A stadium roof that uses the pin-jointed spatial truss system has to be designed by taking into account the unstable phenomenon due to the geometrical non-linearity of the long span. This phenomenon is mainly studied in the single-free-node model (SFN) or double-free-node model (DFN). Unlike the simple SFN model, the more complex DFN model has a higher order of characteristic equations, making analysis of the system’s stability complicated. However, various symmetric conditions can allow limited analysis of these problems. Thus, this research looks at the stability of the DFN model which is assumed to be symmetric in shape, and its load and equilibrium state. Its governing system is expressed by nonlinear differential equations to show the double Duffing effect. To investigate the dynamic behavior and characteristics, we normalize the system of the model in terms of space and time. The equilibrium points of the system unloaded or symmetrically loaded are calculated exactly. Furthermore, the stability of these points via the roots of the characteristic equation of a Jacobian matrix are classified.

Abstract
1. 서론
2. 2-자유절점 모델의 지배 방정식
3. 대칭 조건의 평형점
    3.1 Λ* = 0 일 때의 평형점
    3.2 Λ* ≠ 0, û = v^ 일 때의 평형점
4. 시스템 평형점의 안정성
    4.1 u1 = v1에서의 안정성
    4.2 u1 =-v1에서의 안정성
    4.3 평형점 근처에서의 안정성
5. 결론
References

• 하준홍(한국기술교육대학교 교양학부) | Ha Junhong (Liberal arts, Koreatech Univ.)
• 손수덕(한국기술교육대학교 건축공학과) | Shon Sudeok (Dept. of Architectural Engineering, Koreatech Univ.)
• 이승재(한국기술교육대학교 건축공학과) | Lee Seungjae (Dept. of Architectural Engineering, Koreatech Univ.)
• 황경주(서울시립대학교 건축학과) | Hwang Kyung-Ju (School of Architecture, University of Seoul) 교신저자