Bridge Flutter Derivatives at Vortex Lock-In

Abstract

Many structures with extended spans undergo the flow-induced oscillation commonly called vortex shedding. In the case of long-span bridges, Ehsan and Scanlan, employing basic data from deck section wind tunnel model tests, offered a nonlinear theoretical framework for predicting maximum vortex-induced amplitude under the lock-in condition. The present paper replaces the Ehsan-Scanlan model with a linearized form that brings it into conformity with the familiar flutter derivative expressions now commonly used in bridge wind-stability analyses. The essential new addition is an active sinusoidal lift term. Converting some of the Ehsan-Scanlan experimental section data to the new form, the suggested replacement model successfully reidentifies the range of original Tacoma Narrows vortex-induced response amplitudes that were experimentally obtained by Farquharson on a full-bridge model. Either vertical or torsional motions at vortex lock-in can be assayed by the method discussed.