Shake Table Tests Of A Self-centering RC Frame Structure Under Mainshock-aftershock Ground Motions

Many earthquake damage surveys at home and abroad have shown that structures designed following current norms are severely damaged after an earthquake,often resulting in huge repair and reconstruction costs.The self-centering structural system has attracted more and more attention due to its conceptual design with small residual deformation after the earthquake and light structural damage.Since the self-centering reinforced concrete(RC)frame structure has been proposed in the Precast Seismic Structural Systems(PRESSS)program,the research focus has shifted from the seismic performance of the self-centering joint to the overall frame structure.At the same time,the additional seismic energy dissipation devices have evolved from built-in mild steel to replaceable external dampers,such as mild steel(MS)dampers,friction dampers.At present,due to the limited research on the seismic performance of the self-centering structure,and lack of practical design method of the structure,the self-centering frame structure is rarely used in practical engineering.In the event of an actual earthquake,there are often many aftershocks occurring shortly after each main shock.In the case that the external damper is too late to be replaced,the seismic performance of the self-centering frame structure needs to be further studied,but there is no such aspect research until now.In this paper,to investigate the seismic performance of self-centering RC frame structures,the shake table tests of a three-dimensional two-story single-span self-centering RC frame structure(1/2 scale ratio)were completed under mainshock-aftershock ground motions at various hazard levels.The entire frame structure includes three types of self-centering joints: column-base joint,beam-column joint with a top hinge(structure Y-direction)and common beam-column joint(structure X-direction).The unbonded post-tensioning(PT)tendons were used to provide the self-centering capability for the joints.The external replaceable MS dampers were installed to the first two types of joints to dissipate seismic energy,while the angle steel was adopted at the common beam-column joints.Two ground motions were selected from the PEER NGA-West2 strong ground motion database: Morgan Hill ground motion(MH,Class IV Site),San Fernando ground motion(SF,Class II Site).Each ground motion consists of two horizontal components and assuming that the horizontal component with larger PGA is the main direction.The test inputs two-way seismic waves,and the amplitude ratio of the main direction to the perpendicular direction is 1:0.85.The acceleration amplitude is increased step by step.Under each amplitude,the main direction ground motion is first input along the ordinary beam frame(structure X direction)and then input along the top hinge beam frame(structure Y direction).After each main-shock,the ground motions with small PGAs were applied twice to simulate the effects of aftershocks on the structure.The experimental phenomena of the structure under each mainshock-aftershock ground motions are recorded and detailed.Finally,the test results are processed and analyzed,including structural dynamic characteristics,floor displacement relative to the base,interstory drift,floor acceleration,joint opening rotation and the response of the PT tendons and MS dampers.By the above results,the following conclusions can be obtained:(1)The floor displacement and interstory drift increase with the amplitude of ground motions increasing,and the interstory drifts are linearly distributed with the structural height.Under the very rare earthquakes(VRE),the maximum interstory drift reaches 2.4%.At this time,the damage of the mainframe is not severe,and only a large number of micro-cracks visible to the naked eye are generated on the concrete beams and columns.The residual deformation of the structure is relatively small,and the structure can be self-centering.The structural displacement responses on the Class IV site(MH ground motion)are greater than that of the Class II site(SF ground motion).(2)The joint opening rotation increases with the amplitude of ground motions increasing.The value is approximately equal to the interstory drift in each direction.The structural deformation is mainly concentrated at the opening position of the joints,and the joints have good self-centering characteristics at the same time.(3)As the joint opening displacement increasing,the increment of the prestress increases gradually.The prestress loss occurs during each mainshock-aftershock loading process,which causes the initial stress decreases to some degree.During the loading process,the PT tendons don't reach the yielding limit state.The MS dampers at the joints are also working at the same time.The dampers are basically in an elastic state under middle earthquakes,and just reach the yielding state under large earthquakes.The dampers haven't been broken under the very rare earthquakes.(4)Due to the structure being not seriously damaged,the times of aftershocks following each amplitude of mainshocks has no obvious influence on the displacement response and acceleration response of the structure.But the end of MS dampers at the beam-column joints with the top hinge is more seriously damaged.