Study On Axial Pressure Behavior Of RU-NC Composite Short Columns

RU-NC column is a new type of composite structure.it composed of reinforced Ultra-high performance concrete(RU)pipe and normal concrete(NC)core column.RU pipe is not only a demolition-free template in construction,but also a protective layer and structural layer of composite columns.It can give full play to the super-high strength and good durability of UHPC,provide tightening force for NC core columns,and improve the efficiency of composite materials.Composite columns have the advantages of good mechanical performance,high durability and convenient construction.They have good application prospects in Piers of sea-crossing bridges.In this paper,the axial compressive behavior of RU-NC composite short columns is studied by means of model test,numerical calculation and theoretical analysis.The main work is as follows:(1)Taking UHPC pipe wall thickness,spiral stirrup spacing and volume fraction of steel fiber as test parameters,the axial compression test of RU-NC composite short columns is carried out.The results show that the stress of RU-NC composite short columns can be divided into three stages: elastic stage,cracked working stage and steel yield stage;the final failure mode is bulging in the middle of the specimen,accompanied by concrete spalling phenomenon,and the spiral stirrups are broken.Core concrete is crushed;both spiral stirrups and UHPC pipe have a certain hoop effect on core concrete;RU-NC composite short columns with steel cages moved from core concrete to UHPC pipe have better crack resistance,ductility and ultimate bearing capacity than UHPC-RC composite short columns.(2)Finite element parameter analysis with ABAQUS software shows that the numerical results are in good agreement with the experimental results,which indicates that the finite element model established in this paper can be used to simulate the axial compression behavior of RU-NC composite short columns;the influence of UHPC pipe wall thickness,spiral stirrup spacing and volume fraction of steel fiber on the ultimate bearing capacity of RU-NC composite short columns is linear,but not linear.There is coupling effect;with the increase of UHPC wall thickness,the ultimate bearing capacity of RU-NC composite short columns increases continuously,and its increasing trend is almost linear growth;with the increase of spiral stirrup spacing,the ultimate bearing capacity decreases gradually,and its decreasing trend is steep and then slow;with the increase of steel fiber content,the ultimate bearing capacity increases continuously,but the increasing range decreases gradually.(3)The theoretical analysis of RU-NC composite short columns is carried out.The results show that four representative confined concrete strength models or algorithms are selected,which are "Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts","Mander Model","Lam and Teng Model" and "Liu Zhi’s Calculation Method".The results of the four algorithms are quite different from those of the finite element parameters,and can not be applied to R-NC composite short columns.Based on the theoretical analysis of UHPC precast pipe confined concrete,considering the effective and non-effective confinement zones of RU-NC composite columns,a calculation model for the bearing capacity of RU-NC composite columns under axial compression is established.Compared with the results of FEM parameters,the proposed method is generally safe and can be used to calculate the bearing capacity of RU-NC composite columns under axial compression.Calculate.