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