For structures with the seismic force resisting system, the linear - inelastic behavior of the structural system under the design earthquake is taken into consideration. Although it is different for each type of steel structrual system, the system is aimed to reach the most appropriate mechanism. Accordingly, linear-inelastic deformations are only allowed in certain members. The structure is dimensioned according to this behavior model.
In moment resisting steel frames, these members are beams. The regions defined for the plastic hinges are certain regions at the ends of the beams connected to the columns.
These parts, which are called as the protected zones, absorb energy with linear - inelastic behavior, while the other parts must remain with linear - elastic behavior.
Column must be stronger than beams in order to prevent brittle collapse, except for the intended mechanism.
In the figure (a) the sway mechanism is the preferred system, while (b) the soft story mechanism is not desired. As can be seen in figure (b), when the hinges are formed at the ends of the columns, this is a situation that leads to brittle collapse.