Torsional Analysis of Boxgirder Suspension Bridges

Theories are presented for the static torsional analysis of the single-span box-girder suspension bridge. The box-girder deck is chosen because of its current popularity and because of the simplicity of its torque-twist relation. An elastic theory is developed and then extended to a deflection theory. It is shown that the torsional elastic theory is usually inadequate in predicting the bridge's behavior under torsional live loading and that recourse should always be made to the torsional deflection theory which is not intrinsically more difficult. This torsional deflection theory provides a natural complement to the classic flexural deflection theory in which the deck is assumed to behave purely in flexure. Three examples are given to show that the torsional analysis is governed primarily by two characteristic dimensionless parameters. These are the relative torsional stiffness parameter (of deck and cables), the parameter involving cable geometry and elasticity, and the torsional stiffness of the towers.