Yu, Ding, and Cai 2007

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This paper present an experimental study on the behavior of circular concrete filled steel tube stub columns with self-compacting concrete and normal concrete concentrically loaded in compression to failure. Four measurement methods on the axial deformation of specimens were compared. Experiments were designed to investigate the effects of concrete strength, notched holes or slots (representing structural elements passing through CFT columns), and different loading conditions on the ultimate capacity and the load-deformation behavior of the columns.

Experimental Study, Results, and Discussion

A total of 17 test specimens were constructed and tested under concentric axial compression loads. All specimens were three times the diameter in length to reduce the end effects and to ensure that the specimens would be stub columns with minimum effect from slenderness. Each tube was welded to a square 5mm thick steel base plate. Many different parameters were varied: A comparison of self consolidating and normal concrete, as well as several different concrete strengths, was made. Wall thickness of steel tubing was either 3 or 5 mm. Four different loading conditions were tested; load was applied to: entire section; concrete section initially followed by the entire section, steel section initially followed by entire section, concrete section only. Some columns were notched with small holes to simulate steel rebar passing through frame joints and others had slots of different heights cut around the perimeter of the steel tube. Four different measurement methods were tested. These methods were strain rosettes with dimension of 3 x 2 mm, strain gauges with dimensions of 100 x 3 mm recording axial shortening and perimeter expansion of specimens in the mid-height region, electronic displacement transducers recording axial shortening of specimens in the mid-height region, and electronic displacement transducers recording the full length axial shortening of specimens.

It was found that electronic displacement transducers recording full length axial shortening was not an accurate means of measuring axial deformation. The other three methods were deemed reliable measurement methods and it was found that electronic displacement transducers recording axial shortening in the mid-height region was the only method capable of recording deformation behavior at the strain softening stage. As cubic compressive strength increased, the limit load of elasticity and ultimate capacity of the stub columns reached a higher value, but the residual capacity almost maintained the same value. Concrete strength did not greatly affect the failure mode of the stub columns. In specimens notched with small holes representing steel rebar passing through joints, the modulus of elasticity was reduced and the deformation increased slightly. The limit load of elasticity decreased, the deformation grew rapidly at inelastic stage, and the strain corresponding to the ultimate capacity increased slightly. There was hardly any influence on the ultimate capacity and residual capacity between the notched and unnotched specimens. Local buckling of the steel tube happened earlier in the notched specimens, the concrete core at the holes was crushed. The authors conclude that the notched holes significantly influence the mechanical behavior of the column and that steel rebar passing through CFT joints should be avoided. When a steel tube was notched with a full perimeter slot in the mid-height region, the stress flows of steel tube could not pass through the slot; the concrete behavior with three-dimensional compressive stress was changed; and the capacity and axial compressive stiffness were also reduced as a whole. The confinement effect was enhanced when the dimension of the full slot was reduced but decreased greatly when the dimension was large. Under different loading conditions the sequence of confinement effect for the specimens changed. Compared to the specimen with the load applied to the steel section initially, the confinement effect appeared earlier but decreased and the ultimate capacity was almost the same. When the load was applied to the concrete section initially or only to the concrete section, the confinement effect appeared later but enhanced and the ultimate capcity increased slightly. The residual capacity of the stub columns under the four loading conditions was hardly influenced.

References

  • Yu, Z.-W., Ding, F.-X., and Cai, C. S. (2007). “Experimental Behavior of Circular Concrete-Filled Steel Tube Stub Columns.” Journal of Constructional Steel Research, 63(2), 165–174. doi:10.1016/j.jcsr.2006.03.009