NATURAL FREQUENCY OF STRUCTURES IN FRESH, DAMAGED AND REINFORCED STATES PART 2, SHAKING TABLE MODEL AND ANSYS SIMULATION COMPARED WITH REAL BUILDING

摘要

In part 1, vibration experiments on real building have been carried out to obtain the variation of the natural frequencies. The results have made it clear that the natural frequency will not change even if some of the beams or columns be damaged. In special cases the damage of columns or beams may cause the natural frequency increase, which turned to be in opposition to the assumption. For this reason, it becomes necessary to show simulations on natural frequency while the stiffness of the columns or beams to be changed, for shaking table models and for simulations using both real building size and model size of shaking table. Shaking table model has been tested in our University to obtain the change of natural frequencies. The model is 1/20 of the real building, Sweep test has been used to obtain the natural frequencies for the model in fresh, damaged and reinforced states. In the same time the FEM soft ANSYS has been used to build the models and to do vibration experiment simulations. Two kinds of the models have been made in ANSYS to simulate the real building and the models for shaking table model. After modal calculation, the natural frequencies have been obtained for the fresh, damaged and reinforced structures for the two kinds of the ANSYS models, which coincide with the variation of natural frequencies of the real building and the shaking table model respectively. In more, earthquake waves have been input to check the response and the fact that the weak and strong points of the structures have been found, which also coincide with the result of shaking table models. The final result of this study including part 1 and part 2 clearly indicated that for complicated structures, the variation of the natural frequencies will not simply depend on the change of the stiffness of beams and columns of the structure. They may be influenced by other factors. The assumption that the decrease of stiffness will simply decrease the natural frequency seems not always correct. In the same time, reinforcement will not always cause an increase of the natural frequency.