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The different frequency variations result in the modal crossover phenomena, which reflect the instability of the dynamic characteristics during the long deck erection period. With more deck unites erected, the main girder is built up gradually, and thus the frequencies of deck-dominated mode would arise. Both the in-plane and out-of-plane modes including in-phase and out-of-phase modes of the two main cables in the same span or symmetry and anti-symmetry modes of the neighboring-span cables as well as their corresponding combinations can be classified into groups. Due to the strong interactions between deck unites and the cables, both the in-plane and out-of-plane vibrations of cables would excite the swing, lifting or torsional modes of the deck units. In the beginning of the deck units erection stage, the tower-cable-dominated modes are the primary modes and the deck-dominated modes come to next.

The results demonstrate that the natural frequencies of each erection stage are closely distributed in the low-order range. Then, the dynamic characteristics of each of the tower-cable-deck systems are analyzed to study the modal properties of TTTS suspension bridge in various deck erection stages. Based on the finite element (FE) model of a triple-tower twin-span (TTTS) suspension bridge in the completed stage, the FE models of the tower-cable-deck system corresponding to 0 %, 5 %, 10 %, 20 %, 30 %, and 40 % deck units erection stage are established respectively by a backward dismantling method. Modal analysis of large span suspension bridge in different construction stages has to be carried out for the aerodynamic instability analysis.