抄録
Square steel tubular columns are widely used in moment resisting frames. The moment resisting frame system will resist the horizontal load action through the flexural manner of the columns. Therefore, the columns will subject the axial force and the bending moment simultaneously. Under the seismic action, the effect of the bending moment that is the result of the shear force subjected to the column will be significant; the structural safety of the column at the ultimate limit state (ULS) shall be guaranteed. Recommendation for Limit State Design of Steel Structures and Recommendation for Plastic Design of Steel Structures specifies the requirements for the column to guarantee sufficient strength and ductility at the ULS. However, the appropriateness of requirements for square steel tubular column is not verified clearly. The test results, where the square steel tubular columns are under compressive axial force with different moment distribution, were reported in the previous papers. In this paper, structural performance evaluation of square steel tubular column is conducted based on the existing test results. Collapse mode, maximum bending moment, plastic deformation capacity, and moment amplification factor are evaluated; comparison study between the current design recommendation is also shown.
This study revealed the following results.
1) The design requirement (Eq. (2)) that is specified for the column expected to form the plastic hinge provided conservative results. The gap between the collapse mode observed from the testing and that is expected in the design requirements gets larger when the moment distribution will get close to antisymmetric bending moment. In this study, Eq. (10) is shown to evaluate the collapse mode reasonably. Moreover, Eq. (10) still fulfill the required minimum plastic deformation capacity that is stipulated in the recommendations.
2) Design strength of the column which will not fulfill the requirements for the column to form plastic hinge showed conservative results when it was computed from current recommendations. The design strength computed from the old version of the recommendation provided more reasonable results.
3) Moment amplification factor that is used to evaluate the Pδ effects was compared with the test result. It was confirmed that some columns showed unconservative results at the ultimate limit state. The column categorized to have no Pδ effects per Eq. (14) but could not resist full plastic moment (i.e., Mpc) during the test showed large second-order effects. In this case, the maximum bending moment through the column cannot be evaluated appropriately. However, this kind of phenomenon was observed in two specimens only. A further study shall be needed for the evaluation.
4) Large deviation was observed at the plastic deformation capacity of the column under the double curvature bending when it was compared with the formula that is proposed for one end bending moment condition. From the regression analysis, the formulae that will evaluate the plastic deformation capacity of the column were shown. The derived formulae are not including the parameter of the width-to-thickness ratio; therefore, further study shall be needed for the proposal.
