| Reinforced concrete slab-column structures as a common structural form are widely used in the actual projects, but the slab-column connections which are considered as the major weak links of this structural system as they are prone to fail in punching shear failure. Punching shear failure is a brittle failure, which occurs with almost no warning signs. In addition, the internal force redistribution will emerge in order to achieve a new stable equilibrium after a local failure, the load carried by the failure connection would redistribute to adjacent connections. Adjacent component or connection which will likely be overloaded once the substructure does not have enough alternative load path or strength reserve, and finally caused partial or total collapse of the structure in a manner analogous to the chain reaction of "domino effect". Over the past decades, several building collapse accidents occurred due to punching shear failure resulting in a large number of human casualties and economic loss in the whole world. These accidents revealed some shortcoming in codes of practice and the necessity of reconsidering punching provision. Therefore, it should be essential to study the collapse resistant capacity and post-punching behavior which can help adopting reasonable structural measures to avoid progressive collapse.The designers hoped that the slab-column structure had an alternative load path under the accidental load similar to the catenary action of longitudinal reinforcement in the frame structure. Thus, design code placed a certain amount of continuous bottom bars through the column which was termed "integrity reinforcements". Integrity reinforcements were provided to give the failure connection some residual strength after punching shear failure. However, there were significant differences in the mechanism between the suspension effect after punching shear failure and the tensile membrane action of the bending plate. Therefore, this paper investigates the post-pucnhing behaviour of reinforced concrete slab-column connections with a goal of avoiding progressive collapse of slab-column structure.1. Post-punching behavior of 8 concentrically loaded slab-column connections with various shear-span ratio, longitudinal reinforcement ratio and integrity reinforcement layout was studied. Some structural measures could be adopted to improve the post-punching bearing capacity and enhance the capacity of structure to resist progressive collapse. Static test method was used to simulate the damage process of interior slab-column connections after the failure of the supporting column. The axial load applied in a sustainable way to estimate the post-punching bearing capacity of slab-column connections after the initial punching shear failure occurred. Based on the test results, the whole damage process and forced state of slab-column connections was analyzed, the conversion process of force mechanism pre and post punching shear failure and the formation process of the alternate load path was also discussed. The test results show that the post-punching bearing capacity provided by the longitudinal reinforcement is insignificant because of the anchorage invalidation at an early stage. The presence of integrity reinforcement plays a crucial role in carrying load after the punching shear failure of slab-column connections.2. Post-punching behavior of 6 interior slab-column connections subjected to punching shear failure in the presence of the extra shear force and an unbalanced moment was presented, which characterized in the study by an eccentric load on the column. In addition, the behavior and failure mechanism of slab-column connections with various longitudinal reinforcement ratio and integrity reinforcement layout the was studied. Meanwhile, a comparative analysis between axial and eccentric loading specimens with the same shear-span ratio was also performed. Moreover, the performance and reliability of the integrity reinforcement were investigated to obtain physical explanations of the post-punching load-carrying mechanism.3. Nonlinear finite-element analysis of the loading process and stress state of slab-column connections was made by using ATENA program basis for the test in this dissertation. The simulation results show that the predicted results accord well with the test results. Load exerted by displacement control method could obtain the post-punching behavior of slab-column connection, which has a good stability. The stress state of the whole loading process was analyzed by comparing with the test results. Moreover, the stress distribution and the formation process of alternate load paths are revealed.4. The sample database for interior slab-column connections which subjected to concentric gravity load were collected. Then, a comparison with the punching bearing capacity of the formulas proposed by codes of practice and the database was presented. Based on the regression analysis, a simple suggested formula to estimate punching bearing capacity of interior slab-column connections without shear reinforcement was proposed. The prediction was in a good agreement with the database.5. A theoretical analysis model was established based on the failure mechanism of post-punching failure. Moreover, a computerized method and calculation flow of post-punching bearing capacity was proposed by comparing the concrete breakout strength with the vertical bearing capacity of reinforcement in the plate. Then, the rationality of the calculation method was verified, the predicted post-punching behavior in accordance with the test results.6. On the basis of test and analysis, the reasonable structural measures and design suggestions are proposed in order to improve the collapse-resistant capacity and the disaster-resistance ability of the slab-column structure by using the strength reserve of compressive membrane action and catenary action. |
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