The Research On Reinforcement Optimization Of Reinforced Concrete Member Under Complex Stress State

Designs of the reinforcement layouts of reinforced concrete (RC) members under complex stress state are always difficult. Compared with the previous proposed methods based on experiments, the stress methods are becoming more and more popular in these years. This is a great progress for the stress methods are more consistent with mechanical concept. However, there are still many problems exist in the stress design methods for their immaturity. In order to solve these problems and promote the design method of RC members under complex stress state from empirical and semi-empirical method to elastic stress method, then to elastic-plastic stress method, a more reasonable and convenient design method based on the existing genetic evolutionary structural optimization (GESO) is proposed. At the same time, various aspects of performance of the new method are also studied herein. In this thesis, the main contents are as follows:The advantages and disadvantages of present GESO, which is based on analysis of integrated elements model, are discussed according to some existing tests results of deep beam with openings. The integrated elements model GESO can be used to construct topological structures, but several questions are still exist. For example, the analysis results are inaccurate since the linear elastic analysis and ignored nonlinear stress features of RC structures; the process of establishing a simplified strut-and-tie model (STM) is subjective for the topology visualization is always not very clear; the centralized reinforcement layout in the final design will result in unsatisfactory failure mode. Therefore, a new application of GESO method, specifically a separated elements model GESO, is proposed in this thesis, which promotes the GESO develops from elastic stress method to elastic-plastic stress method. The new method considers the influence of material nonlinearity and reduces the subjective factor of the conversion, so the precision and rationality of analysis are improved. It is extended from a single load case to multiple load cases and the optimization can be completed based on stress envelope values, so the practicability is also improved and the method is closer to the needs of engineering designs.In the thesis, some members under complex stress state, such as deep beams, shear walls, corbels and so on, are considered as numerical examples to complete the corresponding topological construction through separated elements model GESO, which is compared to sorts of existing methods in terms of design steel consumption. Accordingly, the stability, feasibility and economy of the separated elements model GESO have been preliminarily verified. Through the analytical analysis of ATENA nonlinear finite element (FE) simulation, the comparison in the performances of ultimate bearing capacity (UBC), deformation, cracks, failure mode of shear walls designed by empirical method, elastic stress method and separated elements model GESO respectively, are investigated. The contrast static test of 4 shear walls with openings are studied, on one hand, the UBC, ductility capacity, stiffness degradation, reinforcements stress state and failure mode of shear walls designed by separated elements model GESO are discussed; on the other hand, the comparison is made between the specimens designed by elastic stress method and separated elements model GESO, which is an elastic-plastic stress method. At the same time, the results provide the experimental basis for the research on the mechanical properties and designs of RC shear walls. Thus the reliability and superiority of the stress methods, especially that of the separated elements model GESO as a new method, are further proved.At last, the stain and crack data recorded in the static test are analyzed. Accordingly, several problems about the stress redistribution, changes of load transfer paths with the damage in members under complex stress state, the effects of the UBC when the internal stress fields change, the relation between failure modes and changes of load transfer paths of members designed by different methods, are discussed. The research demonstrates that designers should ensure the load transfers smoothly and the load transfer path changes moderately, when they are designing members under complex stress state. It means that cracks and damage of members should be controlled by reinforcement in moderation. If the control is too loose, the load transfer path might be narrowed down rapidly which results in premature failure; if the control is too strict, the stress distribution might drastically change which leads to an unpredictable brittle failure. In the test, the specimens designed by elastic stress method happened brittle failure finally, because it always generates a centralized reinforcement layout and the crack are controlled too strict to develop, The separated elements model GESO as an elastic-plastic stress method result in a more scattered reinforcement layout, so the cracks of the specimens designed by it could fully develop, the corresponding load transfer path might be narrowed down moderately and failure mode could be expectable. In this way, the ductility capacity is improved at least. Therefore, it is very necessary to promote the development from elastic stress method to elastic-plastic stress method.In a word, the separated elements model GESO was proposed to design the members under complex stress state. At the same time, large sized shear walls with opening were tested, which are combined with nonlinear finite element analysis (FEA) to verify the new method. Based on the discussion of load transfer paths, the damage characteristics of members under complex stress state and how to use the new method to improve the current method are studied at last.