The design procedures implemented in bridge engineering after the dramatic collapse of the first Tacoma Narrows Bridge in 1940, includes the investigation of various aeroelastic phenomena, e.g. flutter of the bridge deck in its final stage. Bridge deck flutter is a dynamic instability caused by the motion induced wind load of the bridge deck when it is subjected to cross winds. The displacements are characterized by harmonic angular displacements, which increase for each oscillation cycle and eventually lead to structural failure.
The coupling of torsional and vertical modes of long span bridges with a single bridge deck has been known to cause dynamic instability in terms of classical flutter since the Tacoma Narrows investigations. The torsional stiffness of the bridge deck girder does usually ensure that the torsional frequency is considerably higher than the vertical frequency and thus postpones the onset of flutter to higher wind velocities. The effect of the torsional deck stiffness decreases when the span increases. For very long bridges this means that the torsional and vertical modes become very closely spaced. As the aerodynamic stiffness tends to decrease the torsional frequency in wind, the aerodynamic coupling between vertical and torsional modes is unavoidable. However, if the torsional still air frequency is below the vertical still air frequency, the modes will be decoupled with increasing frequency separation at higher wind velocities.
The coupling of torsional and vertical modes of long span bridges with a single bridge deck has been known to cause dynamic instability in terms of classical flutter since the Tacoma Narrows investigations. The torsional stiffness of the bridge deck girder does usually ensure that the torsional frequency is considerably higher than the vertical frequency and thus postpones the onset of flutter to higher wind velocities. The effect of the torsional deck stiffness decreases when the span increases. For very long bridges this means that the torsional and vertical modes become very closely spaced. As the aerodynamic stiffness tends to decrease the torsional frequency in wind, the aerodynamic coupling between vertical and torsional modes is unavoidable. However, if the torsional still air frequency is below the vertical still air frequency, the modes will be decoupled with increasing frequency separation at higher wind velocities.