Modal analysis of suspension bridge deck units in erection stage

A long suspension bridge located in a typhoon region may be exposed to a high risk of wind-induced large vibrations during the construction stage because of its long construction period. The dynamic properties of the bridge in different construction stages therefore have to be well understood in bridge design, so that a proper wind tunnel test or further dynamic response analysis can be performed. To this end, this paper presents a detailed finite element modeling and modal analysis of the Tsing Ma long suspension bridge when the first few deck units are erected at the bridge midspan. The three-dimensional finite element model includes not only the deck units but also the main cables to which the deck units are suspended through hangers, the bridge towers, and the side span cables. The results from the numerical analysis show a clear picture of how dynamic properties are transformed from the tower-cable system to the tower-cable-deck unit system. The results indicate that wind-induced vibration of a cable suspension bridge during the construction stage may be more critical than that of the completed bridge. The predicted natural frequencies and mode shapes of the tower-cable-deck unit system are also compared with the measured results, and the comparison is satisfactory.