The Tacoma Narrows Bridge failure in 1940, although it
was not the first bridge to be destroyed by wind, gave
rise to serious initial research on aerodynamic stability
of suspension bridges..
This research is a part of the above PhD emphasized the following issues
- Natural Mode shapes and Frequencies
- Wind Loads on Bridges and Flutter
- Suspension Bridge Flutter for Girder with Separate
Control Flaps
- Buffeting Response of Suspension Bridge
2. Natural Mode shapes and Frequencies The natural mode shapes and frequencies of suspension bridges are the first items to be computed in design and analysis of the aerodynamics of suspension bridges.
The following notations of the bridge mode shapes are
used.
SV : Symmetric vertical mode
ST : Symmetric torsional mode
ASV : Asymmetric vertical mode
AST : Asymmetric torsional mode The key input to configure a suspension bridge using FEM is the cable sag fm in the main span centre and the cable sag fs (vertical) in the side span centre. The relation between fm and fs can be approximately obtained by eq. (2.1) 123456789_123456789_123456789_123456789_12345678
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Figure: Principle of deck girder with separate control flaps, [00,0]
Once the final configuration of the suspension bridge is obtained through iteration, the natural mode shapes and frequencies can be performed. 123456789_123456789_123456789_12
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When the total horizontal cable force is acting on a single joint at the pylon top, the longitudinal movement of the pylon top (the flexural stiffness of each pylon leg) will have influence on the torsional modes of the bridge.
In the ST mode, one main span cable vibrates downwards and its corresponding pylon tops vibrate towards each other in longitudinal direction due to the cable inextensibility in the quasi-static state. In the contract, the other main span cable vibrates upwards and its corresponding pylon tops vibrate away from each other, Figure 2.5
The increased constraining effects of the pylons of the
bridge in ST modes can increase the torsional frequency
A numerical example for suspension bridge span 1000+2500+1000m is employed, with and without active vibration control. The Akashi Kaikyo Bridge and the Great Belt Bridge are investigated for the key data of the example, Figure 2.2 to Figure 2.4.
