STUDY ON STRENGTH AND OUT-OF-PLANE BUCKLING OF STEEL HEXAGONALLY LATTICED PLATES SUBJECTED TO CYCLIC SHEAR DEFORMATION

Abstract

 Some new multi-story buildings constructed in recent years employs single-layer latticed walls as the outer-tube, because of attractive appearance and great in-plane stiffness. In the region at high seismicity as Japan, structural walls are used as earthquake resistant shear walls and subjected to severe cyclic shear deformation during strong earthquakes. Ductility is the most critical factor to be considered as well as strength for such shear walls. The present paper gives a summary on the cyclic tests of steel hexagonally latticed shear plates and discusses on the inelastic strength and buckling characteristics based on the theoretical and FE analysis.
 Four steel hexagonally latticed plates (denoted by H1, H2, H3 and H4) were manufactured. The width and height of the plates were 1484mm and 1284mm, respectively. The thickness was 22mm (H1) and 16mm (others). The number of hexagon was 6 in both width and height. The H1 and H2 plates were composed of regular hexagon. The H3 plate comprised orthotropic slender hexagon and the H4 plates was composed of the irregular hexagon determined by the Voronoi rule. Forced cyclic shear was given to these for plates. The shear strain (denoted by γ) amplitude in each cycle gradually increased from ±1/900 to ±1/25. Two cycles were given in one amplitude.
 Three plates except H3 yielded at the cycle where γ=1/300 but the H3 did at γ=1/450. After yield, the plastic response behavior of the H1 and H2 plates was very stable without out-of-plane buckling until the final cycle. On the other hand, out-of-plane buckling was clear in the tests of the H3 and H4 plates. Buckling caused from the cycle where γ=1/75 in the H3 test and during the final cycle in the H4 test. Moreover, after buckling, break of three lattice members occurred in the H3 test during the first cycle where γ=1/25. Crack was also observed in the H2 and H4 tests. All these breaks and cracks occurred on the vertical members.
 The hysteresis curves were not slip but spindle shape even in the H3 test where severe buckling was observed. Significant strain hardening was observed in all the tests. The following FE analysis shows the appearance of yield hinges in the vertical members from the early stage. This is a characteristic of hexagonally latticed plates when subjected to shear. As shown in the theoretical analysis, the rotation of yield hinge is amplified to almost twice the shear strain even on the regular hexagon. The FE analysis also shows that the bending moment at the yield hinges increases significantly due to strain hardening and the accumulated plastic strain was extreme at the spots where break occurred in the H3 plates.
 An effective framework to estimate the shear strain when out-of-plane buckling initially occurs, has been proposed by Tamai on steel shear panel dampers. In this study, the estimation formula is modified to a function of the generalized slenderness to apply to buckling of latticed plates. The regression coefficient in the formula is re-determined by the test results of latticed plates including the past tests on triangularly latticed plates. The modified formula well estimates the buckling shear strain and that shows the effectiveness of this framework proposed for steel panel dampers.