Study On The Performance Of Bond-Slip Between Steel Shape And Concrete In CFSST Structures

Concrete-filled square steel tubes (CFSST) with preeminent advantage, have already become a kind of rapidly and widely developed structural forms and directions in the modern high-rise and the skyscraper. The bond strength and the bond-slip performance of the interface between the steel tube and the core concrete of the CFSST is always one of the insurmountable concerns among the construction engineers and managers. The mechanism of bond and basic bond-slip performances studied in this paper are based on the axial push-out test of CFSST and theory analysis.This test has been done upon 14 CFSST specimens including 9 standard push-out test specimens,3 different connection push-out test ones and 2 different axial load transfer test ones. The tube longitudinal strains of the outer surface were tested at various load levels through the electronic strain gauges installed on the surface at close intervals along the length, from which the bond stress distributing rules were obtained. In all these 14 specimens, the updated Embedded Electronic Steel-Concrete Slip Transfers were respectively embedded on the outside of steel tube at certain intervals along the embedment length to measure the distributions of the interior slip. According to the experimental results, the load-slip curves ( curves) were completely analyzed, and the measured strains as well as measured interior slip at both the loading phase and unloading phase of all the 9 specimens were analyzed. The ultimate bond strength was calculated precisely based on unascertained mathematics theory and fully reflects the bond behavior in CFSST structures. In order to find out P ?S the mechanism of bond-slip, the whole push-out experiment process had been fully observed. The first slip time, the development progress, and the final patterns were carefully observed and depicted, then the mechanism of bond-slip were analyzed.According to the experimental results of the 9 push-out specimens, together with the mathematical analysis of the P ? Scurves, three characteristics of bond strength, i.e., first-slip bond strength, maximum bond strength and the remnant bond strength, were summed up. The relationships between 4 main factors and the three characteristics of bond strength were analyzed. Two set of bond strength calculation formulae considering several bond-anchoring factors were statistically regressed, the comparison and error analysis of the calculated results were conducted to evaluate the formulae accuracy, by which the calculation results were verified in good agreement with the experimental results. Moreover, theses two set of bond strength calculation formulae were adopted to calculate the bond strength of the former experimental specimens, and compared with the experimental results, from which the calculation formulae were corrected and revised.According to the experimental P -S Scurves, the relations of the bond strength and load-end slip (?-S relations) were studied and mathematically modeled. Based on the measured results of the tube strain, the longitudinal distributions of the bond stresses were consequently established according to the force balance equations. The longitudinal distributions of the interior slip along the embedment length were also obtained from the experimental results, which were measured by the Embedded Electronic Steel-Concrete Slip Transfer, and the characteristic slip values were established by statistically regression. According to the distributions of the bond stresses and the interior slip, a new type of bond-slip constitutive relation model including two longitudinal position functions, F ( x ) and G ( x),was established and expressed as formula?(x) =S{C₁ , ..., C_i ,... Cn,F(x),G(x)}, in which the longitudinal difference of the bond-slip constitutive relations along the embedment were fully considered.Based on the limit equilibrium method, the ultimate bearing capacity was analyzed, and the simple and practical calculation formula has been proposed. Compared with the former test, theory value matches surveying value quite well. For the CFSST, through analyzing and calculating the ultimate bearing capacity, the lateral pressure improves coefficient of core concrete was proposed to be 2.67 desirably.With the analysis and summaries of the inner distribution, the rules of relative inner force and length were obtained. According to the regression analysis the formula of concrete axis force was supposed. With the application of the distribution formula, the internal force of steel and concrete was obtained separately along the transfer direction in different position. Based on the limit equilibrium method, the critical transfer length Lcs r of push-out member was analyzed with the relatively ones . According to the test result, the transfer length and its limit were carried out, and the limited transfer length was proposed which is the smaller one of 750mm and 2.5B order to spread shear force.Considering the transfer length, some other bonding anchors suggestion such as the distortion section steel were put forward. Together with the analysis of the CFSST bond-slip performance, the design of shear section was discussed, and then the corresponding calculation formula was given for the reference of actual projects design.The numerical simulation of the push-out test specimens and the axial load specimens were conducted by the ANSYS according to the bond-slip constitutive relations obtained, the simulation results well matched the experimental results. The two CFSST numerical simulation cases under axial load by ANSYS considering and no considering the bond-slip performance were both conducted and compared with the experimental results. The influence of the bond-slip and different types of loading conditions upon mechanical behaviors of CFSST under axial load is analyzed.