THE EFFECT OF JOINTS ROTATIONS ON THE NATURAL PERIOD OF BUILDING FRAMES

In conventional dynamic analysis of building frames under lateral loading the rotations of joints are neglected to simplify the calculation of the stiffness matrix. In this paper a methodology is presented in which consideration of rotations and the change of angle between columns and beams at beam-to-column connections becomes possible in calculation of the stiffness matrix and the natural period of the structure. To avoid the excessive errors due to omission of joint rotations, it is suggested that the stiffness matrix of frames with flexible girders should be calculated using a method of averaging the values of every three stories.