Gerber bridge on roadway bridge †

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1.Field of application †

Gerber bridge on the roadway bridge, Japan.

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2.Circumstances of Repair †

Fatigue cracks were found in the coped corner at hinge location of the Gerber bridge in 1984 after 29 years of service.

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3.Types of structure †

The outline of the bridge structure is as follows.

Elevation and plan views are shown in figure 1. The bridge is composed of the box-girder in the center of the cross section and the I-beam girders on both sides of the box-girder as shown in figure 1. The longitudinal ribs on the steel plate floor are Y-type ribs. The structure with Y-type ribs were few used.


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Fig.1 Elevation and plan views.


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4.Details of loading †

Fluctuating loads due to highway live load.

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5.Description of damage †

The location and the number of fatigue cracks which were found as a result of the inspection are shown in table 1. The cracks were observed at the connections of the members as shown in figure 2.

Table.1 Number of cracks and ratio of the occurrence.

Member  Total location  crack location Ratio
 At the weld between the deck plate and vertical stiffener.    144    143   99%
 At the weld between the floorbeam bottom flange and stiffener.      76      34   45%
 At the intersection of the longitudinal rib and transverse rib.   3160    256     8%
 At the boxing of the floorbeam web stiffener.       32      25   78%
 At the manual weld of transverse rib.    292        2     1%
 the gusset at the cross bracing location.    152      24   16%
 the gusset at the diagonal member  ( I-girder side)      38      20   53%
 the gusset at the diagonal member (box-girder side)      38        6   16%
Total       510  


These cracks showed the characteristic as follows (Fig.2).

1. The cracks of type A were only observed in the I-beam sections(Fig.3). No the cracks of type A occurred in the welds of the vertical stiffener ends, which were connected to the steel plate deck, in the inside webs of the box-girder.

2. The difference of the tendency in the crack occurrence were not observed in both ways.

3. Fatigue cracking did not show a tendency in the axial direction of the bridge. The crack at the weld of the vertical stiffener top end.

Type A of the cracks were classified in three patterns (Fig.4). However most cracks were crack-1 that originated from the boxing toe at the vertical stiffener side. Some of them propagated into the steel plate deck (Fig.4:crack-1').

Cause of fatigue cracks was thought as follows:



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Fig.2 Crack location.


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Fig.3 Crack at the weld between the stiffener and deck plate.


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Fig.4 Crack location at the weld.


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6.Repair method applied †

After frame cutting the vertical stiffener upper parts, the fillet weld terminations exposed by frame cutting (Fig.5:A point) were welded as repair. The finishing of the weld toe was performed by TIG dressing.

In case of having propagated into the deck plate, 22mm diameter stop holes were drilled at the crack tips. T-type plates were weld connected to the deck plate (Fig.5).

Repair was performed by TIG dressing and R process to the locations where the stress concentrated.

The original cross bracings were strengthened to the double warren structure due to the addition of the members.


Follow-up inspection have been executed because of confirming the effect of the repair and the strengthening. Periodic inspection will be continued too.

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Fig.5 Repair procedure.
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