Restoring Force Characteristics and Axial Deformations of Columns in RC Portal Frames Subjected to the Both Vertical Loads and Horizontal Loads of Alternately Cyclic Reversals

The axial force of columns may undergo a change severely under strong motion earthquakes. This change which might be caused by the overturning moments, higher order modal components of vibration and/or up-down ground motions, may show up remarkably in columns especially at lower story of multi-story building structures. It can be considered that the change of axial force should exert influence on the restoring force characteristics and also on the earthquake response behavior severely during the strong motion earthquakes. The main aim of this experimental research is to obtain the load-deformation relationships and also to take measure of the column length cased by the varying axial forces. A one bay portal RC(Reinforced Concrete) frame of 1/3 scale is used as the test specimen which was designed to simulate the bottom story of one bay middle-low rise RC building structure. It is described in this paper that the restoring force characteristics and axial deformations of columns are influenced significantly by varying vertical loads in large story deformation region. The restoring force characteristics should be evaluated with consideration of P-△ effect for overall deformation region. Two vertical loads and one horizontal load were acting simultaneously on the specimen by means of three loading actuators. The alternately cyclic load reversals were controlled by the computer program to operate accurately. This research has two special features. One is the wide deformation rang to the extent of 6% of story drift angle to be concerned. The other is to adopt not only push down force but also pull out force (axial force ratio of 2.5％) as the vertical loads acting on the column heads