| During an earthquake,plastic deformations in the structures are occurred to absorb the energy released by earthquake.After a violent earthquake,substantial residual deformation may be a huge problem for the repair and reinforcement for the traditional concrete beam-column joints.Fortunately,the self-centering concrete joint is an outstanding alternative to solve the problem.Many researches demonstrated that the self-centering joints can achieve excellent seismic performance and self-centering capacity with little residual deformation.At the same time,they are easier to repair.It is well known to us that the unbonded prestressed tendons are the main source of the self-centering,when the angle-bolt connections are the main contributor for the energy dissipation.However,the significant degradation in the mechanical properties of the steel at elevated temperature are destructive to the joint,especially that of unbonded prestressed tendons.With the rapid development of the modern city,the urban pipe networks become more complex.It means the lager probability of the fire when the earthquake occurs.Hence,it is significant to investigate the mechanical properties of the self-centering concrete joint under the earthquake-induced fire.Furthermore,the assessment of the fire-induced damage to the mechanical properties after fire is the foundation of the reinforcement and the guarantee for the safe reuse of the structures.In order to solve the problem,the thesis is investigated in the following section:(1)In order to obtain the tensile properties and creep properties,a series of tests at elevated temperatures were performed on two representative structural steels Q345 B and Q460 GJ.The application of artificial neuron network(ANN)was proposed to predict the yield stress,ultimate stress,flow stress and creep strain of both steels at elevated temperatures.It provides the practical foundation for the material selection and theoretical analysis of the joint.(2)12 reinforced concrete(RC)joints connected with bolted angle,prestressed tendons,and hybrid connection(both of them)were designed and tested to investigate the influence of the angle thickness,angle with stiffener,bolt diameter,column-bolt gauge,beam-bolt row,grouted beam-bolts,and prestressed tendons under cycle loading regime.Their seismic performances were elaborately investigated from the point of the failure mode and hysteretic behavior such as hysteretic curves,self-centering capacity,skeleton curves,stiffness degradation and energy dissipation per cycle.It provides a strong basis for the subsequent design and analysis of self-centering joints.(3)Based on the experimental results,the finite element modeling was proposed by using software ABAQUS.The obtained results from the modeling were in good concordance with the experimental results.This means the possibility and reliability of the model to predict the hysteretic response of the joints with the corresponding parameters.Then,a parametric study was conducted to delineate the contribution of the initial prestressed force and area of the prestressed tendon,the height and width of the beam,the steel grade of the angle,the initial bolt force of the bolt to the hysteretic response.It lays a foundation for the further theoretical analysis of self-centering joints.(4)The damage and degradation properties of the tests were described using eight damage index models.The applicability of the index models to the semi-rigid joint connected with bolted angles was investigated.Based on the code for seismic design of building in China,the damage indices proposed in the damage models were used to assess the seismic performance of the specimens.The reliability of the specimens with the connections were validated.(5)Based on the experimental and numerical investigations,the self-centering joint was modified by the addition of corbel.Three interior RC self-centering joints with corbel were designed and fabricated.Three loading protocols were prescribed to the three specimens respectively.Firstly,A quasi-static test was conducted to validate the seismic performance of the modified joint.Secondly,the fire resistance was investigated through fire test.Finally,the earthquake-induced fire test was carried out on the remaining specimen to evaluate the seismic pre-damage on the fire resistance of the self-centering joint.(6)Experiment is still an effective method to evaluate the fire-induced damage to structures.After the fire tests,the quasi-static tests were carried out on the two post-fire specimens.The fire-induced damage and the coupling effect of the earthquake and fire induced damage on the seismic response of the self-centering were investigated,respectively.Their failure mode,hysteretic behavior,stiffness,ductility,energy dissipation,and self-centering capacity were examined.(7)The morphology and mechanism of the joints with different connections were analyzed based on the component method.A simplified mechanical model was proposed.The calculation formula of the initial stiffness and ultimate bearing capacity for the joints with different connections were derived.(8)According to the post-fire material characteristic,the damage model was suggested to evaluate the fire-induced damage on the post-fire self-centering joints.The post-fire mechanism characteristic of the self-centering joints were analyzed.On the basis of the simplified model at ambient temperature and post-fire damage model,the calculation formula of the initial stiffness and ultimate bearing capacity were also introduced.In this study,self-centering joints have good seismic performance and selfcentering ability.They can effectively control the damage in the target region with limited residual drift.When the connections are in well fire protection,the fire resistance of the self-centering joints meets the requirements of fire-resistant design code in China.Furthermore,the residual seismic performance of the joints is still excellent after being exposed to multi-hazard environment.In conclusion,the application of the joint is practicable in seismic zone. |
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