Seismic Performance Study On The Brace Connector In Anchor Block Type Based On Hybrid Controlled Experimental Method

At present a large number of existing reinforced concrete frame structures(RC frame),which were constructed before 1990,may encounter great safety trouble under the threat of strong earthquake.These structures need to be retrofitted to improve the overall seismic performance,and to meet the basic requirements in the design codes as "strong column and weak beam" and "no collapse under rarely occurred earthquake".With the rapid development of energy dissipation technology in recent years,the idea of structural seismic design is constantly updating.Embedding energy-dissipated braces have been widely concerned by engineering and academic circles because of the convenient construction,no big change to the structure,low cost and additional stiffness along with significant energy dissipation capacity.However,previous experimental studies indicated that adding embedded braces will attach greater axial force,shear and bending moment to the beam and column near the core area of the existing RC frame,especially under the huge earthquake.The above mentioned additional force may cause big change to the stress state and deformation ability compared with the unretrofited RC frame,which lead to the decrease of ductility and the unexpected failure of braces,even structure collapse.Moreover,the complex control requirements of quasi-static and pseudo-dynamic tests can not be satisfied by the traditional single-loop control method of hydraulic servo actuators,which make it difficult to simulate the muti-dimensional boundary condition accurately.Focused on the problems above,this paper mainly carried out the seismic performance study on the braced RC frames retrofitted by the anchor block type connectors based on hybrid controlled experimental method.The main research contents and conclusions were as follows:(1)In order to reduce the adverse effects of additional brace force,a new composite structure braced connector in anchor block type,which is termed as anchor block type connector for short in the following paper,was proposed to connect the RC frames and the energy-dissipated braces in the retrofitting project.The construction details was introduced.And in accordance with mechanical analysis and the ABAQUS finite element results,a simplified model of the anchor block type connector was established,and the failure mode was also simulated.(2)A dual-loop hybrid controlled method was proposed to simulate the boundary condition reasonable and accurate in the multistory braced frame tests.Force-displacement hybrid control method for multi-actuators synergic control in quasi-static test and LVDT-outerloop hybrid control method in pseudo-dynamic testwere separately indicated.Furthermore,the implementation of hybrid control method through MTS793 platform and the design method of outerloop PI controller were also studied.A series of tests proved that many complicated control problems in seismic testings can be solved by the proposed hybrid control method,which were very practical and accurate.(3)To compare the seismic performance between brace frames connected by anchor block and steel anchor plate,three 1/2 reduce scaled two-layer one-span specimens were constructed.One of which was unretrofited,one was retrofitted by anchor block type connectors and BRBs,and the other was retrofitted by traditional embedded steel anchor plates and BRBs.By using the hybrid test platform named HyTest,the specimen above were tested by the pseudo-dynamic methods under frequently-occurred and rarely-occurred earthquakes,and the actuators in the test were controlled by LVDT-outerloop hybrid control method.The test results showed that the unqualified RC frame retrofitted by anchor block type connectors and BRBs can achieve the requirements that elastic in frequently-occurred earthquakes and no collapse in rarely-occurred earthquakes.HyTest can execute the muti-degree of freedom hybrid testing of RC structures perfectly.The LVDT-outerloop hybrid control method was an important technical guarantee for accurately carrying out the pseudo-dynamic test.The global seismic performance of braced frame retrofitted by the anchor block type connectors and steel anchor plate type connectors were almost the same.(4)For the sake of further study on the performance of brace connector in anchor block type under large inter-story drift ratio,and to explore the failure mode of the tested braced frames,and to reveal the effect of anchored block type connector on reducing the additional force of brace-retrofitting,quasi-static testings were carried out to the three pre-damaged specimens under hybrid control method.The hysteretic behavior,stiffness and strength degradation,ductility capacity,energy dissipation,damage phenomenons,BRB performance and the strain results were compared and analyzed.The experimental results showed that the anchor block type connector can better protect the existing RC frame joints and its nearby column-beam region compared with the steel anchor plate type connector.It was proved that the anchor block type connector can effectively reduce the adverse effect by additional braced forces.The proposed hybrid controlled loading rule can reflect the failure characteristic more close to the real earthquake response.