In Japan, steel moment frames, which consist of H-shaped beams and square hollow section columns with through diaphragms, are often used for building structures. It is commonly known that panel zones of these steel moment frames often yield under strong ground motion and have high deformation capacity. In order to take into account high deformation capacity of panel zone for seismic design, it is needed to calculate the full plastic strength of panel zone. Several methods for calculating the full plastic strength of panel zone have been proposed by previous researches [Ref. 5]-7]]. For example, in the most frequently used method, which is adopted in the recommendation [Ref. 3]], only axial force and shear force is considered, and the effect of bending moments which act upon the top and bottom of panel zone is ignored. Thus, this method can only be applied to the panel with aspect ratio up to 1.6 [Ref. 3]]. On the other hand, most methods proposed by previous researches can not correspond to arbitrary directional shear force. Therefore, this paper proposes a new method to calculate the full plastic strength of square hollow section panel zones with through diaphragms under axial force, bi-directional shear forces and bi-axial bending moments.
Firstly, cyclic loading test was conducted in order to investigate effects of aspect ratio, axial force and input direction of shear force on elasto-plastic behavior of square hollow section panel zones, as described in Chapter 2. From the test results, it is clarified that ultimate states are classified as ductile crack, shear buckling, and local buckling. And the deformation capacity with shear buckling is larger than the cases with other ultimate states. Furthermore, it is verified that maximum strengths of panel with small aspect ratio and small axial force ratio under 45 degrees input are larger than those in other cases.
Secondly, finite element analysis (FEA) was conducted in order to verify full plastic strengths with various parameters more than the cyclic loading test. The validity of FEA model is confirmed by comparing between FEA results and loading test results, as discussed in Chapter 3. Strength interaction curves of bi-directional shear force, which are obtained by FEA, are shown in Fig. 12-14. From these figures, while the aspect ratio and axial force ratio are small, the strength interaction curve looks like square with a bit round at corner because shear yielding mainly occurs at all over the panel. On the other hand, while either aspect ratio or axial force ratio is large, the strength interaction curve is circlelike due to yielding under shear stress and normal stress at ends of panel.
Finally, a method to estimate the full plastic strength of square hollow section panel with through diaphragms, considered with correlation among axial force, arbitrary bi-directional shear force and bi-axial bending moment, is proposed. The full plastic strength is obtained as the smallest one between strength at center section and strength at end section. In Fig. 19 and 20, calculation results by the proposed method are compared to analysis results from Chapter 4. From these figures, in case aspect ratio is over 2.5 and input direction of 0 degrees without axial force, calculation results by proposed method underestimates full plastic strength of panel as much as 20%. However, aspect ratio of panel zones, which is used in steel moment frames, are usually under 2.0, in this case, calculation results by proposed method corresponds well to finite elements analysis results.
