Study On Punching Shear Behavior Of Pile Caps Under CFST Columns

Punching shear bearing capacity and failure forms of pile caps are studied under the embedded CFST columns by contrast. Five specimens are designed with no strengthen measures, halving the buried depth of column, studs reinforcement, shear annular plate reinforcement and adding headed bars in pile cap. To provide the experimental basis for design theory of the pile caps under the CFST columns, the punching shear effect of pile cap is analysed under the different construction measures by monotonic static loading.The researches of thick pile cap design theory models are referred from home and abroad, this paper proposes modification suggestions for Strut-and-Tie model calculation formula of pile caps under the CFST columns. The research results are obtained as follows:1)The pile caps under different strengthening measures are analysed and found: the specimen with no strengthen measures happen to punching failure under the lower load and the angle between inclined surface of destruction cone and the bottom of cap is less than o45.The shearing bearing capacity of specimen with studs reinforcement is greatly improved, but still instantlypunching failure with deflection surging. And studs nearly bear 66% of theentire specimen capacity. The shearing bearing capacity of specimen with shear annular plate reinforcement is the highest with the relatively long destruction process, which belongs to ductile failure. And shear annular plate nearly bear 71% of the entire specimen capacity.2)To the specimen with halving the buried depth of column, shearing bearing capacity of pile cap has be increased by almost one times, and the failure phenomenon is similar to the control group specimen. The failure type belongs to brittle punching failure.To the specimen with adding headed bars in pile cap, headed bars can bear some punching force from column base plate, greatly improves shearing bearing capacity of the whole specimen.Headed bars are not only conducive to the construction layout of cap, but also conducive to reaching ultimate bearing capacity of the pile cap with slowly deflection increasing, showing some ductility.3)Comparing the measured values and calculated values, the calculation of existing specifications for pile caps of punching shear capacity is too conservative and does not fully comply with the actual situation. It not only costs labours and materials at the time of construction, but also not conducive to the study of the theory of pile caps. Based on the research of two and three dimensional Strut-and-Tie model, two dimensional Strut-and-Tie model can reference the rules of 《Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts》(JTG D62-2004)and only applies to the angle that larger than o34. When the angle reacheso34, the bearing capacity calculation results of the Ministry of Communications specification are higher than the actual results, so it still need to be modified. The research rules of three-dimensional Strut-and-Tie model for pile caps by Guo Honglei don’t apply to pile caps under the CFST columns. The oblique compression bar damage is the marks of pile caps punching failure and the rule of pressure bar section area is not suitable for this study. So the pressure bar section area is defined roughly as pile section area of 2.5 times that is more appropriate.