Exposed column-base had double flag type restoring force characteristics, and it strength, rotational rigidity & hysteresis were affected by the column axial force. That characteristics understand obtain from partial frame experiment. Moreover, the influence of its characteristics on steel structure seismic response examined by time history analysis of steel structure with rotational spring, that rotational spring had double flag type restoring force characteristics.

However, partial frame experiment was no agreement boundary condition of column-base in structure on ground motion, and rotational spring was no agreement influence of axial force. Therefore, seismic response & working stress of exposed column-base with steel structure included many uncertainty about building structural design.

The author discussed exposed column-base behavior and structure response on ground motion, that uses seismic response analysis results obtain from low-rise steel structure with model that can reproduce the column-base behavior.

In this paper, elucidate and estimate for exposed column-base mechanism and effective working stress, frame response. The following conclusions were drawn.

1. Exposed column-base around plastic deformation are affected by variable axial force caused shear force gap of beams of layer direction. As a results, yield element changes around of column-base in column-base strength ratio range from 0.8 to 1.2.

2.Exposed column-base around plastic deformation are unaffected by up-down acceleration of earthquake. On the other hand, gravity acceleration are very important consideration in its understand.

3. Exposed column-base had semi-rigid connect behavi　, if its strength over the column full-plastic strength.

4. A change in rotational rigidity change in variable axial force were few effect to frame response.

5. Shear force of anchor-bolts and shear-key occurs external column and internal column. And its values approximately equal yield shear strength of single anchor-bolt.

6. Maximum of base plate rotation evaluated method shown, obtain from maximum story drift angle and mechanics model of column with rotational spring.