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UBC Theses and Dissertations

Impact loading of timber beams Jansson, Bengt


Static tests as well as impact tests were performed to build up our knowledge about impact loading of timber beams. The investigation was focused on failure modes and failure stresses. The static tests were conducted using both deformation and load controlled regimes at three different loading rates in a MTS test machine. The single blow impact tests were carried out with three different drop-heights using a drop-weight impact machine built at the University of British Columbia. A total of 651 specimens (38 x 89 x 1145 mm) were sorted into two categories. The specimens were then grouped into the various loading groups according to the modulus of elasticity obtained from non-destructive bending tests. Three different dynamic failure stress analyses were applied and compared with each other. The distributed inertial forces were replaced with a mid-point inertial force in the first analysis, which was based upon accelerometer measurements. The accelerometer measurements from these tests were considered very unreliable. The second analysis employed a very detailed finite element program which went beyond the objectives of this thesis. The third analysis was a modal analysis, which was a mode-superposition analysis of a distributed-parameter system. The modal analysis was used throughout the remaining analysis because its solution was easily divided into its static and dynamic parts, and it was more efficient than the second analysis. The strength ratio, which was the ratio between dynamic and static failure stress, reflected the effect of impact loading better than the traditional duration of load theory. Non-parametric percentile values of the strength ratio based on the distribution of strength ratios were found more appropriate than if these percentile values were based on the distribution of failure stresses. No differences in failure stresses were observed between the different static tests. A strength decrease of 15 % was experienced for the weaker specimen. The overall tendency was that the number of compression initiated failures de-creased with decreasing failure time. The decrease in failure strength with decreasing failure time was explained by the decreasing amount of compression initiated failure modes.

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