Kawaguchi et al. 2002

An experimental study on portal frames consisting of steel I-girders framing into square CFTs subjected to cyclic loading was presented in two papers (see also Kawaguchi et al., 1997). Using the test results, the elasto-plastic behavior of the CFT frames was examined.

Experimental Study, Results, and Discussions

Four specimens having square CFT columns and wide flange steel girders were prepared. The columns had a D/t ratio of 20.83 and an L/D ratio of 8. The measured yield strength of the steel was 58.5 ksi for the columns and 49.7 ksi for the girders. The measured average concrete strength was 2.67 ksi. The columns were fixed at the base and connected to the girders using fully-restrained connections. Through type diaphragms were used to connect the girders to the CFT columns. Two of the specimens were designed to have a shear type of failure in the panel zone of the CFTs prior to failure due to combined axial force and flexure took place in the CFT columns. These specimens were designated as panel-yielding specimens. The other two were proportioned to have failure along the length of the CFT columns occur earlier than failure within the connection region; these specimens were designated as column-yielding specimens. The girders were designed to remain in the elastic range.

The frames were tested under constant axial load, with 15% and 30% of nominal axial strength applied to the CFT columns. The base-beam of the columns was subjected to a cyclic horizontal load while the original position of the column top ends was kept stationary. The column-yielding specimen that was loaded with 15% of the nominal axial load experienced a rapid reduction in strength after achieving its peak strength. After fracture took place at the weld connecting the external diaphragm to the column, the strength of the frame decreased severely. The response of the panel-yielding specimen subjected to 15% of its nominal axial load was more stable. Local buckling did not significantly affect the response of the panel yielding-specimen. However, weld fracture between column and diaphragm also took place for this specimen. This type of fracture observed in the welded connections was attributed to defects in welding process. The remaining two specimens were subjected to 30% nominal axial load. The failure of the column-yielding specimen was governed by local buckling. The failure of the panel-yielding specimen was dominated by shear buckling of the panel zone. However, for both of the frames, no significant strength degradation was observed, and their post-peak behavior was ductile and stable.

The strengths of the frames were predicted conservatively using the AIJ design code provisions. It was concluded that local buckling of the column ends and shear buckling of the panel zone did not affect the strength of the frames significantly.

Analytical Study

The use of a Ds factor in Japanese practice to reduce the required strength of a ductile frame was discussed. This factor was dependent on the plastic deformation capacity of the frame. The cyclic hysteresis loops were utilized to obtain the plastic deformation capacity of a tested frame. For this purpose, the positive halves of the loops at each cycle were added sequentially and a curve tangent to these loops was assumed to represent the monotonic load–deformation curve of that particular frame. This curve was then fitted to an elasto-plastic curve having the same area. Using the ratio of the yield deformation to the ultimate deformation, both plastic deformation capacity and the Ds factor were calculated. This procedure was repeated for each specimen. The Ds factors were found to be less than 0.25, which was the minimum value specified for the most ductile steel structures according to the Building Standard of Japan. Thus, it was concluded that the deformation response of the CFT frames was similar to that of the steel frames.

Reference

Kawaguchi, J., Morino, S., Sugimoto, T., and Shirai, J. (2002). “Experimental Study on Structural Characteristics of Portal Frames Consisting of Square CFT Columns,” Composite Construction in Steel and Concrete IV, Hajjar, J. F., Hosain, M., Easterling, W. S., and Shahrooz, B. M. (eds.), United Engineering Foundation, American Society of Civil Engineers, Reston, Virginia.