*Section L-2C [#n4672e7c]

**1.Field of application [#cb022eba]

>Washington Metropolitan Authority's metrorail near new Carrollton, Washington, D.C.

>The area is section L-2c.

**2.Circumstances of repair [#y20e2529]

>The structure was built in the late 1970's. Fatigue cracking was found in the back-up bar welds in 1987. A site inspection indicated that retrofit was required.

**3.Types of structure [#m4b5fbb4]

>The structure is composed of multiple simple spans. Each span consists of a steel box girder supporting a reinforced concrete deck. The plan and elevation views are shown in [[figure 1>#fig1]]. Within the box girder, X-type diaphragms are bolt connected to transverse connection plates which are fillet weld connected to the box girder webs and to the tension and compression flanges.   The web-flange welds of the box girder are full penetration groove welds with a back-up bar ([[Fig.2>#fig2]]).   The back-up bar is continuous and the butt splice of the back-up bar is a groove weld ([[Fig.2>#fig2]]).

>The structural steel is ASTM A588.

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CENTER:&aname(fig1);&attachref(Sectio40.gif);&br;
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CENTER:Fig.1 Plan and elevation views.
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CENTER:&aname(fig2);&attachref(Sectio41.gif);&br;
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CENTER:Fig.2 Schematic of the groove-weld butt-splice connection and the web-flange groove weld.
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**4.Details of loading [#ae259d81]

>Fluctuating loads due to train (Metrorail) live load.

**5.Description of damage [#ca65640a]

>Fatigue cracking was found in the groove-weld butt-splice connections of the back-up bars.

>Cracking occurred adjacent to the tension and compression flange. The crack had propagated into the box girder web-flange groove weld.

>Cause of fatigue crack was as follows:
-Luck of fusion in the groove-weld butt-splice connections of the back-up bars.
-Lack of fusion in the groove-weld butt-splice connections of the back-up bars.

**6.Repair method applied [#u225a865]

>The retrofit was to remove all of the groove-weld butt-splice connections adjacent to the tension flange. The procedure was to use a hole saw and to remove a 38mm hole encompassing the groove-weld butt-splice connection and the crack tip.   The crack tips which remained in the web-flange welds were to be ground out.

>The fatigue cracking adjacent to the compression flange was not critical and did not require retrofit.
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