CFT Members Subjected to Bending Moment

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Concrete-filled steel tubes subjected to pure bending behave much like hollow tubes. In fact, several authors suggest using the plastic moment capacity of the steel tube as a lower bound for the strength of a CFT in pure bending. The steel contributes a large portion of the stiffness and strength since it lies at the periphery of the section where the material has the most influence. In general, CFT beams fail in a ductile manner. A limited number of tests have been performed regarding CFTs under pure flexural bending since their primary application thus far has been as columns.

Strength

The tensile resistance of a CFT depends primarily on the steel alone. Therefore, moment resistance is highly influenced by the steel tube. The only contribution of the concrete to moment resistance occurs due to the movement of the neutral axis of the cross section toward the compression face of the beam with the addition of concrete. This effect can be enhanced by using thinner tubes or higher strength concrete (Furlong, 1967). Tests by Bridge (1976) showed that the concrete core only provides approximately 7.5% of the capacity in member under pure bending. However, Kitada (1992) reported (without quantifying) that substantial increases in strength, and especially ductility, could be achieved by filling hollow steel tubes with concrete.

Pure bending tests of CFTs by Lu and Kennedy (1994) indicated an increase in moment capacity due to concrete infill for the square and rectangular beams of between 10 and 35% as compared to hollow tubes. The amount of improvement was larger for the thinner tubes. In addition, the maximum curvature improved substantially. This was attributed to the high deformation of the top flange of the tube as local buckling was delayed by contact with the concrete. Lu and Kennedy (1994) also examined the effect of moment gradient on the flexural strength by varying the shear span of the specimens. They found that it has little effect on the moment and deformation capacity. The steel and concrete strains along the depth of the beam were similar, so that the slip between the steel and concrete was negligible and has little influence on the moment capacity.

Elchalakani et al. (2001) obtained similar results as Lu and Kennedy (1994) for circular CFT beams and found that concrete infill enhanced the moment capacity between 3 and 37%, with larger improvement for the thinner sections. The slip between the steel and concrete was insignificant, and an a/D ratio of 2.7 was specified to provide full load transfer without any slip. Elchalakani et al. (2001) also presented a D/t ratio limit of 112 for circular CFT beams to achieve plastic flexural strength.

Failure Mechanism

In pure bending tests by Lu and Kennedy (1994) with medium strength concrete and low D/t ratios, the specimens exhibited a linear moment-curvature response followed by a nonlinear stiffness degradation region, approaching the maximum moment asymptotically. The failure took place by local buckling in the compression flange of the tube, concrete crushing in the locally buckled area, and often yielding of the tube in tension. Beams containing high strength concrete or beams having high D/t ratios begin failing when the concrete fractures in shear after the steel has begun to yield. The concrete shearing causes further stretching and then a subsequent rupture of the steel tube. Local buckling in the compression region also occurs near failure (Prion and Boehme, 1989). The beams tested by Prion and Boehme (1989) showed very ductile behavior and did not seem affected by the slip that occurred between the steel and concrete (i.e., the ultimate moment capacity was not lowered). The CFT beams with large D/t ratios tested by Elchalakani et al. (2001) exhibited local buckling distributed along the compression flange, and failure took place with tensile fracture at the bottom fiber of the section that had the most severe local buckling.

Stiffness

The stiffness of a CFT beam depends to some degree on whether or not bond exists at the interface of the two materials. In the absence of bond, there will be no interaction between the materials, little or no concrete augmentation, and the composite stiffness will depend heavily on the stiffness of the steel tube. Tests by Furlong (1968) showed that specimens exhibited a lower stiffness than that calculated assuming plane sections remain plane in bending (i.e., that bond between the materials exists). He concluded after testing greased and non-greased specimens that little or no bond existed at the interface between the steel and the concrete in his tests. The only interaction between the two materials was the physical pressure. Although Prion and Boehme (1989) questioned the existence of bond in beams, their analytical results assuming strain compatibility most accurately predicted the behavior of specimens in pure bending. Lu and Kennedy (1994) found that the flexural stiffness of CFT beams was approximately 1.1 times greater than that of hollow tubular beams, on average. Therefore, they recommended using the flexural stiffness of steel for the composite section as a conservative approach. With respect to initial stiffness early in the loading history, Gourley et al. (1994) suggest adding the steel and concrete flexural rigidities to get an estimate of the initial composite flexural rigidity, essentially assuming initially perfect bond between the materials.

Cyclic Loading

Concrete-filled steel tubes in bending dissipate a significant amount of energy with only a slight decrease in strength as the loading is cycled. While the strength of CFTs during subsequent cycles is not greatly affected by the slip between the two materials, beam specimens do show a loss of stiffness due to a lack of bond and cracking of the concrete after the first cycle (Prion and Boehme, 1989). Beyond the first cycle, the stiffness of the CFT is primarily due to the steel alone since the concrete is cracked on both faces of the beam. As the specimen is cycled, the gap in the tensile zone of the concrete increases as the member approaches zero curvature, and the member strength and stiffness decrease. As the gap closes upon reverse loading, strength and stiffness increase again. This sometimes results in some pinching behavior, with the stiffening occurring due to strain-hardening, compressive concrete re-engaging, and possibly interaction between the tube and the core (Prion and Boehme, 1989). With each subsequent cycle, the tensile cracks in the concrete form more quickly, which also contributes to an overall decrease in strength and stiffness.