Seismic Design of Steel Column Tree Moment Resisting Frames 97

One of the most common types of steel structural system is the moment resisting framing
system shown in Figure 1.1. Depending on their ductility, steel moment resisting frames
are divided into two categories of "Special" and "Ordinary". Figure 1.2 shows typical
behavior of Special and Ordinary moment-resisting frames under lateral load. Special
moment-resisting frames are designed to have higher ductility and be able to deform
inelasticly during earthquakes. Such inelastic ductile deformations increases damping and
reduces stiffness of the structure resulting in smaller seismic forces generated in the
structure. As a result, current codes allow special moment resisting frames to be designed
for smaller seismic forces than similar but ordinary moment frames.
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