Abstract
The damage of steel moment resisting frames (MRFs) with exposed-type column bases which have weak anchor bolts concentrates to the upper end of columns and/or to the beam ends of the first story under seismic excitations. This happens because the exposed-type column base with weak anchor bolts shows slip-type restoring force characteristics which decreases the energy dissipation in comparison to when the bottom end of columns of the first story is yielding. In current Japanese seismic design of the weak column base type for steel MRFs, Ds value of the firsts story has to be increased to 0.05 to reduce the damage concentration at the first story. However, it remains to be clarified whether the increase of Ds value of the first story to 0.05 is suitable or not in various cases. In this paper, a series of earthquake response analyses of plane frames weres carried out to evaluate the effect of different Ds values of the first story.
