No. 28 County road Br. のバックアップ(No.1)

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  • 1 (2013-09-24 (火) 15:32:54)

Dan Ryan Express †

1.Field of application †

Dan Ryan elevated bridge of the Lake-Dan Ryan express line in Chicago, Illinois.

2.Circumstances of repair †

Fatigue crack was found in the transverse box beam cap of the rigid-frame steel pier in Jan. 1978 after 9 years of service. After finding this crack, repair was immediately recommended.

3.Types of structure †

This bridge is curved and elevated structure (Fig.1,2). The superstructure is composed of four continuous plate-girders and field-placed concrete deck (Fig.3). The transverse beams of the rigid-frame steel pier which support four plate-girders and columns are box sections (Fig.4). The girder bottom flanges pass through the slot-holes, which were formed near the box beam lower web, and is partial penetration groove weld connected to the box beam webs. The welded girder members, which corresponded to 0.9~1.22mm length from both side of the transverse beam, was fabricated with the pier and was pin-link or bolt connected to the adjacent girder members. The structural steel was applied to the quality of A36 steel as a result of the tensile test and so on.

4.Details of loading †

Fluctuating loads due to rail live load.

5.Description of damage †

Fatigue cracks was found in the groove weld at the box beam web connection with the girder bottom girder flange (Fig.5). In the fabrication, the slot holes were firstly opened to the box beam webs by frame-cutting in order to permit the penetration of the girder flanges. Secondly , the girder bottom flanges were inserted in the slot hole and were groove weld connected to the box beam webs.

Causes of fatigue crack were as follows:

• Lack of fusion in the groove weld which connected the girder bottom flange to the transverse box beam cap of the pier.
• Detail of the weld joint indicating the low fatigue strength.

6.Repair method applied †

After the discovery of the crack, temporary supports were immediately constructed and stop holes were drilled at the crack tips as a emergency repair.

Strengthening was recommended to lessen the high stress concentration and residual stress at the girder flange-box beam web connections where fatigue crack did not initiate yet. Two 64mm diameter holes were drilled on side of the box beam web adjacent to up-and-down girder flange termination. The two holes were connected by a vertical saw cut (Fig.6).

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