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The Fourth Line of The Metropolitan Expressway, Japan.
Fatigue cracks were found in the girder web gap plates at the floorbeam connection in 1988 after 15 years over of service.
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.
Fig.1 Framing plan and cross section.
Fluctuating loads due to highway live load.
Fatigue cracks were found in the welds of the web gap plates at the girder-floorbeam locations (FIg.2,3).
The characteristic of the fatigue cracks were as follows.
- The frequency of the cracks (A1,A2) initiation at the top web gap plate weld are high.
- The cracks which occurred at the girder web-floorbeam top flange weld were observed in the weld beads and in the weld toes of the girder both sides web.
- In the bottom web gap plates, the cracks at the floorbeam bottom flanges connections were high frequency and initiated from the weld toes of the web gap plate side.
Cause of fatigue cracks was as follows:
- Out-of-plane distortion resulted from the differential deflection between the girders.
Fig.2 Crack at the girder-floorbeam locations.
Fig.3 Crack around the rib (web gap) plates at the girder-floorbeam locations.
Repair of parts subjected to cracking was performed as follows.
- Replacement to the thicker web gap plates for the cracks which occurred in the girder web.
- For the cracks except the above cracks, full length of the web gap plates welds to the girder top flange and the floorbeam top flange was full penetration welded and also TIG dressed (Fig.4).
Fig.4 Detail of repair.
