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Steel bridge of The Metropolitan Expressway, Tokyo, Japan.
Fatigue cracks were found in the coped end of the plate girder in 1978 after 14 years of service.
The outline of the bridge structure is as follows.
- Span length : 31.0 m
- Structure : Simple composite plate girder bridge.
- Deck : Reinforced concrete deck ( 18.0 cm thickness )
- Structural steel : SS 41
Elevation and cross section are shown in figure 1. The number of the main girders which are spaced at 2.4 to 3.2 m and are seven. The main girders are deep at 1.8m and are coped at the ends.
Fig.1 Elevation and cross section
Fluctuating loads due to highway live load.
Fatigue cracks were found in the coped end of the main girder and the swells of the girder webs also occurred in the girder ends due to buckling (Fig.2).
Cause of fatigue cracks was as follows:
- Concentration of stress unforeseen in design.
- Functional disorder of the movable support.
Fig.2 Crack location and swell range of the girder web.
Repair of parts subjected to cracking had been performed by stop hole and bolt connection as shown in figure 3. After completion of repair, inspection indicated that fatigue crack re-initiated from the stop hole. Therefore, additional repair and strengthening were required.
New plates with the rib plates were bolt connected to both sides of the girder webs (Fig.4). This repair made the swell of the girder web reform. These new plates of 9mm thickness played the role of transmitting the load.
Fig.3 Primary repair.
Fig.4 Final repair
