U.S.51 Br. のバックアップ(No.1)

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  • 1 (2013-09-17 (火) 15:17:22)

I-93 †

1.Field of application †

The Central Artery of I-93 in Boston, Massachusetts.

2.Circumstances of repair †

• Horizontal cracks were found along the longitudinal girder bottom web-flange weld at the flange termination in 1984 after approximately 10 years of service.

• Vertical cracks were found in the longitudinal girder webs at the reentrant corner of the bottom cope and along the bolt fixity line of the connection angle in 1984 after approximately 10 years of service.

• Fatigue cracks were found at the intersection of the fillet weld and the groove weld in 1987 after approximately 13 years of service.

3.Types of structure †

The bridge is a viaduct structure with bi-level rigid steel frame bents supporting the reinforced concrete decks, two longitudinal I-girder per level ,and multiple longitudinal stringers per level.

Plan view and framing plan is shown in figure 1,2. The columns of the bents are box sections and the transverse beams of the bent are either box section or I-section (Fig.3,4,5). The longitudinal girder top and bottom flanges (an webs) are coped by flame-cutting to permit a bolted web connection, using double angle, to the transverse beams of the bent. The girder webs are bolt connected to the double angles which are bolt connected to the transverse beams of the bent (Fig.6). The structural steel is ASTM A588.

4.Details of loading †

Fluctuating loads due to vehicles live load. This resulted in significant cyclic stress occurred from the longitudinal traction and braking forces.

5.Description of damage †

Ⅰ. Horizontal cracks were found along the longitudinal girder bottom web-flange weld at the flange termination (Fig.7).

Ⅱ. Vertical cracks were found in the longitudinal girder webs at the reentrant corner of the bottom cope and along the bolt fixity line of the connection angle (Fig.8).

The fatigue cracks occurred at locations where the girder web was bolted full depth to the connection angle.

Other locations where the girder web was not bolted full depth to the connection angles did not contain fatigue cracking in either the cope or the connection angles (Fig.9).

Ⅲ.A crack was found at the intersection of the fillet weld, which connected the closure plate to the column flange, and the groove weld, which connected the top flange of the box beam to the column flange.

Causes of fatigue crack were as follows:

Ⅰ.The lateral displacement of the longitudinal girder bottom flanges originated during fabrication (Fig.8) and the corresponding web gap distortion at the connection between the longitudinal girder and transverse beam.

Ⅱ.End fixity of the double angle connections resulted in large cyclic stresses acting on the web plate.

·Coping by flame-cutting which resulted in high tensile residual stress along the cut edge and a sharp.

Ⅲ.A fabricated lack of fusion plane at the corner fillet welds of the bent transverse beams.

6.Repair method applied †

• The 1984 recommended retrofit for fatigue cracking in the web copes of longitudinal girders and the floorbeams was 25mm hole-drilling at the crack tips.

A further retrofit was required. Holes were drilled in the girder web and flange and an angle, 200 by 200 by 29mm, was bolted to the girder inside web and flange and to the connection angle (Fig.10,11,12). 16mm fill plate was used on the web and the lower bolt holes ware reamed for bolting.

• Removing the lower nine bolt rows in the web connection for relaxing the compression web and moving the bottom flange toward normal alignment.
• The 1987 recommended retrofit was the drilling of a retrofit hole, 51mm, through the web thickness at the crack tip. For the lower beam-column connections of the bents, the retrofit was the transfer of the live load force from the lower beam top flange to the column.

Both procedure needed to enable the flanges of the wide-flange to be bolted to the webs of the column (Fig.13,14).

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