NATURAL FREQUENCY OF STRUCTURES IN FRESH, DAMAGED AND REINFORCED STATES PART 1, THE CASE OF REAL BUILDING

摘要

Generally when a structural construction finished, the natural frequencies of the structures are thought to be constant values and will not change unless the structure itself (including beams and columns) be changed. Such changes may be caused from natural accidents like earthquake, snow and wind, or in turn may from reinforcement. Thus recently, the natural frequencies are believed to become an important index to identify the toughness (stiffness) of the structure. In other words variation of natural frequency can be utilized to indicate the health state of a structure, and most important to be suitable for a complicated structure. After the WCEE in Mexico 1996, many researchers have believed that following the increase of damage, the stiffness of the structure will decrease, which means the natural frequency will decrease. In the same time, the natural frequency will increase while the structures have been reinforced. Based on such assumption, many calculation methods have been developed theoretically till now to indicate the damage degrees and positions following the variations (generally the decrease) of natural frequency, or to indicate the new life cycle following the increase of natural frequency while the structure has been reinforced. Although calculation methods have been developed, the problem still remains to check the correctness of the assumption, also especially for complicated structure. Based on our real 3F wooden structure in the campus of Kinki University, in Osaka City, in this part 1 the recent research results have been reported. The tendency of the variation of the natural frequencies obtained from the state of fresh structures, structures with damages and reinforced structures have been clearly shown. Natural frequencies of fresh structure, structures with damages decreased following the increase of damage for real structure and model structure in general. But in some stages natural frequency remains unchanged or increased even the damage increases. In the same time, comparisons between damaged structures and reinforced structures also have shown that the natural frequency increase in general cases but in some cases remains unchanged, after reinforcement.