Flutter, Galloping, and Vortex-Induced Vibrations of H-Section Hangers: Fid-Bau Portal

Flutter, Galloping, and Vortex-Induced Vibrations of H-Section Hangers

Chen, Z. Q. / Liu, M. G. / Hua, X. G. / Mou, T. M.

Abstract

Hangers in arch bridges and other long vertical bars in truss bridges are typically slender members and often adopt an aerodynamically unfavorable H-section due to the conveniences in manufacture, construction, and maintenance. In contrast to the very narrow range of wind attack angles of horizontal members, hangers may suffer wider wind attack angles, possibly from 0° to 360°, and are therefore more prone to wind-induced vibrations. In August 2006, large torsional vibration with severe damage was observed on the 13 longest hangers of the Dongping arch bridge in China during a strong wind. While vibrations of hangers were usually caused by galloping, and vortex shedding excitation, the present case was likely to be a kind of torsional flutter instability. Therefore an in-depth investigation on the hangers’ aerodynamic performances in forms of flutter, galloping, and vortex shedding has been conducted through a series of wind tunnel tests. First, with section model and aeroelastic models tests of the longest hanger in the bridge, the observed field vibration is confirmed as torsional flutter under large attack angles from 15° to 25°, and the experimental onset velocity coincides well with the field observation; the flutter derivative $A_{2}^{*}$ becomes positive at a low reduced wind velocity, which further implies that the H-section is prone to flutter instability. Then, the influences of web perforation on flutter, galloping, and vortex shedding are studied with four section models having different web perforation ratios but the same depth-to-width ratio $D / B = 0.416$ (flange depth $D$ to web width $B$ ). It is found that the web perforation may increase the galloping critical velocity to some extent but have no obvious effects on flutter instability and the Strouhal numbers, at least for the shallow H-section with $D / B = 0.416$ . Next, a total of 16 H-section models with different $D / B$ ratios, web perforation ratios, and flange perforation ratios are tested to investigate their effects on aerodynamic behaviors of hangers. A comparison of the experimental results with previous work is made, which may explain why the flutter instability of H-shaped hangers under large attack angles was not treated by earlier investigators.