The Research On Structural Robustness Coefficient Of RC Frames Considering The Catenary Action

As a widely used structure,RC frames may suffer from accidents during service,causing the severe damage or progressive collapse.The process of collapse experiences the beam mechanism(the elastic phase and the arch action)and the catenary action.The catenary action is the last line of defense against the structural collapse.Adopting appropriate construction methods to improve the load bearing capacity and the energy dissipation capacity of the catenary action is an effective way to improve the resistance to progressive collapse of the RC frames.The load-displacement curve,which takes the catenary action into consideration,has the characteristics of double peak points,i.e.the peak point of the arch action and the peak point of the catenary action.The traditional structural robustness evaluation method only evaluates the anti-collapse capacity based on the first peak point,which cannot reflect the mechanism of the structural collapse.The collapse process of structures is a dynamic process.Thus,based on the mechanism of structural collapse and the energy dissipation view,this paper proposes a robustness evaluation method of RC frames considering the catenary action.The research contents and the results are as follows:(1)To investigate the impact of whether the reinforcement bars at the upper part of the beam were cut off on the catenary action,two one-layer and four-span RC plane frames were designed.The structural collapse mechanism test for simulating the failure of the middle column was carried out using the MTS electro-hydraulic servo loading system.The failure modes,load-displacement curves and the strain of the reinforcement bars were obtained.The test results show that whether the reinforcement bars at the upper part of the beams are cut off has no significant influence on the peak bearing capacity and the energy dissipation capacity at the beam mechanism.But it has a great influence at the catenary action.Although the two frames have high bearing capacity and ductility in the catenary action,the peak bearing capacity at the catenary action of the frame,which the upper part of the reinforcement bars are perforated bars,reaches 67.75 k N,which is 46% higher than that of another frame.Besides,the ductility increases by 37%.(2)To further investigate the collapsed mechanism of RC frames,and to lay a foundation for studying the structural robustness coefficient,the ABAQUS finite element model was verified and calibrated according to the experimental results.Then 15three-layer and four-span RC plane frames were established for parameter analysis.The parameters include the failure of the middle column of the different layers,the reinforcement ratio at the bottom of the beams,and the reinforcement ratio at the top of the beams.The results show that the catenary action is more difficult to function when the failure column is at the lower layer.The improvement of the reinforcement ratio at the bottom of the beams has no positive effect on the catenary action.The catenary action of the structure is weakened when the reinforcement ratio is too high.The increase of the reinforcement ratio of the upper part of the beams can significantly enhance the catenary action of the structure.The energy dissipation at the catenary action accounts for at least 70%of the total energy dissipation during the collapse process.Thus,the contribution of the catenary action should be considered when evaluating the structural robustness.(3)To assess the ability to resist the progressive collapse,a concept of robustness is introduced.An energy-based robustness evaluation index considering the catenary action is proposed.The calculation method of the robustness coefficient is introduced in detail by several typical numerical examples.By analyzing and comparing the robustness coefficients of the model,some measures to improve the robustness of the structures are proposed.