Influence of Cracks at the Middle-Height Layer Position on the Load-Bearing Capacity of Timber Beams: A Study Based on Small-Sized Specimens and a Large Sample Size

The anisotropic properties of wood make timber beams prone to developing longitudinal cracks. Notably, cracks occurring at the mid-height position are both highly common and critically detrimental to structural load-bearing capacity. This study focuses on the influence mechanisms of such cracks through four-point bending tests on 860 specimens, finite element simulations, and fracture morphology analyses. By introducing a horizontal crack location parameter called crack eccentricity (e), the influence of cracks at different horizontal positions on timber beam load-bearing capacity was investigated. The experimental results show that the horizontal position of the crack has a “critical eccentricity effect”: when e is below the critical value, cracks will not propagate and will have a minor impact on the load-bearing performance of timber beams; when e exceeds the critical value, cracks will propagate and their harmfulness will increase dramatically. As a special case, specimens with side-opening cracks (e = 1) exhibit “damage saturation” characteristics. That is, when the crack length exceeds the threshold (half of the beam span), regardless of further lengthening, the bearing capacity is unchanged and the damage evolution reaches a saturation plateau. The above analysis results suggest that the influence of cracks on the bearing capacity of timber beams may be counterintuitive. The calculation method for the “crack hazard coefficient” proposed in this study can provide a reference for crack hazard assessments.