구조시스템에 따른 텐세그러티형 케이블 돔의 정적 불안정 거동 특성에 관한 연구
(A) study on the static unstable characteristics of the tensegrity-type cable dome structures according to the structural system
건물구조 강철구조 구조시스템;
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After civilization began, large spacial structures were in continuous demand, because human beings pursued life in larger and more affluent spaces. Changes in roof structures are the most important factor for the possibility of making large space structures. Roofs gradually lightened with the change of materials and technical developments, and the present technique can exceed a span of 300 meters. Following such a trend, the construction of large spacial structures is rapidly increasing all over the world and accordingly, incidents of collapse are on the rise. To build safer and more economical buildings, we have to know the precise collapse mechanism of large space structures, and many researchers have focused on this problem. A shell structure, having a curvature with a curved surface, is an extremely efficient mechanical creation regard to the external load. A basic structural resistance mechanism is the structural system, which is resisted the out-of-plane direction load by in-plane forces using the structure's curvature. Therefore, it has a merit to make thin and lightweight large spacial structures using minimum materials. Among the large space structural system, the rapid development of the membrane structures, cable structures and the hybrid structures are watched recently. Fuller firstly tried this system for the lightweight and rationalization of structure by reducing compression members and using cable for the tension members. Morto, Vilnay, Otto and other researchers have been studied this system continuously.[16,29,63] '88 Seoul Olympic Gymnastics Arena and Georgia Dome which designed by Geiger(1986) is the successful cases of these structural system. Also, the construction of cable dome is continuously increasing all around the world, the studies about the cable dome structures are in active. But, this kind of structural system shows the unstable phenomenon by snap-through or bifurcation according to the shape of structure, and the understanding of the collapse mechanism by this phenomenon is very important to the design process. Korea has the light weight hybrid cable dome structure in 88 Seoul Olympic for the first time. After that, the studies of cable dome have been focused on the shape analysis that find the best design shape. Because cable dome structure is initial unstable one and essential to introduce initial stress. So several researches have been reported the structural behavior characteristics due to the need of collapse mechanism. However, it is difficult to analysis this phenomenon because it could be happened by structural shapes, materials and forces. With regard to unstable characteristics, it is not clearly reported yet. Most of all, there are not many studies about unstable behaviors on cable dome. In this study, I investigated the unstable characteristics of the Geiger-type, Zetlin-type and Flower-type hybrid cable dome structures, which is the lightweight hybrid structures using compression and tension elements continuously, according to the difference of structural system. The structural characteristics of soft structure such as hybrid cable dome are different from the rigid one. That is, the rigid system switches over from stable region to unstable by the external forces but the soft system shows unstable phenomenon before introducing initial stresses and after introducing initial stresses, this system switches from unstable region to stable one. That is, the soft structures such as hybrid cable dome need the stage of shape analysis. At this time, the large displacement or large deformation phenomenon is happened. Also, the structural behaviour of the hybrid cable dome shows a strong nonlinearity according to the initial stress and external forces. Therefore, I determine the shape by applying initial stress and then investigate the unstable characteristics of the hybrid cable dome by the static external forces. As the static nonlinear analytic method, I trace the critical load according to step by step analysis using the tangent stiffness equation. Here, I used the displacement incremental method. Also I investigate the bifurcation path of initially imprefected model because this kind of structure is very sensitive to the initial condition. And the followings are observed. 1. Geiger-type and Zetlin-type cable dome did not show unstable phenomenon in the perfectly shaped model in cases of A-type and B-type load mode, but in the initially imperfected model the equilibrium path changes by the main displacement modes are coupling with a new independent buckling mode. And the magnitude of initial imperfection becomes larger, the stabilization of rigidity becomes more rapid and showing softening phenomenon. This phenomenon is clearer in A-type load mode. But, in the case of C-type load mode, the unstable phenomenon is not happened in the perfectly shaped and initially imperfected models. 2. In case of A-type load mode, the unstable phenomenon of the Flower-type cable dome is happened as 1st and 2nd cases. The unstable phenomenon of 1st critical point is happened by bifurcation and 2nd is happened by snap-through. At this time, the snap-through is occurred locally and this phenomenon bring about the incline of whole structure. In cases of B-type and C-type load mode, the rigidity of the structure is increased at the first stage. After that the snap-through is happened suddenly in the local part of the dome so the whole structure is inclined to the (-)X direction. 3. In A-type load mode, the static critical load is Zetlin-type, Geiger-type and Flower-type according to magnitude, and in B-type load mode, Geiger-type, Zetlin-type and Flower-type. Generally, the static critical load of Flower-type cable dome is greater than Geiger-type and Zetlin-type because Flower-type cable dome has the shape which can prevent the in-plane twist. Also, the static critical load level is different according to the load mode. The static critical load level of B-type and C-type load mode is very greater than A-type and the static critical load level of B-type and C-type load mode are far from the design load level. In this study, I tried to make a fundamental data which can be used to the design process of the tensegrity-type cable dome by investigating the static unstable phenomenon of three kinds of tensigrity-type cable domes under various load modes. The generation path of unstable phenomenon is different by the composition system. And the critical load level is greatly different from each other. Also, these models show different behaviour according to the load mode. Therefore I think that it is needed to reflect these sensitivities of structural behaviour by structural system and load mode of cable dome to the design process before determine the structural system.