Strength of Wooden Truss Connections with Nail Plates Under Cyclic Humidity Changes

Metal-plate-connected (MPC) joints govern the stiffness and load-bearing performance of many prefabricated timber roof trusses, yet their response to repeated moisture changes remains critical for serviceability and durability. This study evaluated five continuous C24 Norway spruce reference specimens and fifteen specimens joined with GNA20-MIT nail plates using sequential four-point-bending stiffness measurements and wetting–drying conditioning. Test I was used as the initial stiffness stage, whereas Tests II and III followed successive 24 h water-immersion and 6-day natural-drying intervals; the specimens were subsequently tested to failure. The mean apparent modulus of elasticity of the MPC specimens decreased from 1.39 to 1.22 GPa (approximately 12%), but a Friedman repeated-measures test did not show a statistically significant stage effect (χ2(2) = 4.13, p = 0.127). Because the same specimens were repeatedly loaded, and no unexposed MPC control group was included, this change cannot be attributed exclusively to moisture cycling. In the primary analysis retaining all 15 MPC specimens, the mean apparent bending strength of the connected elements was 16.92 MPa, compared with 34.14 MPa for the structurally different continuous reference specimens; excluding M7 yielded 17.87 MPa only as a sensitivity analysis. Failure of the connected specimens was progressive and dominated by plate slip and partial spike withdrawal, whereas solid specimens failed more abruptly in bending. The results therefore support attention to connection flexibility and serviceability under variable environmental and loading histories, while further controlled testing is required to isolate the specific contribution of moisture cycling.