Dynamic analysis and response of semirigid frames

The dynamic response of semirigid frames is studied using a computer model. The flexibilities of the connections are modelled by rotational springs with bilinear moment-rotation relationships. Geometrical nonlinearities in the form of member (P-δ) and frame (P-Δ) instabilities are incorporated in the model through the use of stability functions in the formulation of the frame stiffness matrix. Material nonlinearity in the form of column inelasticity is accounted for by the use of the tangent modulus concept in the formulation of the column stiffness relationship. Response characteristics of semirigid frames under free vibration, forced vibration and earthquake excitation are studied. Response spectra are generated for frames subjected to forced vibrations and earthquake excitations. The presence of connection flexibility and P-delta effects tends to reduce frame stiffness, and hence increase the frames' natural periods of vibration. Semirigidity and P-delta effects also affect the magnitude of base shear and the amount of energy dissipation of semirigid frames. The extent of these effects is frequency dependent. Based on the results of this study, implications for semirigid frame design will be discussed.