A method for modelling the strength of bonded wood strands which are oriented principally in one direction is proposed. The hypothesis tested was that strand grain orientation data, fitted to a von Mises probability distribution, could be used in an analysis for estimating the potential tensile strength of an ideally bonded composite. The strand strength, resolved at any loading angle with respect to the principal composite strand orientation axis, was multiplied by the distribution probability at that angle. When integrated over all angles, this product yielded the mathematical expectation of strength for the composite. The model predicted composite strength at off-orientation axis angles and represented the material in two dimensions in an orthotropic fashion. A feature of this research is the use of a parametrically quantified strand orientation level in an algorithm developed to estimate composite strength. A practical number of strand angle readings (100) were taken to characterize each composite. These angle readings defined orientation in terms of a parameter which described composites ranging from random to highly oriented. The model input also required microtensile strength means from samples of strands tested in the longitudinal and radial or tangential directions. Comparisons between the model and actual specific strengths were made at five equally spaced-composite principal axis load angles from 0 to 90 degrees. Both tensile and flexural tests were performed to evaluate the model. The evaluations were designated in terms of resin content, distribution, and droplet size. These variables were studied using colorimetry and computerized image analysis. Composite density profiles through the specimens' thickness were obtained from direct reading x-ray densitometry. Composites made of juvenile trembling aspen, red alder, red cedar, mature lodgepole pine and yellow birch were studied. Assumptions concerning wood shear strength and strand length/thickness ratio were discussed in the interpretation of an overlapping strand stress-transfer model. This led to the definition of failure criteria based on stress transfer. A trial of orientation modelling in elasticity estimation was made and a random function model of composite elasticity based on laminated plate theory is outlined in a supplementary proposal for further research. The simplified algorithm for the strength of aligned.wood strand composites provides design targets for reconstituted high strength strand lumber and panel products of the future. / Forestry, Faculty of / Graduate

A comparison of the creep/duration of load (DOL) performance of a new structural wood composite material called Parallam® parallel strand lumber (PSL) to two grades of machine-stress-rated (HSR) Douglas-fir lumber is presented in this thesis. Evaluation of the creep/DOL performance was made on nominal 2x4 members under constant bending stress at three stress levels. A total of 306 test specimens were evaluated for a 15-1/2 month time period. The analysis suggests that the duration of load effect for Parallam PSL was consistent with the Madison curve for the time period studied while the MSR Douglas-fir lumber was consistent with recent duration of load models developed for structural lumber. The analysis also indicates that the current duration of load adjustment factors can be applied to develop working stresses for Parallam. The creep behaviour of the Parallam PSL was found to be equivalent or better than the two MSR lumber grades under dry-service conditions. Furthermore, evidence of linear viscoelastic behaviour was found for all test materials within the range of applied stresses evaluated. Two mathematical models of creep were fitted to the creep data and compared. A '4-parameter linear viscoelastic' model fitted the creep data better than an empirical 'power curve' model. The model parameters developed provide a basis for estimating the mean creep behaviour and variability in creep response for these materials under in-service load conditions for dry-service environments. / Forestry, Faculty of / Graduate

Using state of the art information concerning the statistical nature of loads acting on light frame structures, distributions of maximum lifetime roof snow load and maximum lifetime floor live load were developed for use in differential reliability analyses of lumber properties data. The dead load was combined with the live load to obtain the total roof or floor load. Utilizing these total loads, contrasting sets of lumber data were analyzed based on the concept of equal reliability. The sensitivity of the reliability analysis to changing load distributions was studied. Subsequently, load distributions were recommended for use in differential reliability analyses of lumber properties data. / Master of Science

The torsion test was evaluated as a method for determining the shear strength of full-size structural lumber. The evaluation involved an experimental length study, an experimental depth study, and a finite element study. The length study consisted of fifty nominal 2x4 specimens, ten specimens for each length, and ten American Society for Testing and Materials (ASTM) shear blocks. One 14 foot long board yielded one specimen for each length: (a) 21.0", (b) 28.5", (c) 32.0", (d) 35.5", (e) 39.0", and (f) an ASTM D143-94 shear block. The statistical analysis revealed no evidence that the length affected the shear strength. The depth study consisted of fifty specimens, ten specimens for each depth: (a) 2x4, (b) 2x6, (c) 2x8, (d) 2x10, and (e) 2x12. In addition, fifty ASTM shear blocks, one block for each specimen, were tested. The statistical study did not reveal convincing evidence of a depth effect on shear strength, even after accounting for specific gravity and shear span as covariates. Failure modes for the torsion samples involved a longitudinal shear crack at the mid-point of the longest side, which propagated toward the ends of the specimen and through the cross section perpendicular to the growth rings. The finite element model revealed that uniform shear stress occurs within the shear span, which begins and ends a distance of approximately two times the depth plus the grip distance away from each end of the member. In addition, torsion theory verified that the experimental shear failure plane that occurs within the shear span is parallel to the grain and the shear slippage is also parallel to the grain, similar to the known shear failure in specimens subjected to bending loads. Based on the results of this study, the torsion test is the best practical method to determine the pure shear strength of full-size structural lumber, because the test yields 100% shear failures and the specimen is in a state of pure shear stress. / Graduation date: 1998

A testing procedure was developed to measure the effectiveness of tension proof testing, given that the gripped ends of the lumber do not fully experience the proof stress during a tension proof test. First the middle portion of each 14-foot 2X6 No 2 KDIS Southern Pine lumber specimen was tension proof tested, as if the pieces were only 10 feet long. Then the pips of the testing machine were moved out so that the entire middle 10-foot, section including the portions that had been gripped during the initial proof test would be fully stressed during a second tension test Some specimens, referred to as falldown, were broken in the second test at a load level below the proof load. This indicates that because of the effect of gripped ends the, initial proof test was ineffective in destroying some pieces of lumber with tension strength below the proof load level. The experimental design and analysis were aimed at validating the Showalter et al. (1987) tensile strength - length effect model as adapted to this specialized problem. Although the model predicted that falldown would occur, it was unable to predict the amounts of falldown subsequent to the amounts of proof-test breakage occurring for a sample of lumber. Through the use of end-grading rules, falldown amounts can be reduced to acceptable levels. A method was proposed for developing end-grading rules and deriving concomitant allowable tension stresses for proof-tested lumber. A significant finding was that the histogram for the tension strength of the surviving end-graded lumber had an abruptly-ending left tail, indicating that there were no severely reduced tension strength values for any of the falldown specimens remaining in the sample. / Master of Science

Research has established that correlations exist between bending and tension, and bending and compression strength of lumber. Because of this correlation, improvement in bending strength may be realized from proofloading in tension or compression. The data utilized in a reliability analysis was from Galligan et al. (1986) that characterized the properties of 2-inch softwood dimension lumber with regressions and probability distributions. Randomly selected groups of 2 by 4 1650f-1.5E Hem-fir and No.2 KD Southern Pine were evaluated for bending strength. One group from each species was selected as a control and tested in bending. The other groups were proofloaded in tension and compression at two stress levels and the survivors were tested in bending to failure. Based on the concept of equal reliability and utilizing the load distributions from Thurmond (1986), the tensile and compressive proofloaded strength distributions were compared to the control. The probability of failure for the control group is found, then with an iterative approach, the bending strength values of the proofloaded sample distribution are artificially altered by a factor K until the probabilities of failure for the proofloaded and control groups are similar. The K is a shift factor relating the amount the proofloaded strength distribution must be shifted on the x-axis to give the same reliability as the control. Simple 5th percentile comparisons, the advanced first order second moment (AFOSM) and numerical integration analysis methods were used to evaluate increases in allowable bending strength from proofloading in tension and compression. Proofloading in tension or in compression both produced significant increases in allowable bending strength for the Hem-fir grade. Proofloading in tension to a target 15 percent breakage level, or 2,838. psi, yielded for the survivors an increase of 72 percent in allowable bending strength. The allowable bending strength increased 60 percent due to a compressive proofloading to a target 15 percent breakage level. The allowable bending strength increased as the proofloading level increased for both tension and compression proofloading with the Hem-fir grade. The southern pine visual stress grade did not show a consistent trend between proofloading level and improvement in allowable bending strength. The lack of a trend between proofloading level and allowable bending strength was attributed to possible sampling error. The fifth percentile analysis method, the AFOSM method and numerical integration method were compared. For lumber strength comparisons, a simple fifth percentile analysis was not the preferred method. The AFOSM method and the numerical integration method provided identical results in terms of their application in adjusting allowable bending stresses. It was not possible to show that the approximate AFOSM method can be used exclusively in lieu of the numerical integration method for reliability calculations. / Master of Science

M. S.