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- 1 (2013-07-24 (水) 13:33:51)

Minato-Shin bridge of R-23 †

1.Field of application †

Minato-Shin bridge of Route 23, Nagoya, Aichi.

2.Circumstances of Repair †

Fatigue cracks were found at the corners of the diaphragms in the box-girders and at the intersection of the ribs on the steel plate floor deck in 1987 after 23 years of service.

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3.Types of structure †

The outline of the bridge structure is as follows.

Bridge length : 250.0 m

Span length : 77.0 + 96.0 + 77.0 m

Structure : Tree spans continuous two box-girders bridge with steel plate floor.

Deck : Steel plate floor (12mm thickness)

Structural steel : SS 41 , SM 50

Elevation and cross section are shown in figure 1. The box girders are spaced at 11.0 m and depth is from 2.5 m to 5.6 m. The interior of the box-girders contain the diaphragms. Diaphragms were composed of the vertical stiffeners and the transverse ribs. In the vertical stiffeners at the diaphragm locations, the flanges had been installed to increase the stiffness. On the other hand no the flange plates existed in the vertical stiffeners at the intermediate transverse rib locations.

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Fig.1 Elevation and plan views.

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4.Details of loading †

Fluctuating loads due to vehicles live load.

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5.Description of damage †

Fatigue cracks were found in the floorbeam top web gaps at the bottom chord member connections (Fig.2). These cracks initiated from the web weld toes at the floorbeam web-flange connections. The cracks horizontally propagated along the web-flange weld. Propagating over the width of the connection plates which connected the floorbeams to the main girders, the cracks diagonally propagated down (Fig.3). The crack of the web gap at and near the end floorbeam locations propagated more than the crack at the center span.

Cause of fatigue cracks was thought as follows:

Out-of-plane deformation which resulted from the horizontal relative displacement between the bottom chord member and floor system.

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Fig.2 Crack at the floorbeam top web gap.

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Fig.3 Crack location.

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6.Repair method applied †

The original deck at the warping was separated from the floorbeam. The floorbeam top flanges were bolt spliced to the main girder top flanges (Fig.4).

Repair of parts subjected to cracking was performed by re-welding. Filled by welding, the finishing was performed by grinding.

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Fig.4 Connection detail between the floorbeam top flange and the chord member to flange.