Kilpatrick and Rangan 1999

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In this paper, an experimental program was presented to study the effect of bond on the load-deformation response of circular CFT columns and girders. To represent typical steel-concrete interfaces, different bond conditions were prepared. Moreover, tubular columns were tested and the results were compared with the CFT specimens.

Experimental Study, Results and Discussions

A total of thirteen circular CFT columns and girders were tested. The main parameters of the tests were the bond conditions, the L/D ratio, and the type of axial loading. Three bond conditions were prepared and they were classified as maximum bond, partial bond, and minimum bond. For the maximum bond condition, self-tapping screws were inserted into the steel tube and the specimens were treated in an acid bath and then in an alkaline bath to improve the chemical bond between steel and concrete. In the case of the partial bond condition, the grease on the inside of the steel tube was removed. To prepare the minimum bond condition, the specimens were kept in as-received condition and the inside of steel tube was oiled. The D/t ratio of the specimens was 42.3. The nominal yield strength of steel was 50.8 ksi and the average measured compressive strength of concrete was 15.30 ksi. A displacement-controlled axial load was applied to three CFT stub columns and one HT stub column. The L/D ratio of the stub columns was equal to 3.5. A total of eight short columns and slender columns, which included 2 HT columns, were tested under eccentric displacement-controlled axial load. The eccentricity was provided by displacing the columns from the axis of loading. The approximate L/D ratios for the short and slender columns were 10 and 19, respectively. The girders were simply supported and had an 82.67 in. span length. The total number of the CFT girder specimens was three. They were loaded at their third points and displacement-controlled loading was utilized. All of the girder specimens, excluding one of them, were capped at the ends.

For the column specimens, the bond strength did not influence axial load capacity and load deformation response significantly. The difference was not more than 3.6% for the minimum and maximum bond conditions when the axial load capacities were compared. All of the column specimens exhibited stable and ductile load deformation responses. The largest enhancement in axial load capacity and load deformation response, when the bond conditions were improved, was observed in the stub columns. However, these improvements were mainly attributed to discrepancies in the compaction of the concrete, which was harder to achieve in short tubes. The concrete infill increased the axial load capacity by a factor of 3.86 for the stub columns while this factor became 2.61 and 2.13 for the short columns and slender columns, respectively. The reduced improvement of capacity was due to the fact that less amount of concrete was under compression in the short columns and slender columns as they were loaded eccentrically. Similar to the column specimens, the bond strength was also found to have insignificant effect on the behavior of the CFT girders. For the girders having capped ends, the capacity was higher by approximately 8% versus the comparable specimen with uncapped ends. In addition, the specimen with uncapped end experienced reduced stiffness at an elevated load . These findings showed that the longitudinal confinement could affect the behavior of CFT girders.

References

Kilpatrick, A. E. and Rangan, B. V. (1999). “Influence of Interfacial Shear Transfer on Behavior of Concrete-Filled Steel Tubular Columns,” Structures Journal, ACI, Vol. 96, No. 4, July-August, pp. 642-648.