Plug shear failure in nailed timber connections avoiding brittle and promoting ductile failures

Sammanfattning: Mechanical timber connections can fail both in a brittle and a ductile manner depending on the joint layout. Plug shear failure in nailed timber connections is a brittle failure mode, which shall be avoided in timber structures. This failure mode occurs in nailed connections loaded in tension parallel to the grain, for instance in the lower chord of a timber truss. The failure mode is not described in the Swedish building code of today, but there is an informative annex in the upcoming Eurocode 5 concerning the subject. In the thesis, plug shear failure is studied and a new prediction model developed based on experimental work on nailed steel-to-timber connections. The experiments were short-term experiments and the effect of long-term conditions is not considered. The total amount of joints tested was 86, which together with results from other researchers formed the base for the prediction model. The stress state and the effect of fastener placement were analysed and showed that plug shear failure is initiated at the nail farthest from the free end of timber. Increased spacing between fasteners lessens the risk for plug shear failure to occur, especially the spacing perpendicular to the grain is important. The prediction model is based on structural mechanics and suggests that the plug shear resistance R = blfv, where b is the joint width, l the joint length and fv the shear strength of wood. The shear strength is determined through fv = KAs-0.25, where As = bl and K is a constant. Furthermore, an analysis based on fracture mechanics was performed showing the importance of the softening behaviour of the joint failure. The overall goal with the development of a prediction model is to avoid brittle failures and promote ductile failures. As a tool to promote ductile failures, a measure was developed to aid the designer in designing ductile and strong joints. Recommendations on nail spacing and overall design of the joint are finally given with the aim to avoid plug shear failure.