Static bending properties of structural wood-base panels: largepanel versus small-specimen tests

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1 Static bending properties of structural wood-base panels: largepanel versus small-specimen tests J. Dobbin McNatt Abstract The use of small-specimen bending tests on some particle panel products may be inappropriate because the relative size of the specimen and wood elements in the panel (wafers, flakes, strands) can result in unnecessarily large variability in test values. In this study, eight different structural panel products were tested in static bending using quarter-point loading of four different test panel sizes. The goal was to assess the effect of panel size on bending strength and variability of test values. Products tested included veneered composite panels, oriented strandboard, waferboards, and flakeboards. For comparison, tests were made on 3-inch-wide midspan-loaded specimens specified in ASTM D Bending strengths (MOR) and stiffnesses (MOE) were essentially the same for all sizes of quarter-pointloaded test panels for each of the eight products. MOE from ASTM-size specimens was somewhat lower, whereas MOR was somewhat higher than corresponding values from the other test Variability of test results generally increased as test panel size decreased. Strength characteristics of sheathinggrade face veneers were overriding factors in variability of composite The study results suggest that large-panel bending tests should be used when developing design stresses for some structural-use The static bending test method in ASTM Standard D 1037 (2) calls for a 2- or 3-inch-wide specimen to be loaded at the center of a span 24 times the panel thickness. This standard, first printed by ASTM in 1949, is still applicable to many of the wood-base panel products covered by D However, the suitability of this method for some of the newer structural panels (waferboard, oriented strandboard (OSB), structural flakeboard) has been questioned because the specimen size relative to the size of the wood elements used such as flakes, wafers, or strands can result in unnecessarily high variability in property values (8,13). The same can be said of other test methods in D A similar condition exists with construction plywood. Strength-reducing characteristics occur in such a manner that results are extremely variable if small specimens are used. For example, a 2-inch knot in the face ply would cause a much greater reduction in strength and stiffness in a 3-inch-wide bending specimen than it would in a large panel loaded in bending. Areas of higher and lower strength and stiffness are averaged out by large-panel testing (1), which gives values useful in construction. Szabo (13)suggested that perhaps structural panels such as waferboard, OSB, and flakeboard should also be tested in large-size panels rather than in small-specimen sizes. Therefore, the purpose of this study was to evaluate the effects of panel/specimen size on the magnitude and variability of bending properties of a variety of structural wood-base Literature Little published information is available on largepanel testing of structural particle panels in bending. Pellerin and Morschauser (11) measured dynamic modulus of elasticity (MOE)of three grades of particleboard nondestructively using transverse vibrations and longitudinal stress waves. The 2- by 4-foot test panels were not loaded in static bending. Instead, the dynamic MOE values were compared to static MOE values of 3-inch-wide bending specimens cut from the larger McLain and Bodig (7) measured MOE on 4- by 8-foot sheets of plywood and underlayment-grade particleboard using both static bending and transverse The author is a Research Forest Products Technologist, USDA Forest Serv., Forest Prod. Lab., P.O. Box 5130, Madison, WI This paper was received for publication in August Forest Products Research Society Forest Prod. J. 34(4): APRIL 1984

2 vibrations. However,they did not correlate the values to small-specimen tests. They did find that modulus of rigidity (G) values determined from full-size panels were two to six times those determined from the standard ASTM-size specimens, and they questioned whether G determined from small specimens is characteristic of full-size panel use. Szabo (13)determined bending properties of three thicknesses (5/16, 7/16, and 5/8 in.) of waferboard from five different manufacturing plants using two test methods: The large-panel test specified in Method C of ASTM D 3043, (1) and the small-specimen test described in the Canadian Standard CSA (3). MOE values from the large panel (48by 42 in.) averaged 19 percent higher than those from the small-specimen tests. However, modulus of rupture (MOR)values from the small-specimen tests averaged 19 percent higher than those from the large-panel tests. Coefficients of variation (COV) for both MOE and MOR were consistently greater for the small-specimen tests. Szabo (12) also compared bending strength and stiffness values obtained from 610- by 1,220-mm (2 by 4 ft.) and 1,220- by 1,220-mm (4 by 4 ft.) waferboard test panels using the large-panel test procedure of ASTM D 3043, Method C (1).He found no significant differences in strength or stiffness between the two panel sizes. The larger test panel data showed slightly lower COVs (8.5%as compared to 10.5% for the smaller panels). Szabo concluded that the smaller and more economical panels can be used to adequately represent the bending stiffness and strength of commercial-size waferboard Lehmann and Schaffer (6)found that 2- by 4-foot flakeboard panels loaded in bending at midspan gave MOE values averaging 23 percent higher than those from small-specimen tests. As a part of the Forest Products Laboratory (FPL) Light-Frame Research Program (5), MOE values were determined on 4- by 8-foot sheets of 7/16-inch waferboard panels from two manufacturers. The study measured deflection of the panels under their own weight over a 95-inch span (sagmethod). MOE from these sag data averaged 654,000 psi (8.9%COV) and 640,000 psi (8.6%COV) for waferboards Nos. 2 and 3 (as identified below).no small-specimen destructive tests were made. Additional panels from these same two manufacturers (waferboard Nos. 1 and 3) were, however, included in the study reported in this paper. FPL also determined MOE values on 4- by 8-foot sheets of lab-made structural flakeboards using the same sag method (4).The average MOE obtained in this way - 880,000 psi - was 23 percent higher than that determined from small ASTM-size (2) specimens cut from these panels - 714,000 psi (9). Materials Twelve 4- by 8-foot sheets of eight different panel materials were included in this study: -Three different commercial waferboards, all 7/16 inch thick, 41 to 42 pcf; -One commercial phenolic-bonded flakeboard, 314 inch thick, 51 pcf; -AnFPL experimental, random three-layer flakeboard made from forest residues, 1/2 inch thick, 43 pcf (9); - One OSB, face and core material cross-aligned, made in a pilot plant facility, 1/2 inch thick, 41 pcf; - Two composite panels with sheathing-grade veneer face and back; one with cross-aligned strand core material, 38 pcf; the other with random particle core material, 41 pcf. Both 112 inch thick. Flexure test methods Before testing, all the panels were conditioned to equilibrium moisture content at 65 percent relative humidity. The first test was a quarter-point loading of each of the 4- by 8-foot sheets over a 7-foot span to determine stiffness only. None of the full-size sheets were loaded to failure. Next, each sheet was cut into quarter-size panels (2 by 4 ft.) with two parallel and two perpendicular to the 8-foot direction of the original sheet (Fig. 1). The four 2- by 4-foot panels were tested in bending using quarterpoint loading on a 44-inch span. Two 2- by 4-foot samples from each original sheet (marked with dashed-line X S through the diagonals in Fig. 1) were loaded to failure. Load-deflectioncurves as well as load at failure were recorded. The other two 2- by 4-foot samples were loaded for stiffness only, after which they were cut into quarter-size (1 by 2 ft.) panels; two parallel and two perpendicular to the length of the original sheet (Fig. 1). Half of these (marked with solid-line X S in Fig. 1)were loaded to failure using quarter-point loading on a 22-inch span. The others were loaded for stiffness only. From the stiffness samples, two 6- by 12-inch test panels were cut (Fig. 1). Again, half were loaded to failure using quarter-point loading on an 11-inch span. Half were saved for possible future testing. Three-inchwide specimens for testing in bending using midspan loading as given in ASTM Standard D 1037 were cut from the remaining material (Fig. 1). The total number of bending tests from each 4- by 8-foot sheet are listed in Table 1. In all cases, MOR and MOE calculationswere based on total panel thickness as if the panels were homogeneous through the thickness. Results and discussion Modulus of elasticity Results of the static bending tests are summarized in Tables 2 and 3. Average MOE values for the eight different materials from five different panel/specimen sizes are shown in Table 2. As would be expected, the composite panels with Douglas-fir sheathing-grade veneer faces were at least twice as stiff as any of the other panel types (1.6 to 1.7 million psi MOE for the quarterpoint-loaded panels). However, the two veneered composite panels were 9 to 10 times stiffer in the panel length (parallel) direction due to the orientation of the grain in the face veneers. MOE of the OSB panels in the direction of panel length (parallelto face material alignment) averaged about 850,000 psi. This was nearly twice the stiffness in the cross-panel direction. FOREST PRODUCTS JOURNAL Vol. 34, No. 4 51

3 TABLE 1. - Number of tests of each panel size made from each 4- by 8-foot sheet. Figure 1. - Method of cutting 4- by 8-foot panels for bending tests. For the "random" structural flakeboards and waferboards, stiffness in the forming direction of the panels as manufactured was up to 15 percent greater than in the cross-forming (perpendicular) direction. However, since most of these products are commercially manufactured in panel sizes which are multiples of 4 by 8 feet, the forming direction does not necessarily correspond to the 8-foot direction of a finished 4- by 8-foot sheet. For example, one of the waferboards in this study was 5 to 10 percent stiffer and stronger in the crosspanel direction since the 4- by 8-foot sheets were cut crosswise from an 8-foot-wide panel. Average MOE values for waferboard Nos. 1and 3 tested by full-panel quarter-point loading in this study (631,000and 616,000 psi, respectively) were quite close to the values determined earlier (5) by full-panel deflection under their own weight (654,000 and 640,000 psi, respectively). MOE values determined by midspan loading of the 3-inch-wide specimens according to ASTM D 1037 (2) averaged 10 to 20 percent lower than those determined from the quarter-point-loaded These results agree with values reported by Szabo (13) for waferboard and with values from the FPL study on structural flakeboard (4, 6, 9). Results of a current study at FPL (10) indicate that given the span-depth ratios used and interlaminar shear moduli of these panel types, the effect of shear deflection on MOE for midspan loading as compared to quarter-point loading is only 1or 2 percent. Variability of MOE data The COVs for the individual test values, shown in parentheses in Table 2, indicate the variability of the MOE data. Specific statements as to the statistical significance of the data cannot be made since COVs were calculated from different sample sizes (Table 1), and the various panel sizes were not from independent samples; however, the trend indicates an increase in variability as panel size decreases. As noted earlier, Szabo (12, 13) reported somewhat larger COVs for small-specimen test data for waferboard as compared to large-panel test data, as well as for 2-by 4-foot panels as compared to 4- by 4-foot The unexpected result was that the COVs for the 3-inch-wide specimens were generally less than those for the 6- by 12-inch test This is probably in part due to the fact that the 3-inch-wide ASTM D 1037 (2) specimens were tested by midspan loading and the others were tested by quarterpoint loading. For all test panel sizes, the COVs were least for 3/4-inch-thick commercial flakeboard. This reflects the uniformity in properties resulting from the small size of wood elements used as furnish in manufacture as compared to the larger wafers, etc., used in the other flakeboards and waferboards, and the characteristics of the sheathing-grade veneer used as facings on the composite Modulus of rupture Average MOR values for the eight wood-base panels determined on four panel/specimen sizes are shown in Table 3. (Note that there are no values for the 4- by 52 APRIL 1984

4 TABLE 2. - Modulus of elasticity (MOE) of eight structural wood-base panel products from five test panel (specimen) sizes. a 8-ft. panels since none ofthe full-size sheets were loaded to failure.) For all sizes of quarter-point-loaded panels, MOR values for the composite panels with the random and aligned core layer averaged about 7,700 and 6,000 psi, respectively, parallel to the grain of the face veneers. The ratios of MOR parallel and perpendicular to the grain of the face veneers averaged 8:l for the randomcore panels and 4.5:l for the aligned-core Surprisingly, for the OSB panels, the bending strength ratio parallel and perpendicular to face-layer alignment averaged only 1.25:l. In all cases, MOE ratios were greater than MOR ratios, as shown: Ratios for stiffness (MOE) and strength (MOR) parallel and perpendicular to the 8-foot direction Ratio Panel type MOE MOR Composites Random core 10.4:1 8.1:l Aligned core 9.2:1 4.5:l OSB 1.9:1 1.25:1 MOR values for the waferboards and flakeboards were 5 to 15 percent higher in the forming direction of the For all eight panel types, bending strengths were essentially the same for all three sizes of quarterpoint-loaded In contrast to MOE, MOR determined from the 3-inch-wide, midspan-loaded specimens was larger than that determined from the larger quarter-pointloaded panels - as much as 29 percent larger for some panel types. Again, these results agree with those reported by Szabo (13)who found that bending strength of waferboard averaged 19 percent less for large panels than for small specimens. Variability of MOR data The COV for the MOR data are shown in parentheses in Table 3. Variability of the nonveneered panels generally increased as panel/specimen size decreased. Again, the variability of the 3/4-inch flakeboard was less than that of any other panel type. COV for the composite panels did not follow the trend of the other panel types, which reflects the overriding effects of knots, knot holes, etc., in the sheathing-grade veneer used. The trend of the COV data for the cross-panel direction was much the same as that for the 8-foot direction except that variability of the veneered panels was no greater than for some of the other panel types. The influence of veneer characteristics would have a much smaller effect on panel strength across the grain of the faces than along the grain. With few exceptions, variability of MOR data for all panel types was greater than variability of MOE data. Conclusions and recommendations 1. Bending strength and stiffness of the eight structural wood-base panels investigated, measured on quarter-point-loaded panels, are affected little or none by decreasing the test panel size from 4 by 8 feet to 1/2 by 1 foot. However, variability ofthe individual test specimen values does increase. 2. When compared to values from small ASTMsize, midspan-loaded specimens, MOR values from the FOREST PRODUCTS JOURNAL Vol. 34, No. 4 53

5 TABLE 3. - Modulus of rupture (MOR) of eight structural wood-base panel products from five test panel (specimen) sizes. a larger quarter-point-loaded panels were lower, but MOE values were greater. 3. For reasons listed below, large-panel tests should be considered for products such as waferboard, flakeboard, and OSB when test results will be used to develop design values: A. These panel products are used primarily as large sheets (4 by 8 ft.) in construction. B. Test data variability decreases as test specimen/panelsizeincreases. C. Test results must be adjusted for variability in deriving design values and variability factor should reflect end-use conditions. The current ASTM pure-moment test for large panels of plywood (1) is one such method. It is currently being used to test waferboard in Canada, as noted earlier in this paper (12, 13). Literature cited 54 APRIL 1984