Load-bearing Capacity of Longitudinally Cracked Timber Beams Under Different Loading Conditions: Investigation of Three Different Loading Point Distances and Three Different Grain Directions

Abstract

This study investigated the effects of loading point distance, grain direction, and crack depth ratio on the load-bearing capacity of longitudinally cracked timber beams. Pinus sylvestris var. mongolica was selected for specimen preparation, and fracture tests were conducted under three loading configurations: three-point bending, four-point bending with a large loading-point distance, and four-point bending with a small loading-point distance. Specimens with grain directions of T(tangential), R(radial),and TR(tangential-radial) in crosssection were used. A total of 720 defect-free specimens and 1,200 specimens containing a longitudinal crack at the middle-height layer were tested. Three coefficients, namely load-bearing capacity coefficient, load-bearing capacity degradation coefficient, and crack hazard effect coefficient, were defined to quantify the crack influence. The results showed that T-specimens had the highest load-bearing capacity, followed by R-specimens, and TR-specimens were the lowest. The increase of loading-point distance in four-point bending aggravates the degradation of load-bearing capacity. There exists a critical crack depth, and the load-bearing capacity decreases significantly only when the crack depth exceeds the critical value; the larger the loading point distance, the smaller the critical crack depth. These findings offer a scientific basis for damage assessment and safety control of timber structures with longitudinal cracks.