Yellow Mill Pond Br. (Cover plate ends, stress concentration) †

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1.Field of application †

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2.Circumstances of repair †

Fatigue cracking was found in the cover plate fillet weld toe in Nov. 1970 after 12 years of service. In following site inspections of the other structure, fatigue cracks were found in various locations.

Inspection and strain measurement of the bridge structures indicated that repair was required.

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3.Types of structure †

The structure is composed of multiple simple spans with multiple plate girders (Fig.1). This bridge is rolled-section with the cover plated beam and composite girder with a reinforced concrete deck. The framing plane and typical girders' cross section of span 10 are shown in figure 2,3.The cover plates are partially installed in the main girders except for M88 and M83 girders. All of the cover plates' end are vertically cut and those edge-corners are 75mm radius roundness. The structural steel in span 10 is A242.

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Fig.1 Plan and elevation view.

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Fig.2 Framing plan and cracked location.

(x: cracking location, o: not-finding crack location by UT, φ: minor crack not-identified by UT)
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Fig.3 Typical cross section.

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4.Details of loading †

Fluctuating loads due to vehicles live load.

Total traffic from 1958 to 1976 is 259*108(with 35*10^8 trucks)

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5.Description of damage and Repair method applied †

The fatigue crack initiated at transverse fillet weld toe, which connected cover plates with lower tension flange. The crack propagated through the lower flange and extended into web(400mm)(Fig.4).

At every inspection, similar fatigue cracks, which does not extend into main girder web, were found. And fatigue cracks, which initiated from weld root and grew 152mm, were also found(Fig.5).

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Fig.4.

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Fig.5 Fatigue crack.

Cause of fatigue crack was as follows:

Because a few of cyclic stress ranges were over the fatigue limit under constant loading condition (fig .6) and because of the too much traffic, the fatigue cracks initiated and propagated at weld which connected the cover plate with main plate lower flange.

Analysis results about the fatigue cracks/failures were equal with the laboratory fatigue test results.

The fracture toughness of material of girders is sufficient enough to avoid collapse of girder(Fig.7).

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Fig.6 Result of the field measurement.

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Fig.7 The fracture toughness of material of girders

The recommended retrofit for improving the cover plate girder in span 10 was peening the weld ends and Gas Tungsten re-weld (Fig.8,9). The retrofits of the fatigue cracking which are more than 38mm in length along the weld toe were bolt spliced (Fig.10) and hole-drilling in the girder webs just above the cracking.

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Fig.8 Weld toe re-welded by Gas Tungsten arc.

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Fig.9 Weld toe peened.

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Fig.10 Detail of the bolt splice.


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