Friction between wood and steel in the embedment area of timber connections

According to Eurocode 5, the rope effect in nails is limited by the axial withdrawal resistance and the friction between the connected members at the shear plane. However, at high load levels, gaps can form between the fastener and the surrounding wood in the embedment area, reducing the effective contact area. Recent studies have highlighted that friction between the fastener and the wood surface can become the limiting factor for the rope effect. This study aims to experimentally quantify this friction, providing parameters that can enhance existing models and contribute to more accurate predictions of the lateral load-bearing capacity of timber connections. A test setup was developed to measure normal and friction forces simultaneously, allowing for the determination of the coefficient of friction (). The experimental testing involved two sets with 281 specimens, focusing on critical factors including loading protocol level, nail surface condition, nail diameter, contact length and wood moisture content. The wood materials tested included softwood (C24), laminated veneer lumber (spruce) and oriented strand board. The tests were conducted using three types of nails: smooth, corroded and profiled. The results show that the -values range from 0.107 to 0.804, depending on the surface conditions of the materials. The impacts of different key factors are also discussed in detail.