In this study, we tested for identifying dimensional effects of rotational stiffness of wedged and halved scarfed joint. At first, a model of stress distribution in bending was made from observation of joints fracture under four-point bending tests. Proposal of rotational stiffness design formula(M-Θ) was derived from this model. Under the tests, M-Θ equation values well corresponded to all types of experimental ones. In particular, rotational stiffness rose in proportion to the cube of the materials height and the cogging width and had asymptotic limit value. These tendencies corresponded to the experiment results.
