Johansson and Gylltoft 2002
The behavior of circular CFT stub columns subjected to monotonic compression was analyzed and compared to three-dimensional nonlinear finite element models. The primary focus was the effect load placement on the structural behavior of CFTs.
Experimental Study and Results
A total of 13 circular columns were tested, four of which were HTs. All of the steel tubes were the same size, with an outer diameter of 6.25 in., thickness of 3/16 in., and length of 25.6 in. The measured yield stress of the steel tube was 63 ksi and the measured concrete compressive strength was 9.4 ksi. The placement of the load (steel only, concrete only, entire cross-section) was the only test parameter for the CFTs, accordingly, three sets of three identical CFTs were tested. To achieve the steel only load placement 0.4 in. of both ends of columns were left unfilled. To achieve the concrete only load placement 0.4 in. of both ends of columns were left unfilled and 2 in. long cylinders were placed at the column ends, only in contact with the concrete.
Each of the four different sets of tests had a typical load-deformation relationship, with various characteristics. Most interestingly, while both reached the same peak axial load, the CFT with loading only on the concrete had a lower initial stiffness than the CFT with the entire cross-section loaded. Also, the CFT with loading only on the steel reached a plateau at the same axial load as the peak load of the HT but after deformation equal to the unfilled space, the load resistance increased to the approximately that of the other CFTs.
Analytical Study and Discussions
A three-dimensional nonlinear finite element model using ABAQUS was created for further analysis. Three symmetry planes were used; hence only one-eighth of the column was modeled. The concrete material model used a smeared crack technique. The steel material model was elastic-plastic model based on the stress-strain relationship from uniaxial tension tests performed on sample coupons. The concrete-steel interface was modeled with interface elements using Coulomb friction. Load-deformation relationships from the finite element models were compared those from the experiment with reasonable accuracy.
Detailed discussion was provided on the results of the tests and analyses, particularly, the differences in behavior associated with each of the three methods of load application. The distribution of the load between the steel tube and concrete core at the mid-height of the column was illustrated for the different loading configurations. While the peak axial capacity of the CFTs with their entire cross-section loaded and the CFTs with only their concrete loaded is nearly the same, the distribution of the loads is different. When only the concrete is loaded, the steel tube carries at most 30% of the load, while, when the entire cross-section is loaded, the steel tube carries about 40% of the load. When only the steel was loaded, no redistribution of the force was noted and the steel carried 100% of the load. Examination of the load distribution also gave insight to the difference in initial stiffness between CFTs loaded on the concrete only and the CFTs loaded on the entire cross-section. When loaded on the entire cross-section, the load is distributed by bearing from the start. When loaded on the concrete only, stress in the steel is developed through friction at the interface, a more gradual process.
By adjusting the coefficient of friction between the steel tube and concrete core in the ABAQUS models, the effect of bond strength on the behavior of the CFTs was investigated. Bond strength was found to have little influence on the CFTs which were loaded on the entire cross-section or on the steel only. However, for the CFTs with only the concrete loaded, bond strength was found to have a significant effect. Increased efficiency of the concrete due to confinement was quantified as the amount of axial force in the concrete core above the nominal plastic resistance (the product of the concrete compressive strength and the area of the concrete core). This increase was found to be larger when the load was applied only to the concrete. Despite the increased efficiency in the concrete, the authors concluded by suggesting that the natural bond should not be relied upon to obtain full composite action and that it is better to force the entire section to undergo the same deformations.
Reference
Johansson, M. and Gylltoft, K. (2002). “Mechanical Behavior of Circular Steel-Concrete Composite Stub Columns,” Journal of Structural Engineering, ASCE, Vol. 128, No. 8, August, pp. 1073-1081.