Ft. Duquesne Bridge, Pennsylvania. †

1.Field of application

Northern approach Ramp of Ft Duquesne Bridge in Pittsburgh, Pennsylvania.

2.Circumstances of Repair

Fatigue cracks were found in the welds ,which connected the lower transverse beam to the column in the rigid -frame piers, in Sep. 1978 after 10 years of service.

3.Types of structure

Approach Ramp is the double-deck structure which is composed of the composite box girders, which are continuous for two and three spans, supported by the steel rigid-frame piers (Fig.1). Plan and side views are shown in figure2. The box girders and piers were individually fabricated by shop welding.
The box girders were connected to the piers by field welding. Center parts of the transverse beam of the pier were bolt connected on site. The differential cross sections of the box girders depended on span length. Typical cross sections of the box girder with the internal diaphragms are shown in figure 3. Typical detail of the rigid-frame pier is shown in figure 4. The transverse beam flanges were continuous and had penetrated the column. The column inside flanges were groove weld connected to the transverse beam flanges at the web corner. Most of piers were fabricated from A36 and A441 steel. Piers with the relative long span were from S517 steel.

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Fig.1 Rigid-frame pier and box-beam girder.

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Fig.2 Plan and side views.

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Fig.3 Typical cross section.

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Fig.4 Detail of the rigid-frame pier

4.Details of loading

Fluctuating loads due to vehicles live load.

5.Description of damage

Fatigue cracks were found in the weld joint which connected the box-section transverse beam flanges to the box-section column flanges in the rigid -frame piers.
There were two types of the fatigue cracks (Fig.5, Fig.6)
Fatigue cracking like lamellar tearing which occurred in the transverse beam flange plate center at the penetrating locations.
(Fatigue cracking was observed in A517 built-up members.)
Fatigue cracking which occurred in the groove weld toe which connected the column flange to the transverse beam flange. (Fatigue cracking was observed in both A517 and A 441 built-up members.)
The cracks occurred in both the transverse beam tension flange and the compression flanges. The cracks which occurred in the transverse beam tension flanges indicated the trace of the fatigue crack growth.
Cause of fatigue cracks was as follows:

6.Repair method applied

All the right-angled directional cracks were ground out and were removed by hole-drilling or core-drilling (Fig.5). The ground areas were checked by dye penetration. It was verified that no the crack tips remained.

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Fig.5 Repair part.
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