The Fourth Line of The Metropolitan Expressway のバックアップ(No.1)

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- 1 (2013-07-20 (土) 11:00:14)

The Fourth Line of The Metropolitan Expressway †

1.Field of application †

The Fourth Line of The Metropolitan Expressway, Japan.

2.Circumstances of Repair †

Fatigue cracks were found in the girder web gap plates at the floorbeam connection in 1988 after 15 years over of service.

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3.Types of structure †

The outline of the bridge structure is as follows.

Bridge length : 30.0 m ~ 40.0m
Structure : steel simple I-beam girder bridge
Deck : Light-weight concrete deck ( 19 cm ~ 20 cm thickness)
Structural steel : SS41

Framing plan and cross section were shown in figure 1. The number of the main girders which are spaced at 2.9m is six. The floor system is composed of the floorbeams, transverse cross bracings and the lateral cross bracings . Web gap plates were weld connected between the floorbeam flanges and the girder flanges.

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Fig.1 Side view and cross section

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4.Details of loading †

Fluctuating loads due to highway live load.

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5.Description of damage †

Fatigue cracks were found in the welds of the web gap plates at the floorbeam and sway bracing locations. Fatigue cracks were classified in four types as follows (Fig.2,3).

(A) Fatigue cracking which occurred in the web gap plate welds at the connection with the girder top flanges.

(B) Fatigue cracking which occurred in the scallop of the web gap plates.

(C) Fatigue cracking which occurred in the web gap plate welds at the connection with the girder webs.

(D) Fatigue cracking which occurred in the girder web-flange weld at the scallop location.

Type A cracks are reported that were the most frequent observed in four types of the cracks.

Cause of fatigue cracks was as follows:

Out-of-plane distortion of the girder web gap resulted from the differential rotation between the deck and the floorbeam.

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Fig.2 Types of crack..

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Fig.3 Types of crack..

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6.Repair method applied †

Repair of parts subjected to cracking was performed by welding and replacement of the web gap plates.

The repair procedure is as follows.

The original web gap plates were removed and the new plates of 19mm thickness were weld connected at the web gap locations (Fig.4). The additional rib plates were installed on the outside web face of the exterior girders (Fig.4(a)). Gouging out, crack area was full penetration welded.

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(a)Repair for the exterior girder.

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(b)Repair for the interior girder.

Fig.4 Repair method.