Assessment of long-term behavior of tuned mass dampers by system identification

Two pedestrian bridges, which were equipped with tuned mass dampers almost two decades ago, have been evaluated for their long-term performance. The response due to pedestrian action is compared to the response at installation. Forced vibration tests are used to identify system parameters of the bridges and the tuned mass dampers. The assessment shows that both bridges and tuned mass dampers are still working well. Detuning has been observed in one of the two bridges, which is, however believed to have existed already at installation. Parameters of the tuned mass dampers are shown to depend significantly on seasonal temperatures, which gives some practical limits to exact optimal tuning.Difficulties may arise in system identification when only the deck response can be measured. Since both inherent bridge damping and absorber effects reduce the overall response, it is difficult to separate the two effects. In some cases even a negative bridge damping together with a high absorber mass has been identified. For reliable results, one of these parameters has to be fixed to a reasonable value. The concept of effective damping is used to show the correlation between the two effects. This concept permits separating the effects of bridge damping, absorber mass, and detuning in an approximate manner.