Seismic analysis and response of multistory semirigid frames

The responses of multistory flexibly connected frames subjected to earthquake excitations are studied using a computer model. The model incorporates connection flexibility as well as geometrical and material nonlinearities in the analyses. Connections are modeled as rotational springs with bilinear hysteretic moment-rotation relationships. Geometrical nonlinearities in the form of member and frame P-delta effects are considered by the use of stability functions in a stiffness formulation. Material nonlinearity in the form of column inelasticity is accounted for by the use of the tangent modulus concept. Response characteristics of two multistory frames with three connection types (rigid, semirigid and flexible) subjected to two earthquakes are studied with reference to their modal response attributes. The study indicates that connection flexibility tends to increase upper stories' interstory drifts but reduce base shears and base overturning moments for multistory frames. Connection flexibility also causes the frame periods to spread over a wider spectrum and increases the importance of higher mode contributions to structural response. When the Complete Quadratic Combination (CQC) rule of spectral analysis is applied to flexibly connected frames, it is found that noticeable errors may be produced.