EXPERIMENTAL STUDY ON BASIC MECHANICAL PERFORMANCE OF TORSIONAL STEEL TUBE IN LATTICE DAMPING WALL SYSTEM

Abstract

 1. Introduction
 It is known that the thin circular steel tube under cyclic torsional moment shows a stable energy absorption capacity. The reason is because its cylindrical shape prevents plate buckling, up to large shear deformation. By taking these advantages, development of the lattice damping wall system has also been carried out. (Fig. 1) In this paper, cyclic loading tests are performed in order to confirm basic mechanical performance of single torsional steel tube for the lattice damping wall system.

 2. Experimental design
 The test parameters are steel grade, diameter-thickness ratio, length-diameter ratio and loading pattern. The standard specimen has 48.6mm diameter, 40mm length, 2.3mm plate thickness, and its steel grade is STK400. The loading patterns are “increasing load” and “increasing load after decreasing load”. Also, specimens without welded-joint are manufactured in order to improve the construction workability. And its damping performance is examined to.

 3. Experimental Results
 The full plastic torsional moment and the initial rotational stiffness of each specimen are almost same as the calculated value. In the case of cyclic increasing load to the torsional rotation angle θ = 1/20, all of the specimens draw stable hysteretic curve of spindle shape. According to the increasing of torsional rotation angle, the value of the torsional moment increases too. Also, compared at each amplitude, as the diameter-thickness ratio and length-diameter ratio becomes small, the strength and energy absorption capacity increases. In the case of monotonic loading, the strength of ST2-8 and SUS1-4 dropped and we can see large buckling displacements on the entire steel tube. In the case of the increasing load after decreasing load, a large difference of the strength between the decreasing load (1 to 8 cycles) and increasing load (9 to 16 cycles) could not be found, so stable hysteretic behavior was confirmed. Moreover, stable hysteretic behaviors in specimens without welded-joint could be confirmed. However, the initial rotational stiffness by experiment was lower than calculated value.

 4. The torsional characteristics of circular steel tube
 The initial rotational stiffness of specimens with welded-joint corresponds well with the calculated value, so it can be evaluated by Equation (2). Therefore, the initial rotational stiffness of the steel tube can be adjusted by diameter, length and thickness. A tendency can be see that strength’s increasing rate increases as the length-diameter ratio is reduced. Also, we can see that strength’s increasing rate increases as the diameter-thickness ratio increases. All plastic rotational angle was less than 0.01 in all of the specimens. The plasticity start time can be adjusted by the length-diameter ratio, diameter-thickness ratio and strength of the steel. It can be seen that cumulative plastic deformation magnification increases as length-diameter ratio is smaller, and the energy absorbing capacity is excellent. In the same degree of length-diameter ratio, the cumulative plastic deformation magnification of LY225 and SUS304 is higher than that STK400.

 5. Conclusion
 The stable history behavior could be found in all specimens with no difference in welded-joint or without welded-joint. It was shown that the yield strength and the initial rotational stiffness of the steel tube correspond well to the calculated value, and the strength and the energy absorbing capacity increase as the diameter-thickness ratio and length-diameter ratio become smaller. However, we found the buckling deformation on the specimen with large diameter-thickness ratio and length-diameter ratio, and the strength of those were decrease.