Impact of higher modes and inelastic behavior on the equivalent static loading of steel frame

In the equivalent static analysis (ESA) procedure the response of a structure is determined based on the first vibrational mode and elastic spectrum acceleration. In this procedure, the assumption of not contributing all vibrational modes in a structure behavior and using a constant behavior factor for a specific structural system may lead to an incorrect estimation of story ductility demands in intermediate- and long-period structures. In this thesis, in order to take into account the effects of higher modes in the linear and nonlinear response of steel moment resisting frames (MRFs), concentrically braced frames (CBFs) and eccentrically braced frames (EBFs), the equivalent static base shear correction factor was estimated. In linear regime, the base shear correction factor is determined as the ratio of spectral base shear to the ESA base shear. These factors were determined for the studied systems located on soil types I, II, III and IV. The obtained results for elastic regime showed that the correction factor depends on the spectral shape and the ratio of modal periods. The effect of higher modes increases as the ratio of the periods of second mode to first mode decreases and also increases as the spectral acceleration drops more rapidly with an increase in period. For inelastic regime, the base shear correction factor was determined using nonlinear time-history dynamic analysis and adjusting story ductility demands to a target value based on an iteration procedure. To this end, some 3-. 10- and 20-story steel MRFs, 7-, 20- and 30-story CBFs and 4-, 15- and 25-story EBFs on soil types I and II were designed. The inelastic regime results showed that applying the base shear correction factor takes into account the effects of higher modes as well as limits story ductility demands to the target ductility.