Conventional expressions for the strength of T -joints do not allow for the interaction between chord bending and the local joint strength. Using finite element methods a series of diamond bird beak T -joints were analysed, where chord length, width ratio and chord slenderness are all varied. Using these results, this paper develops an idea by G.J. van der Vegte and adapts it to diamond bird beak joints so that an interaction contour is produced which relates the local joint strength and the moment capacity of the chord.
The bird beak is still a relatively new joint system in which the chord and maybe the brace have each been rotated by 45o about their own centre line axis. It should be recognised that while the second moment of area of the square cross section is the same about any axis through the centroid, the elastic and plastic moduli are decreased for bending about a diagonal, compared with the conventional arrangement. Previous work on bird beak X-joints (Davies and Kelly 1995) has shown how dependent the joint strength is on the chord length and the restraint imposed at the ends of the chord. Physical testing of diamond bird beak T-joints was carried out and reported by Ono et al (1991) and Ishida et at (1993). The physical limitations of testing requires a short chord length, so that the failure observed is a local failure of the joint, rather than of the chord. End plates also have to be provided at the ends of the chord, to accommodate the reaction, caused by a compressive loading on the brace, to prevent a local support failure of the chord. These conditions also effect the strength of the joints tested. Under present design procedures, the bending moments in the chord are not considered when assessing the design strength of the Tee joint.
