Council on Tall Buildings and Urban Habitat 1979

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A comprehensive summary of CFT behavior and corresponding design procedure was presented. Several references were made to previous papers. Much of what this article described can be found in other summaries contained in this report.

CFT Behavior

Short Axially Loaded Columns. The authors describe the relationship between the Poisson ratio of concrete and that of steel as the strains in the CFT section increase. At longitudinal strains of 0.001, the concrete begins to expand at a more rapid rate than the steel tube. Knowles and Park (1969), suggested that at a strain of 0.002 (about 0.95 of the maximum load), contact between the materials is initiated and interactive stresses between the concrete and the steel result. The concrete will be triaxially confined and the steel will be in a state of biaxial stress, and thus unable to sustain the initial uniaxial yield stress in the longitudinal direction because of the secondary hoop stress. At failure, experiments have shown that the longitudinal stress is about three-fourths of the uniaxial yield stress and the hoop stress is about one-half of the uniaxial yield stress. Triaxially confined concrete may attain strengths double the unconfined strength in circular sections, although little strength gain has been recognized in rectangular specimens.

Failure of short CFT specimens was described as a primary result of local buckling of the tube in regions of maximum strain. The tube failure is accompanied by either concrete crushing or a diagonal shear fracture in the concrete. The shear failure is initiated in the concrete, and the concrete slides along an inclined shear plane of approximately 64° (Sen, 1969).

Long Axially Loaded Columns. Concrete-filled steel tube columns exhibit much the same behavior as any structural compression member, showing a decrease in strength with an increase in length. Additionally, strength gain due to confinement decreases rapidly with an increase in slenderness. Knowles and Park (1969), computed a K*L/r_c value of 44.3 as the point beyond which no beneficial confinement action takes place. However, even when no confinement action occurs, the confined core has the advantage of guaranteeing ductile behavior for longer columns. Very long columns, which fail by elastic buckling, do not often exhibit ductile behavior. Elastic buckling occurs at a larger slenderness ratio for lower yield strength steels and for columns with a higher percentage of concrete in the cross section.

Combined Compression, Bending, and Shear. The moment-curvature behavior of a CFT section is similar to a plain thick-walled steel tube, with a ductility at least as great as the hollow tube. As the tube begins to yield, the slope of the moment-curvature curve decreases until the point of local buckling of the steel tube. The concrete at the location of buckling will be in a crushed and disintegrated state. The flexural capacity of the section corresponds to the incipient point of tensile failure in the concrete. The addition of concrete to a steel tube greatly increases the shear capacity of the section. Tomii et al. (1972) indicated that the shear strength is the sum of the individual strengths of the concrete and steel. The authors recommended using the sum of the concrete and steel shear rigidities to compute the rigidity of the concrete-filled tube:


It has been shown that CFTs possess a greater shear strength for a given section size than reinforced concrete members. CFT columns subjected to shear demonstrate the ability to sustain shear forces as transverse deflections increase, even under constant axial load.

Cyclic Compression, Bending, and Shear. CFTs under alternately repeated loading have displayed good deformation capacity and spindle-like hysteresis loops typical of stocky steel sections. Tests of specimens with a low D/t ratio have been conducted with no evidence of buckling or concrete crushing (probably due to confinement). Tests of thin-walled specimens with accompanying shear, however, have displayed local buckling before sufficient rotation capacity is attained. To compute the shear rigidity of the composite section under cyclic loading, the authors added a reduction factor to the concrete term of the monotonic formula:


References

Council on Tall Buildings and Urban Habitat (1979). Structural Design of Tall Steel Buildings, Monograph on the Planning and Design of Tall Buildings, Vol. SB, ASCE, New York, pp. 671-680.