AEROSTATIC DIVERGENCE PROPERTIES OF SUSPENSION BRIDGES WITH LOW TORSIONAL STIFFNESS AND ANTI-WIND COUNTERMEASURES

Flexible suspension bridges are susceptible to wind-induced instabilities. The aerostatic stability properties of highway suspension bridges have been extensively investigated. It has been found that the torsional divergence ends up with the abrupt deck rotation of large amplitudes distributed in a symmetric manner along the bridge axis, and what accompanying to the instability is the complete unload of one of the two main cables to a stress state of the cable finish stage. In this work, however, it is found that the pattern of torsional divergence of small- to moderate-span bridges with very low torsional stiffness girders is significantly different from that of a traditional highway bridge due to the relative torsional stiffness of the bridge deck to that provided by the main cable system. In this case, the torsional divergence substantially differs from the traditional one and exhibits a twist-locking phenomenon. It ends up with asymmetric torsional deformation along the bridge axis with one or more inflection points, accompanied by the abrupt tightening up of the main cable system. Countermeasures are numerically investigated. The results show that improving the torsional stiffness of the main girder not only substantially enhances the threshold of divergence, but also changes the pattern of divergence from an asymmetric, twist-locking rotation to a symmetric rotation. Parametric investigations of wind-resistant cables are performed in terms of the cable section area, cable tension, installation angle and sag ratio. It is found that the best installation angle is between 15 to 45 degrees, and that the mitigation effect increases with the cable section area, cable tension and the sag ratio. The combination of the mitigation effect and material costs suggests that the best scheme is the one of wind-resistant cables to be of one third of the main cable’s area, of the amount of tension produced by a quarter of the dead load, and of a sag-ratio as large as allowed by the topography.