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*Prairie Du Chien Br. [#y3c4e4ff] **1.Field of application [#n747a2a3] >Prairie Du Chien Bridge of U.S.18 over Mississippi River between Prairie Du Chien in Wisconsin and Marquette in Iowa. Mississippi River forks here. **2.Circumstances of repair [#pb139bd2] >Fatigue cracks were found in the floorbeam web gaps of the tie girder-floorbeam connections in tied arch bridge in Aug.1979 after 5 years of service. >Fatigue cracks were found in the girder web of the girder-floorbeam connections in plate girder bridge. **3.Types of structure [#q8c7879a] >In the forked west side river, both sides of the tied arch bridge is connected to welded plate girders approach ([[Fig.1>#fig1]]). Typical cross section of the tied arch bridge and welded plate girder is shown in [[figure 2>#fig2]]. The reinforced concrete deck of 203mm thickness forms composite action with the stringers. The stringers are connected to the floorbeams in tied arch bridge and are on the floorbeams in plate girder bridge. The superstructure is fabricated from A36 and A441 steel. &br; CENTER:&aname(fig1);&attachref(Prarie22.gif);&br; &br; CENTER:Fig.1 Plan and elevation views. &br; &br; CENTER:&aname(fig2);&attachref(Prarie23.gif);&br; &br; CENTER:Fig.2 Cross section of the plate girder bridge and the tied arch bridge. &br; **4.Details of loading [#ia28893d] >Fluctuating loads due to vehicles live load. **5.Description of damage [#a7a4107e] >In tied arch bridge, fatigue cracks were found in the floorbeam web-flange welds and in the fillet welds at the connection plate ends of the tie girder-floorbeam connection ([[Fig.3>#fig3]]). >Causes of fatigue cracks were as follows: -Out-of-plane displacement of the web gap which resulted from relative displacement of the floorbeam system. (In case of the crack of the floorbeam web-flange fillet weld.) -End fixity of the floorbeam. (In case of the crack of the connection plate end.) >In plate girder bridge, fatigue cracks were found in the girder webs of the girder-floorbeam connections as shown in figure 4 (Fig.5,6). connections as shown in [[figure 4>#fig4]] ([[Fig.5>#fig5]],[[6>#fig6]]). All the cracks occurred at the negative moment regions where the connection plates are not connected to the girder tension flanges. >Cause of fatigue cracks was as follows: -Out-of-plane displacement of the girder web gap due to deformation of the floorbeam under live load. &br; CENTER:&aname(fig3);&attachref(Prarie15.gif);&br; &br; CENTER:Fig.3 Cracking location of the girder-floorbeam connection. &br; &br; CENTER:&aname(fig4);&attachref(Prarie16.gif);&br; &br; CENTER:Fig.4 The girder-floorbeam connection near the pier. &br; &br; CENTER:&aname(fig5);&attachref(Prarie17.gif);&br; &br; CENTER:Fig.5 Crack of the girder web gap. &br; &br; CENTER:&aname(fig6);&attachref(Prarie18.gif);&br; &br; CENTER:Fig.6 Crack occurred in the floorbeam web gap and stop holes at the crack tips. &br; **6.Repair method applied [#o2b73586] -In tied arch bridge. >The 19mm retrofit holes were drilled at the crack tips in order to prevent the crack propagation at the floorbeam web gaps (Fig.6). In order to prevent the crack initiation, the floorbeam flanges >The 19mm retrofit holes were drilled at the crack tips in order to prevent the crack propagation at the floorbeam web gaps ([[Fig.6>#fig6]]). In order to prevent the crack initiation, the floorbeam flanges were connected to the tie girder web and the diaphragms of the box-section tie girder through the tie girder web by using the Cut-T steel (Fig.7,8). tie girder web by using the Cut-T steel ([[Fig.7>#fig7]],[[8>#fig8]]). -In plate girder bridge. >Repair recommended was as same the method as the repair performed in Polk County Bridge. At the negative moment regions, top ends of the girder-floorbeam connection plates were removed. &br; CENTER:&aname(fig7);&attachref(Prarie19.gif);&br; &br; CENTER:Fig.7 Detail of the repair. &br; &br; CENTER:&aname(fig8);&attachref(Prarie20.gif);&br; &br; CENTER:(a) Cut-T steel and channel at the diaphragm connection with the tied girder web. &br; &br; CENTER:&aname(fig8);&attachref(Prarie21.gif);&br; &br; CENTER:(b) Cut C steel at the floorbeam flange connection with the tied girder. &br; CENTER:Fig.8 Connections between the tied girder and the floorbeam flange after repairing. &br; &br; &br;
