| Traditionally, through the adjustment of "strong column weak beam" coefficient, RC frame structures were ensured to form the reasonable failure mode in the quake, but ignored effects of reasonable matching relationship between the flexural stiffness and strength of beam and column on structural seismic performance and failure mode.Focusing on RC frame structures, main work of this paper is briefly as follows:①According to the characteristics of plastic hinge and the traditional evaluations on failure mode, this paper identified the quantitative damage indicators from members to structures, put forward the method for evaluating failure modes of frame structure overall, based on the revised Park- Ang double damage parameters’ model and the reasonable weighted combined method.②The differences of seismic performance and failure modes between the actural structure which has slabs and the designed structure without slabs, are compared through accurate nonlinear analysis.It is obtained the equivalent model of effective flange width values of the slabs in this paper, and the effectiveness of the equivalent numerical model is verified.③To investigate the influences on seismic performance and failure modes from the flexural stiffness ratio of beam and column or the flexural strength ratio of column and beam, and determining the reasonable matching relationship between the flexural stiffness and strength of beam and column, it is designed 18 equivalent numerical plane examples(6 kinds of flexural stiffness ratio, 3 kinds of flexural strength ratio).The main conclusions are as follows:①The lateral strength and stiffness of frame structure with slab is higher than structure without slab. But with the increase of lateral displacement, the number of column hinges is increased, the damage of hinges is more serious in structure with slabs. Moreover, the deformation and energy dissipation capacity is weakened, and the strength is softened rapidly during strength degradation, because of the inter-storey post-yield mechanism in the underlying storey at ultimate state. The failure mode of frame without slab belongs to reasonable "the column side-way and beam side-way mixed failure mechanism", which is more reasonable, the damage of beam and column is almost equivalent, the distribution of damage is unifiorm between floors, and the ends of side column hinge late. Under earthquake, as the inter-storey post-yield mechanism still hasn’t been formed at floors, the ductility and energy dissipation capacity of the structure without slabs was performed well. It is indicated one of the main reasons for that the pratical failure mode is difficult to achieve the expected mode, is the current design method for frame structure, which was separated the slab from structure.②The results of equivalent model, considering 8 times thickness as the unilateral effective flange width, are in good agreement with model with slab, from aspects of the seismic performance and failure mode. It is indicated the idea of consideration for the effective flange width is feasible in practical structure. On the other hand, the way of the current seismic code, considering 6 times thickness as unilateral effective flange width, could underestimate the effect of slab in combination of the structure under strong quake.③While improving the flexural stiffness ratio of beam and column or the flexural strength ratio of column and beam, the strength of structure have been significantly enhanced. As increasing the flexural stiffness ratio, the structural lateral stiffness and energy dissipation capacity is enhanced significantly, but the deformation ability is weakened. On the contrary, as the flexural strength ratio is increased, it seems no enchanced effect for the lateral stiffness and energy dissipation ability of the structure. As improving the strength ratio, the ductility of the structure deformation is improved, which is unconspicuous as reducing the stiffness ratio, and the structure of the lateral stiffness, energy dissipation ability enhancement effect is not significant.④It shows controlling effects on the failure modes of frame structure, while adjusting the stiffness ratio and the strength ratio. The failure mode of frame structures that the stiffness ratio is more than 1 is more reasonable. As the stiffness ratio decreases, the damage ratio of columns and beams is improved, the overall damage and local floor damage is reduced significantly, the quanity of beam hinge is increased, the number of column hinge is reduced, and the deformation of column hinge is alleviate. For frame structures that the stiffness ratio is less than 1, it has risk to form the inter-storey post-yield mechanism at the second floor. The number of column hinge is increased and damage of the hinge is higher at the second storey, although the damage ratio of columns and beams is higher. As the strength ratio is increased, the number of column hinge is reduced, and the deformation of column hinge is alleviate. It seems to be an effective way to control failure mode, because the evaluation indicator is obviously reduced.⑤According to the result of limited example, it is recommended that: while designed for frame structures, it should be included in the contribution of 8 times thickness width slab on one side at least, and the ratio of column’s and beam’s actual flexural strength should be controlled upon 1.1, when the ratio of beam’s and column’s stiffness is within the reasonable range of 1~4. If the stiffness ratio is not in the range, the strength ratio should be above 1.3, so that it could guarantee the structures to achieve a more reasonable "the column side-way and beam side-way mixed failure mechanism" failure mode under earthquake. |
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