Bending moment capacity of double mortise and tenon joints under diagonal tension and compression

This paper presents an experimental study on both individual and combined effects of tenon thickness and tenon length on the bending moment capacity of double mortise and tenon (M&T) joints under diagonal tension and compression loads. In total, 90 joint specimens were fabricated of Oriental Beech (Fagus orientalis L.) and tested, with three tenon thicknesses (6, 8, and 10 mm) and three tenon lengths (25 mm, 35 mm, and 45 mm) as variables. Results showed that increasing tenon length led to a higher average bending moment capacity under both loading conditions, with statistical significance observed in most groups. In contrast, increasing tenon thickness resulted in a lower average bending moment capacity, although the statistical significance of this effect varied depending on the tenon length. This result was attributed to the thickness reduction of the mortise walls due to the increase in tenon thickness, which can make the joints prone to premature failure of the mortise walls. Among the two variables, tenon length strongly influenced the bending moment capacity more than tenon thickness. Mode I failure of the mortise walls was the primary failure mode under diagonal compression, while bond line failure was predominant under diagonal tension.