The governing equation for a dome-type shallow spatial truss subjected to a transverse load is expressed in the form of the Duffing equation, and it can be derived by considering geometrical non-linearity. When this model under constant load exceeds the critical level, unstable behavior is appeared. This phenomenon changes sensitively as the number of free-nodes increases or depends on the imperfection of the system. When the load is a periodic function, more complex behavior and low critical levels can be expected. Thus, the dynamic unstable behavior and the change in the critical point of the 3-free-nodes space truss system were analyzed in this work. The 4-th order Runge-Kutta method was used in the system analysis, while the change in the frequency domain was analyzed through FFT. The sinusoidal wave and the beating wave were utilized as the periodic load function. This unstable situation was observed by the case when all nodes had same load vector as well as by the case that the load vector had slight difference. The results showed the critical buckling level of the periodic load was lower than that of the constant load. The value is greatly influenced by the period of the load, while a lower critical point was observed when it was closer to the natural frequency in the case of a linear system. The beating wave, which is attributed to the interference of the two frequencies, exhibits slightly more behavior than the sinusoidal wave. And the changing of critical level could be observed even with slight changes in the load vector.

Abstract
1. 서론
2. 지배방정식과 주기하중
3. 동적 좌굴과 임계하중
    3.1 주기외력의 변위응답과 위상공간
    3.2 임계레벨과 주파수 변화
    3.3 주기 파라미터와 임계레벨의 경계
    3.4 비대칭 주기외력의 임계레벨
4. 결론
References

• 손수덕(한국기술교육대학교 건축공학과) | Shon Sudeok (Dept. of Architectural Eng., Koreatech University)
• 황경주(서울시립대학교 건축학부) | Hwang Kyung-Ju (School of Architecture, University of Seoul) 교신저자