When 1 is not long,the whole rigid body is in the well proportioned field of gravity and the whole body′s center of gravity,as the center of mass,is in the middle ofthe rod.

当l不太长 ,整个刚体处于均匀重力场中 ,其重心位于轻杆的中点 ,与质心相重合。

First, there exists torsional component for the motion of ground or phase delay for the different sites of ground under earthquake. Second, structure has eccentricity for the inconsistency between the center of mass and stiffness.

引起扭转振动的原因，一是地面运动存在转动分量，或地震时地面各点的运动存在着相位差；二是结构本身存在偏心，即结构的质量中心与刚度中心不相重合。