A study on the vibration calculation method of portal frame structure with variable sections

변화하는 단면의 문형구조물의 진동계산에 관한 연구

In this paper, some theoretical equations are derived to calculate natural frequencies and their modes of a portal type derrick system and developed a computer program to carry out their calculations. The ship's structures, such as funnels, upper decks, engine structures, shaft systems suffer local vibrations of the ship. The exciting forces of vibrations are induced by the bearing force and the surface force of propeller or by the main engine. For solving the vibration problem of riggings like the derrick system the natural frequency of its system must be exactly estimated as far as possible and its resonance condition must be kept out of the normal engine operating speed range. When some severe resonances are encountered after the ship's launching, it may be required a tremendous cost to amend their condition. An experimental model of the portal type derrick is made, which is composed of two posts and a truss. This experimental model is excited by an electrical-magnet, and its vibration responses are found out. The calculating results of the model by the developed computer program are compared with those of measured values of model experiment, and they show fairly good agreements.