PASSIVE SEISMIC CONTROL OF CABLE-STAYED BRIDGES WITH DAMPED RESONANT APPENDAGES

A study is conducted to investigate the effectiveness of attaching to cable-stayed bridges resonant appendages with a relatively small mass and a high damping ratio as a means to reduce their response to earthq uake excitations. The study is based on a previously developed formula tion that shows that the use of these appendages may increase the inhe rent damping of building structures and, as a result, may reduce their response to seismic disturbances. It includes numerical and experimen tal tests that are conducted to assess the validity of such a formulat ion for the case of cable-stayed bridges and the extent to which such appendages can reduce their seismic response. In the numerical study, an actual cable-stayed bridge is modelled with finite elements and ana lysed with and without the proposed appendages under different earthqu ake ground motions. Appendages with damping ratios of 10, 15, 20 and 3 0 per cent and masses that, respectively, represent 0.67, 1.5, 2.7 and 6.0 per cent of the total mass of the bridge are considered. In the e xperimental test, a 3.7 m long cable-stayed bridge and an appendage co nsisting of a small mass, a small spring and a small viscous damper ar e built and the bridge tested, without and with the appendage, on a pa ir of shaking tables which are set to reproduce ground acceleration re cords from past earthquakes. The damping ratio of the appendage in thi s test is 32 per cent and its mass represents about 8 per cent of the total mass of the bridge model. In the numerical test, it is found tha t the appendages reduce the longitudinal response of the bridge deck u p to 88 per cent. Similarly, in the experimental test it is found that the appendage reduces the longitudinal bridge deck response by about 41 per cent. From these studies, it is concluded that the suggested ap pendages may indeed be effective in reducing the seismic response of c able-stayed bridges as they are for building structures.