Research On Bearing Behavior Of Elastic Foundation-isolated Footing-reinforced Concrete Column Substructures

In the study of footings about carrying capacity and failure characteristics,a method closer to the real situation of bottom column end in frame structure is analyzing the column,footing and foundation as an integrated whole.In the design of multi-storey and high-rise frame structures by anti-bending point method,the end and inflection point of bottom column are assumed as clamped supported and 2/3 away from the bottom of column.Considering the effect of foundation constraint on footing,whether the embedded performance of column bottom is rigid and the change of carrying capacity and failure mode need to be further explored.Five elastic foundation-isolated footing-reinforced concrete column substructures with different axial compression ratio and footing height were made.The substructures were tested as an integrated whole on a device for simulating elastic foundation to take soil-structure interaction into consideration,and its failure mode,load-displacement curves,strain of concrete and rebar,footing slab deflection and subgrade reaction were obtained.The result shows that with the decrease of the footing height or increase of the axial compression ratio,the substructures failure mode changes from the large eccentric compression failure of the column to the flexural failure of the footing.When the substructures reach the peak displacement,its load-carrying capacity is mainly contributed by the structure where the failure control section is located.The footing slab deflection has obvious turning points under the axial force and horizontal displacement.With the decrease of footing height or increase of the axial compression ratio,turning points appear in advance and then increase faster but the embedded performance of column bottom rapidly reducing.Subgrade reaction presents a certain saddle shape under large axial compression ratio or small footing heightA nonlinear analysis model was established and verified in ABAQUS.On the basis of the model,a parametric study was performed to analyze the effect of column longitudinal rebar ratio,column section size,footing side length ratio,footing size on the load-carrying capacity and failure mode.The result shows that increasing the column rebar ratio can effectively improve the load-carrying capacity and embedded performance,and the failure mode changes from the large eccentric compression failure of the column to failure of column and footing.Increasing the column section size can significantly improve the load-carrying capacity and embedded performance,and the failure mode changes from the large eccentric compression failure of the column to the flexural failure of the footing With the increase of the footing side length ratio,the load-carrying capacity and the embedded performance show a trend of decreasing first,then increasing and then decreasing,and the failure mode changes from the large eccentric compression failure of the column to failure of column and footing.Increasing the footing size can partly improve the load-carrying capacity and the embedded performance but the load-carrying capacity will decline earlier,and the failure mode changes from failure of column and footing to the large eccentric compression failure of the column.The carrying capacity of column side section and column bottom section corresponding to the footing failure and column failure of substructures is derived.Based on the comparison of the measured bending moment,calculation bending moment and section bending capacity,the carrying capacity calculated by trapezoid yield line theory in current code is considered not safe,and well-shaped yield theory is suggested instead.The carrying capacity in different failure modes is calculated,and its minimum value can be regarded as the carrying capacity of substructures with certain safety and can also be used for judging the failure modes of the substructures.