Screw Withdrawal Performance of Hybrid Cross-Laminated Timber (CLT) Manufactured from Spruce (Picea abies), Beech (Fagus sylvatica) and Hungarian Poplar (Populus spp.)

Screw withdrawal force is a key mechanical property related to the load-bearing capacity and reliability of mechanically fastened timber structures. This study investigates the screw withdrawal performance of cross-laminated timber (CLT) manufactured from spruce, beech, and poplar, including both homogeneous and hybrid layups. The selected species represent materials with different densities and regional availability in Hungary. A one-component polyurethane adhesive was used for panel manufacturing. Screw withdrawal force was determined using two methods: a universal testing machine (UTM) and a manual portable device (MPD). The highest withdrawal forces were observed in beech-based configurations, while the lowest values were measured for spruce. Poplar-based configurations demonstrated intermediate but competitive performance, exceeding the reference spruce values. Statistical evaluation confirmed a significant effect of layup configuration on withdrawal resistance. The MPD measurements were on average approximately 9% higher than UTM results, indicating a consistent and quantifiable inter-method difference. The results demonstrate that hybrid CLT configurations can be optimized by combining species of different densities and that portable testing methods provide reliable estimates of withdrawal performance. These findings contribute to the understanding of connection behavior in hybrid CLT and support the practical application of semi-destructive testing methods for in-situ assessment.