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In this paper, joint modelling techniques are investigated in a box beam T-joint, which may be viewed as a simplified model of typical vehicle body joints. For low-frequency vibration analysis, joints are typically modelled by torsional spring elements and the importance of reasonable spring rates has been noted in many investigations. The effects of the joint branch lengths on the spring rates are investigated and it is shown that converging results are obtained only with proper branch lengths. We also discuss some facts to consider for estimating consistently the spring rates when the branches of T-joints meet at oblique angles. Finally, a possibility of using short beam elements instead of conventional spring elements to account for the joint flexibility is examined. The consequence of short beam modelling is that the sensitivity analysis on the natural frequencies with respect to the joint flexibility can be easily performed.

Low frequency vibration characteristics of a vehicle are mainly influenced by the stiffnesses of the beam type structures such as pillars and rockers, and by the stiffnesses of the joint structures, at which several beam structures are jointed together. In the early design stage of the car body structure a simple FE model has been used, in which joints are modeled as linear springs to represent the stiffnesses of the joint structures. In this paper a new modeling technique for the joint structure is presented using an equivalent beam, instead of using a spring. The modeling technique proposed is utilized to design optimal joint structures that meet the required vibration performance of the total vehicle structure.