ABSTRACT
The concentrically-braced steel frame has commonly larger stiffness than pure moment frames to limit the lateral deformation. This type of structures, however, is not so popular in Japan. One reason is the architecture limitation, and the other is that they are often implemented by the Japanese seismic design code (BCJ 1997) with a lower ductility, which considers the strength degradation associated with buckling of compressive braces. Consequently, the braced frames are commonly designed with larger sections, thus economically inefficient. The Japanese seismic design code employs the seismic force reduction factor (also called the structural characteristic factor and denoted as Ds) to reflect the trade-off between the strength and the ductility demand. It is selected in a range from 0.25 to 0.5 in accordance with the ductility type of backup frames, the participation ratio β (defined as the seismic force resisted by braces in one story over the strength of the story) and the slenderness ratio λ of braces, which is deemed practically inconvenient.
