Study On Failure Mechanism Of Reinforced Concrete Frame Structure's Plastic Hinge By Rare Earthquake Action

Reinforced concrete frame structure has been generally welcomed and occupied a large proportion in our country and all over the world for its simple structure, clear transmission path, flexible building plane layout, and so on, There is great practical significance to study the damage in the rare earthquake action and the problem might occur. It's found that "beam plastic hinge mechanism" of reinforced concrete frame structure requested in seismic design requirements didn't appear, but abundant "column plastic hinge" appeared in the survey on damage from the WenChuan seismic. Therfore, "strong column weak beam" haven't achieved. "Strong column weak beam" defined in Chinese"Seismic Design Code of Building" GB 50011-2001 means that, inequality∑M cay >∑Mbay is got between the actual flexural capacity of beam-ends and this of column-ends at a node. However, beam-ends flexural capacity increases due to infilled wall, floor, beam-ends distribution over steel and so on. So that, "Strong beam weak column" (∑M cay <∑Mbay) is formed at rare earthquake action. A lot of research institutions and scholars at home and abroad have conducted a great deal of research, most of the conclusions indicate that different column-ends flexural moment enhancement coefficient adopted, in order to ensure that the structure formed "beam plastic hinge mechanism" or "beam and column mixed plastic hinge mechanism" in the rare earthquake action. While the author of this article believes that there is a minimum stiffness matching of column and beam to ensure the realization of "strong column weak beam."By comparing the three-dimensional finite element model, the yield surface model, concentrated plastic hinge model, the distribution of plastic hinge model and the advantages and disadvantages of fiber model, combined with the distribution of plastic hinge and the fiber model of the respective advantages of the establishment of local fiber model to a high precision fiber model Distribution of plastic hinge model of plastic parts, plastic paragraph to improve the yield, the simulated hysteresis accuracy; used to give full consideration to the materials section of the work of state and loading history of the concrete and steel constitutive model, and incremental variable stiffness based on the framework established Dynamic elastic-plastic finite element analysis model. In the MIDAS/GEN analytical procedures, the framework of bar simulation using beam element, the use of fiber model the definition of multi-axis hinge beam element hysteretic model, the distribution used to provide the type of hinge beam element distribution over the definition, the number of integral points selected as 5, the accuracy can meet the analysis requirements, and reducing the enormous amount of calculation.In order to reflect different strainedcondition of the edge and the middle deformation state of the framework, this paper selected a 5×3 cross-typical 6 layer framework, in accordance with the existing "Code for Seismic Design of Building" (GB 50011-2001), "Code for Design of Concrete Structures" (GB 50010-2002) for conventional design, the corresponding measures is taken in design process to avoid beam bending capacity of super-real.Elastic-plastic dynamic time-history analysis program MIDAS/GEN is used as a platform, beam element is used to establish three-dimensional finite element models firstly, and enter the corresponding load; followed by the definition of concrete fiber and steel fiber, enter a concrete and steel strength, elastic modulus and other parameters; divided column and beam fiber cross-section, and define the distribution of plastic hinge of column and beam; thirdly, enter the time-history analysis data, including the Rayleigh damping coefficient, the two natural and one artificial seismic waves in accordance with code and duration of seismic waves; finally, the history analysis.Post treatment is made through the MIDAS/GEN analysis results. Then interlamina- tion angle of displacement, the distribution and the degree of rotation of plastic hinges, and hysteresis curve of beam-ends is summed up, distribution rule of interlamination angle of displacement and local reaction rule of member bar in the effect of rare earthquake action are obtained, and the resufficient and necessary condition of "strong column weak beam" from stiffness matching among members is achieved.Conclusion:1,In earthquake intensity region 7 which design basic acceleration of earthquake is 0.10g, column plastic hinge mechanism is formed in rare earthquake action, and mostly is biaxial yielded, with larger plastic hinge rotation degree while the number of beam-ends plastic hinge lessen. The main reason is: the bottom steel in the middle beam extends into support, which makes beam-end's steel over than needed. "Beam plastic hinge mechanism" is formed in rare earthquake level ground motion through the column is enhanced (cross-section, reinforcement). Structure bearing capacity is increased and beam plastic hinge rotational angle is lessen due to structure globality extra strength.2,0.20g District 8 degrees by conventional design, rarel of earthquake leve ground motion is similar to 7 degrees formed under the column hinge mechanism zone, mainly due to: 8 degree earthquake zone larger effect, coupled with the lower part of steel beam beam-ends distribution over steel. "Beam plastic hinge mechanism" is formed in the rare earthquake level ground motion, through the column is enhanced (cross-section, reinforcement), and beam plastic hinge rotational angle is larger than conventional design.3,9 degrees 0.40g district framework, because a more stringent norms of the comprehensive control measures is adopt, an ideal body beam plastic hinge can be formed.4,From 7 degrees 0.10g, 8 degrees 0.20g District 6-storey reinforced concrete frame columns, cross-section adjustment process, it is seen that, with rare earthquake level ground motion input, when cross-section column is smaller, plastometric set structure mainly of column plastic hinge is formed; with column earthquake-resistance level improved, means column strength increased, the plastometric set structure of beams and columns mixed plastic hinge is formed. With column cross section Increased (now part of column reinforcement is decided by constructional reinforcement), when column and beam's stiffness ratio reached a certain value, the plastometric set structure of beams and columns mixed plastic hinge can be formed too; with the column minimum reinforcement ratio increased, the plastometric set structure of beams plastic hinge can be formed. Which can be drawn, frame column stiffness increases with increasing intensity can change the form of frame damage. Reinforcement in a reasonable range between column and beam column stiffness ratio and the smallest total cross-section of vertical reinforcement ratio, the framework is to ensure the formation of beam plastic hinge deformation institutions necessary and sufficient conditions. Recommendation 7 degrees column and beam framework 0.10g area than the wish to control the stiffness of more than 4.76, 8 degrees 0.20g framework column and beam. District wish to control the stiffness ratio above 2.91, and the column appropriate to the smallest total cross-section steel longitudinal reinforcement ratio increased by one grade earthquake.