Most of structures are designed on the basis of the strong column-weak beam concept that can tolerate large inelastic deformations without failure under strong ground motions. In general, inelastic deformations of the structure are concentrated in the beams. Post-yield stiffness of a beam is believed to be influenced by the moment gradient. The moment gradients at two ends will be different if the gravity load effect is also considered, and the gradients become dependent on the direction of the seismic loads. Therefore, the effects of gravity loads on the inelastic behavior of beams have to be considered to obtain the inelastic response of structures more accurately. In most studies, however, inelastic response of the structures was analyzed without appropriately considering the effects of the moment gradient due to gravity load together with the seismic load. Therefore, the effect of gravity loads on inelastic seismic response of steel framed structures is investigated in this study. For this purpose, a new plastic hinge model is developed to represent inelastic behavior of beams in a structure considering the gravity load effects.
An idealized structural model is used to represent the inelastic behavior of a beam in steel framed structures and analyzed using the layer model which can consider the shape of cross section and the material properties. From the results of inelastic analysis using the layer model and quasi-static tests, it is found that the moment-rotation curves for beams subjected to gravity and cyclic lateral loads are shifted toward the positive moment direction due to gravity load moments and the shift gradually continues as the amplitude of cyclic inelastic rotation increases. A new plastic hinge model is proposed, which can consider the gravity load effect, particularly the shift of the moment-rotation relationship. Two parameters in the conventional bilinear plastic hinge model are added to consider the shift of the moment-rotation relationship due to the gravity load effect. The initial shift of plastic moment is introduced for the global shift of the moment-rotation relationship. A lower boundary line with a reduced stiffness is also employed for the negative plastic moment to consider the gradual shift of moment-rotation relationship as the amplitude of the inelastic cyclic rotation increases. The properties of the proposed model are derived from the results of the inelastic analysis on the idealized 1-story structure for various cases of the gravity and seismic loads using the layer model.
Inelastic seismic analyses on 4 multi-story steel framed structures are performed for various ground motions and strain hardening ratios using the proposed plastic hinge model. Seismic responses such as ductility demand, story drift, damage index, roof displacement, and base shear are estimated from inelastic seismic responses can be reasonably considered using the proposed plastic hinge model. The results indicate that total inelastic deformations induced in a structure during the seismic excitations are not significantly changed though the gravity load effects are considered in the inelastic analysis. Thus, the effect of gravity load on the maximum roof displacement and base shear which represent global behaviors of a structure is not large. However maximum values of ductility demand, story drift and damage index which represent local behaviors increase significantly when the gravity load effects are included in seismic analysis. From the inelastic seismic analyses of example steel framed structures, it can be known that the effect of gravity load should be considered properly in seismic analyses, because the seismic responses such as maximum ductility demand and story drift may be underestimated.