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*108 trucks)

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5.Description of damage and Repair method applied †

Fatigue cracking which was found in the girder web originated from the butt connection groove weld of the longitudinal stiffeners (Fig.3,4). The fracture in the girder web resulted from a fatigue crack (Fig.3).

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Fig.3 Crack originated from the butt connection groove weld of the longitudinal stiffeners.
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Fig.4 Crack propagated in the girder web.

From thorough investigation of fracture, it turned that non-deposited part of butt weld of horizontal stiffener caused and crack were originated at intersection of web plate and stiffener. It is probable that cracks in non-deposited part propagated during transportation and erection.

Web plate was brittle-fractured from the first fatigue crack. The crack became unstable and propagated rapidly and stopped around the center of flange or web plate. It is found there are some stages in which cracks propagates.

The portion crack appeared was cut off and splice plate was attached by bolts(Fig.5). For cracks which did not propagate so much, drill holes were opened on web plate and cracks were separated from the girder(Fig.6).

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Fig.5 Bolt splice repair of fatigue cracked girder web.
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Fig.6 Hole-drilling repair.


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