural design purposes, there is need for inclusion of planar shear properties in the grading of OSB in the European Standard EN CONCLUSIONS The planar shear moduli of rigidity of an 18-mm thick structural-grade oriented strand board determined from a centre-point bending test have been compared to those determined from the direct shear test of EN 789. There is a fifteen-percent difference between the respective values of of rigidity. The levels of orthotropy in planar shear of rigidity of the board (G xz /G yz ) are 1.27 and 1.21 from the shear and bending tests, respectively. The bending test appears suitable for determination of planar shear of rigidity of the test board. Similar validation tests, using other types and thicknesses of OSB panels, are recommended. NOTATIONS A sectional area of panel E m,app apparent bending of elasticity of panel E x, E y bending moduli of elasticity in the major and minor axes of panel, respectively G planar shear of rigidity of panel G xz,g yz planar shear moduli of rigidity in the major and minor axes of panel, respectively I second moment of sectional area of panel K shear coefficient L span of panel t thickness of panel x-axis major axis face grain direction or principal direction in the plane of the panel with greater bending strength and of elasticity x, y, z rectangular Cartesian and material property axes y-axis minor axis direction in the plane of the panel at right angle to the major axis ACKNOWLEDGEMENTS The shear test described in this paper was carried out at the Centre Technique du Bois et de l'ameublement, Paris, France, as part of a programme of research on the mechanical properties of OSB that was supported by the European Committee for Standardization while the bending test was carried out at the University of Surrey, U.K. REFERENCES [1] American Society for Testing and Materials, ASTM Standards, Vol Wood (American Society of Testing and Materials Standards, West Conshohocken, Pa. USA, 1992). [2] European Committee for Standardization, EN 789:1992 Timber structures: Testing of wood-based panels for determination of mechanical properties for structural purposes, European Committee for Standardization (CEN), Brussels, Belgium (1992). [3] European Committee for Standardization, pren 408:1994 Timber structures- Structural timber and glued laminated timber- Determination of some physical and mechanical properties, European Committee for Standardization, Brussels, Belgium (1994). [4] Shrestha, D., properties tests of oriented strandboard panels, Forest Products Journal 49 (10) (1999) [5] Griffiths, D. R. and Wickens, H. G., CEC Programme: Design stresses for OSB. University of Surrey tests and reduction of result, Department of Civil Engineering, University of Surrey, Guildford, Surrey, England (1995). [6] Karacabeyli, E., Lau P., Henderson, C.R., Meakes, F.V. and Deacon, W., Design rated oriented strandboard in CSA standards, Canadian Journal of Civil Engineering 23 (1996) [7] Thomas, W.H., Mechanical properties of structural-grade oriented strand board, Holz als Roh- und Werkstoff 59 (6) (2001) [8] Fridley, K. and French, L. Five-point bending test for determination of edgewise shear in structural panels, Forest Products Journal 50 (5) (2000) [9] American Society of Civil Engineers, Wood structures A design guide and commentary, (American Society of Civil Engineers, Structural Division, New York USA, 1975). [10] European Committee for Standardization, EN 300 oriented strand board (OSB)- Definitions, classification and specification, European Committee for Standardization, Brussels, Belgium (1997). [11] Structural Board Association, OSB design manual: Design rated oriented strand board (Structural Board Association, Ontario, Canada, 1995). Paper received: March 7, 2002; Paper accepted: August 4,

Assessment of test-piece size for punching shear capacity of orthotropic panels: case study of OSB

Materials and Structures/Matériaux et Constructions, Vol. 36, March 2003, pp 139-144 Assessment of test-piece size for punching shear capacity of orthotropic panels: case study of OSB W. H. Thomas Techsult