This study examines the fragility curves of a bridge by two different analytical approaches; one utilizes the time-history analysis and the other uses the capacity spectrum method. The latter approach is one of the simplified nonlinear static procedures recently developed for buildings. In this respect, a sample of 10 nominally identical but statistically different bridges and 80 ground-motion time histories are considered to account for the uncertainties related to the structural capacity and ground motion, respectively. The comparison of fragility curves by the nonlinear static procedure with those by time-history analysis indicates that the agreement is excellent for the state of at least minor damage, but not as good for the state of major damage where nonlinear effects clearly play a crucial role. Overall, however, the agreement is adequate even in the state of major damage considering the large number of typical assumptions under which the analyses of fragility characteristics are performed.

A significant vortex-induced vibration (VIV) was observed in a suspension bridge under a wind velocity of approximately 6 and 7 m/s, and with a maximum amplitude that exceeded the serviceability limitations. Since the observation of VIV in an operating bridge was an infrequent event, this study investigated the cause of unexpected VIV. A series of wind tunnel tests revealed the main cause of the VIV was temporary screens applied over the guardrails to shield the curing surface during replacement of epoxy-coated pavement. The characteristics of the measured VIV were consistent with those from experimental results in terms of amplitude and lock-in wind speed range. The effect of Scruton numbers on VIV amplitude was examined for several dominant modes with different modal mass and damping ratios. Since the VIV amplitude is sensitive to the inherent damping ratio, the modal damping ratios of the bridge were identified from output-only modal analysis by using the frequency domain decomposition. This approach was efficient in identifying the damping ratios of high modes that were challenging due to weakly excited signals. The damping ratio of the triggered mode for VIV was the lowest among the investigated modes and estimated to be the level of the design-damping ratio.