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of the wind tunnel test results, the optimum lower angle of 30° was observed. Figure 13.41 shows the flow visualization around the girders with the blunt fairing with various angles of lower slope. In the case of 20°, the separation flow from the lower side of the leading edge reattaches on the lower surface of the downstream edge. In the case of 30°, the separation flow flows down without reattachment. And in the cases of larger than 40°, the separation flow rolls in the inner space of the open deck. As a result, in the cases of less than 30°, vortex-excited vibration is induced, and in the cases of larger than 40°, stall flutter is induced.

13.3.2.2.2 Galloping

Since a box-girder bridge is usually constructed by using a beam with a rectangular section with a small side ratio, the galloping is easily induced. For suppression of the galloping, it is very useful to mount the horizontal plate on the surface of the lower part of the girder as shown in Figure 13.42 [3]. Figure 13.43 shows one of the examples of the effectiveness of the horizontal plate for suppressing galloping [35]. The top figure is the graph of the aeroelastic response of the original section. Galloping occurs at high wind velocity. The bottom figure is the graph of the aeroelastic response of the improved section by attaching a horizontal plate on the surface of the lower part of the the girder. The figure shows that the galloping which appears in original section is suppressed in the improved section.

13.3.2.2.3 Flutter

The shallow cross-section is susceptible to flutter that is self-excited vibration with torsional vibration. Figure 13.44 shows countermeasures to suppress flutter [3]. The girders with open grating (a) and central opening (b) are used to reduce the pressure difference between upper and lower surfaces of the girder, which is a main cause of occurrence of flutter. The vertical plates (a) called as ''stabilizer'' and ''center barrier'' have a role to prevent the separation flow from the upstream upper chord of the truss-stiffened girder from smoothly flowing down and to raise the flutter onset wind velocity.

= 50°

FIGURE 13.41 Flow visualization of bridge girder with blunt fairing with various lower slope angles. (Kubo et al. 1993. Reprinted with permission from Elsevier.)

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