The studies on natural vibration of a skew plane-framed structure and a skew plate have already been reported. In this paper, however, the characteristics of natural vibration are described about a skew plane-framed structure in consideration of the torsional rigidity of its main beams and that with right grillage system. The periods of this natural vibration are obtained by analyzing the vibration of multi-mass system, and various charactristics are indicated to variation with the skew angle θ and the ratio J_Q/J of the moment of inertia of the cross beam to that of the main beam. Generally the vertical inertia, the rotatory inertia and the torsional inertia must be sumed up exactly in the case of considering the torsional rigidity, but in this study, only effect of first term is taken into account since the others are regarded as negligible small in the equation of motion. These periods are an calculated by a digital computer. In the same way, the charactristics without consideration of the torsional rigidity are also obtained. The results are as follows : (1) When the skew angle changes in the case of considering the torsional rigidity, the periods indicate considerable difference, but in other cases those show little difference. (2) The period increases with θ, and it does rapidly when θ≦45°. In other words, this fact is that the vertical displacement increases with θ. Accordingly, the flexural rigidity decreases inversely with θ.
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