# Inai, Mukai, Kai, Tokinya, Fukumoto and Mori 2004

## Experimental Study, Results, and Discussion

This paper studies the effects of load level, steel strength, width to thickness ratio, and concrete strength on square and circular concrete-filled steel tube beam columns under constant axial load and cyclic horizontal load with increasing lateral deformation. The square beams were also subjected to biaxial bending. Various types of steel were used with strength ranging from 58.02-113.13 ksi, and two types of concrete were used with F’c of 5.80 and 13.05 ksi. A constant axial load of 40% of the nominal compressive strength was applied to the interior of the columns, and ranged from 30% of the nominal yield strength to compression of 70% for the exterior columns. Lateral loading was applied at angles of 22.5o to 45o from the principal axis. A footing and top stub were attached to the top and bottom of each column, and the clear column length was six times that of the depth or diameter of the cross section. Two vertical actuators were used to apply axial load while keeping the top stub parallel to the footing, and a horizontal actuator was used to apply lateral displacement. Calibrated load cells were used to measure the applied forces, and horizontal and axial deformations were monitored. All specimens reached maximum strength after local buckling occurred in the compressive flange, and many of the specimens exhibited ductile behavior for the circular interior columns. For all of the circular exterior columns, fracture of the tensile flange occurred under axial tension. The square interior columns reached the ultimate moment after local buckling occurred in the compressive flange, and exhibited cyclic deterioration of moment resistance. Overall, the ductility of the specimens becomes larger as the steel strength becomes higher, with the exception of the circular specimens, and ductility is reduced as the concrete strength increases. This indicates that the restraining effect of the filled concrete against local buckling does not depend on concrete strength. The circular steel tubes are able to restrain local buckling due to confining filled concrete, and the flexural strength and ductility under constant axial load are superior for circular specimens. The square specimens also exhibited increased axial shortening and deterioration in moment resistance after local buckling occurred. Local buckling occurs at a lower deformation as the diameter to thickness ratio of the specimen increases, and the ductility will also increase. The ductility of the beam reduces under variable axial loads, due to local buckling at early deformation stages.

## References

Inai, E., Mukai, A., Kai, M., Tokinoya, H., Fukumoto, T., and Mori, K. (2004). “Behavior of Concrete-Filled Steel Tube Beam Columns.” Journal of Structural Engineering 130 (2), February pp. 189-202 doi:10.1061/(ASCE)0733-9445(2004)130:2(189)