STUDY ON STRUCTURAL CHARACTERISTICS OF FOLDED BRACE

Investigation of increase of axial yield displacement and buckling restraint effect

Abstract

 In steel framed buildings, braces are widely used as a main earthquake- resistant element. Braced structures are economically superior to pure moment resisting frame structures. In conventional brace structures, however, stiffness is much greater in braces than in the frame and therefore most of the seismic forces concentrate in the brace. Well-balanced arrangement of braces in the building is therefore required. Conventional braces yield due to small deformation at a story drift angle of approximately 1/500 rad. No braces can therefore be arranged in small numbers because of the restrictions of the primary design that allow no yielding of members. In cases where only a few braces can be arranged or no well-balanced arrangement of braces is possible because the appearance or functions of the building are given priority, therefore, pure moment resisting frame structures are adopted abandoning the use of brace structures. To solve the problem, expanding the range of elasticity to prevent seismic forces from concentrating in the brace is effective.

 Therefore, the authors developed a folded brace that would not yield at a story drift angle of less than 1/200 rad. Specifically, the actual length of the member was made approximately 2.5 times larger than the apparent length by folding three steel bars unicursally. Axial yield displacement increase 2.5 times in proportion to the length of the member. Elastic limit deformation can be controlled arbitrarily by varying the length or frequency of folding even in cases where the material strength is at the level in ordinary steel. Thus, braces that would not yield at a story drift angle of less than 1/200 rad are realized. Also, the folded brace has a buckling restraining effect. Thus, it shows a stable historical performance up to large deformation of 1/50 rad. Using folded brace enables the application of fewer braces and the eccentric arrangement of braces.

 In this paper, we examined the increase in axial yield displacement and the buckling restraint effect, which are the structural characteristics of folded braces.

 Chapter 3 describes a full-scale experiment of folded braces. As a result of the experiment, the folded brace showed elastic behavior up to the deformation level of the story drift angle 1/200 rad. Furthermore, even after axial yielding, it showed stable hysteresis performance up to the deformation level of the story drift angle of 1/50 rad.

 Chapter 4 organized the buckling restraint mechanism unique to folded braces. In a folded brace, the medium steel tube (tensile material) restrains the entire buckling of the core steel (compressed material). As a result of organized, the limit value (critical axial force NC) of the axial force not buckling was derived.

 Chapter 5 describes an element experiment that reproduces the relationship between the core steel(compressed material) and the medium steel tube (tensile material). As a result of the experiment, the validity of the buckling restraint mechanism and the critical axial force NC shown in Chapter 4 was confirmed.