Main Seismic Failure Mode Analyses And Optimization Of Reinforced Concrete Frame Structures

Earthquake is one of the worst natural disasters that human beings face and seriously threatens the survival of humans and development of the society, due to its strong randomness, unpredictability and wide-range involvement. In recent years, many building structures are heavily damaged during the frequently-occurred earthquakes, causing serious casualties and economic losses. Furthermore, the seismic hazard level for the future potential earthquakes is actually unpredictable due to its randomness, which raises higher requirements for the better seismic performance of structures. Conventional structural seismic design is based on the equivalent static force procedure and elastic analysis method, which could not take the post-yield behavior of structures and failure mechanism into account. As a result, the materials along the structure height are not fully exploited and unpredictable and unexpected failure modes, such as the local failure mode and soft-story failure mode, usually occur during strong earthquakes, which cannot makes the structural seismic performance maximized and the structures cannot effectively resist the future potential earthquake hazard. Therefore, it is very meaningful and important to enhance the seismic performance by analyzing and optimizing the structural seismic failure modes. In this dissertation, reinforced concrete(RC) frame structures are mainly employed to study the analysis of main seismic failure modes, uniformdamage based design and design method for the optimal seismic failure mode, and the following issues are investigated:(1) Structures have many seismic failure modes due to the randomness of earthquakes, and Pushover analyses with multiple load patterns can identify the main seismic failure modes and the corresponding failure paths. The components located at different positions in the failure path have different damage, and usually the first few members in the failure path have the highest level of damage. Considering the failure probability under multiple load patterns and the failure paths, a procedure for strengthening some local components is proposed to improve the structural seismic failure modes, and the effectiveness of the proposed method is verified by a case study.(2) Based on the phenomenon that the seismic demands of structures under strong earthquakes are not uniformly distributed along the height, the damage of structures are concentrated at some local stories and the material potential is not fully exploited, consecutive modal Pushover analysis is used to obtain the structural seismic demands. Setting the uniform distribution of interstory drift ratio as the target function, using the reinforcements of beams and columns as the design variables and employing the constraints of constant cost and maximum/minimum reinforcement requirements, an optimization design procedure is proposed based on the framework of static Pushover analysis. The effectiveness of the proposed approach in controlling the story drift, component rotation and energy-dissipating mechanism is studied.(3) Considering the influence of different seismic input on the structural seismic failure modes, an optimization design procedure is developed to consider the seismic input of a single ground motion and multiple ground motions. The procedure employs the minimum of interstory drift ratio as the target function and considers the component section sizes and section longitudinal reinforcements as the design variables. Meanwhile, the constraints of maximum/minimum reinforcement ratio, stiffness ratio of beams to columns and the increase of material cost are considered into the optimization procedure. The strengthening strategy of hinged beams and columns is proposed based on their damage levels. The optimization efficiency for a single ground motion input and multiple inputs, the reduction of story drifts and the effectiveness of the proposed method applied to different seismic intensities are investigated.(4) Using the Chinese Design Code Spectrum as the target spectrum, the endurance time acceleration functions(ETAFs) are generated using the unconstrained nonlinear least squares method. The seismic response of elastic single-degree-offreedom(SDOF) system and nonlinear SDOF system are investigated by using the ETAFs as the input, including the dependency of damping ratio, displacement demands and seismic input energy analysis. The seismic response of RC frame structures are systematically studied for different seismic hazards, including roof displacement, base shear, maximum interstory drift ratio and hysteretic energy, and these seismic demands are compared with the results of a scenario of 22 real ground motions. In addition, the distributions of interstory drift and story shear for severe seismic hazard level are studied. The probability of occurrence for plastic hinges and the values and distribution of beam and column rotations are investigated, and the feasibility of structural failure mode analysis using ETAFs is discussed.(5) Using the “strong-column weak-beam” global failure mode as the design target mode, and considering the seismic input for different modes and the influence of hysteretic behaviors of different structural system on the energy-dissipating capacity, a modified energy balance equation is proposed. Based on different hysteretic models, the hysteretic energy modification factor is developed to consider the reduction of energy-dissipating capacity. The effectiveness of current plastic design method is evaluated, and on this basis, the energy balance-based plastic design method is developed to control the structural failure mode. The proposed method is applied to RC frame structure and compared with the code-based design structure. The proposed method is applied to RC frame structures with different configurations to investigate the effectiveness and feasibility of achieving the “strong-column weak-beam” global failure mode.Since the conventional code-based design method cannot guarantee the “strongcolumn weak-beam” global failure mode, the columns are redesigned using the column tree method to achieve the global failure modes, based on the premise that the beam section properties are kept in accordance with the code-based design results to represent the seismic hazard level and vertical gravity load. The proposed procedure is applied to two RC frame structures originally designed by Chinese Seismic Design Code, and the analytical results show the proposed practical procedure can achieve the “strong-column weak-beam” global failure modes.(6) To fully exploit the stable energy-dissipating capacity of buckling restrained braces(BRBs), BRBs are configurated into the RC frame to develop the dual system. To develop the “strong-column weak-beam” global failure mode of the dual system, the energy balance-based plastic design method developed in the above(5) is extended to this system. Firstly, the total system is decomposed into a bare frame system and a bare BRB system, and the yield displacement of the total system can be derived. Then, the BRB configuration along the height can be determined based on the design base shear calculated based on energy balance. Finally, the section design of beams and columns can be conducted based on the applied force demands from BRBs and plastic forces of the frame. The design procedure are applied to two structures and the analytical results illustrate the proposed design procedure can achieve the “strong-column weak-beam” global failure mode, which provides significant reference to applying the energy balance-based plastic design method to other dual structural systems.