Probabilistic Parameters of GL24h Glued Laminated Timber: Experimental Assessment, Statistical Significance, and Consequences for Component Reliability

Probabilistic models of glued laminated timber are a required input to structural reliability analysis, and the JCSS Probabilistic Model Code is the reference most analyses adopt without independent verification. This paper reports an experimental programme of 61 small clear-wood specimens of GL24h glulam produced in Croatia, cut from three parent elements from one producer, delivered together as a single lot, covering bending, tension and compression parallel to grain, shear of wood and of the adhesive line, compression perpendicular to grain, and finger joints in tension and bending. All eight strength properties are tested against the reference model with confidence intervals and a family-wise error correction across the eight comparisons. Exactly one deviation survives: the coefficient of variation (COV) of tensile strength parallel to grain is 0.339 against the reference 0.180, with a Holm-adjusted p = 0.0064; no other property is distinguishable from the reference. The measured density COV of 0.026, against a reference value of 0.10, shows directly that the specimens are a cluster sample from three parent elements rather than independent draws from the GL24h population, which makes the elevated tensile scatter the more notable, since clustering suppresses observed variability rather than inflating it. The reliability consequence is then shown to depend on an assumption usually left implicit. Holding the characteristic value at the declared 19.2 MPa, as grading enforces, the reliability index of a tension-critical element falls from 3.30 to 3.09; holding the mean constant, it falls to 2.41. The absolute values depend on the load-duration class assumed for snow, but the contrast between the two treatments does not. Carrying the sampling uncertainty of the COV estimate through the calculation lowers these to predictive values of 2.88 and 1.82. The conclusion is correspondingly narrow: elevated tensile variability matters for the assessment of existing structures, where the mean is measured rather than declared, and for reliability studies that adopt generic parameters and vary only dispersion; it matters little where grading against a fractile is active.