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*Lafayette Street Br. [#l42366aa] **1.Field of application [#j702dd68] >Lafayette Street Bridge over Mississippi River located in St. Paul , Minnesota. **2.Circumstances of repair [#ye2247cb] >This bridge was opened in Nov. 1968. In May 1975, fatigue cracking was found in the girder web. **3.Types of structure [#n977ffd5] >The structure is continuous for three spans consisting of two longitudinal main girders ([[Fig.1>#fig1]]). The floorbeam frames into two main girders and the transverse cross bracings exist between the girders as shown in cross section of [[figure 1>#fig1]]. Two stringers with the wide-flanges are supported by the transverse floorbeam. The structural steel of the girder flanges and webs was ASTM A441. &br; CENTER:&aname(fig1);&attachref(I-93_h69.gif);&br; CENTER:&aname(fig1);&attachref(Lafaye87.gif);&br; &br; CENTER:Fig.1 Plan view of bents&br; CENTER:Fig.1 Elevation view and cross section.&br; &br; CENTER:&aname(fig2);&attachref(I-93_h70.gif);&br; &br; CENTER:Fig.2 Typical framing plan for one span.&br; &br; CENTER:&aname(fig3);&attachref(I-93_h71.gif);&br; &br; CENTER:Fig.3 Bent 7 elevation (with beam I-section).&br; &br; CENTER:&aname(fig4);&attachref(I-93_h72.gif);&br; &br; CENTER:Fig.4 &br; &br; CENTER:&aname(fig5);&attachref(I-93_h73.gif);&br; &br; CENTER:Fig.5 &br; &br; CENTER:&aname(fig6);&attachref(I-93_h74.gif);&br; &br; CENTER:Fig.6 &br; **4.Details of loading [#ie997659] >Fluctuating loads due to vehicles live load. This resulted in significant cyclic stress occurred from the longitudinal traction and braking forces. >Fluctuating loads due to vehicles live load. &br; **5.Description of damage [#t1dbed76] >Ⅰ. Horizontal cracks were found along the longitudinal girder bottom web-flange weld at the flange termination ([[Fig.7>#fig7]]). >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>#fig8]]). >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>#fig9]]). >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. >Fatigue crack initiated from the weld intersection which connected the web gusset plate to the vertical stiffener ([[Fig.2>#fig2]]). &br; >Causes of fatigue crack were as follows: >Cause of fatigue crack was as follows: -The lack of fusion which had included in the one-side ?-type groove weld that connected the web gusset plate to the vertical stiffener.&br; &br; >Ⅰ.The lateral displacement of the longitudinal girder bottom flanges originated during fabrication ([[Fig.8>#fig8]]) 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. CENTER:&aname(fig2);&attachref(Lafaye88.gif);&br; &br; CENTER:&aname(fig7);&attachref(I-93_h75.gif);&br; CENTER:Fig.2 Crack initiated from the girder web gusset plate weld. &br; CENTER:Fig.7 Vertical and horizontal fatigue cracks at the bottom web cope.&br; &br; CENTER:&aname(fig8);&attachref(I-93_h76.gif);&br; &br; CENTER:Fig.8 Cross-section view of angle with fatigue cracks.&br; **6.Repair method applied [#rfc5e1c0] &br; CENTER:&aname(fig9);&attachref(I-93_h77.gif);&br; &br; CENTER:Fig.9 Longitudinal girder not bolted full depth to the connection angles.&br; >At the time of discovery, the crack had propagated part way up the web and the bottom flange had completely fractured ([[Fig.2>#fig2]]). The repair of parts subjected to cracking was the bolted splice. **6.Repair method applied [#rfc5e1c0] -For the other bridges which had suffered the same cracking. -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. >Strengthening was carried out for the other bridges in which the same damages were observed in the weld intersection connected the web gusset plate to the vertical stiffener. >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>#fig10]],[[11>#fig11]],[[12>#fig12]]). 16mm fill plate was used on the web and the lower bolt holes ware reamed for bolting. >Two 32mm diameter vertical holes were drilled through the gusset plate at the intersection of girder web, vertical stiffener, and gusset plate ([[Fig.3>#fig3]],[[4>#fig4]]). The hole edges were ground smooth. Two vertical holes which are smaller than those holes were drilled through the gusset plate at the intersection of the gusset plate and the vertical stiffener. The exposed toes of the groove weld, which connected the gusset plate to the vertical stiffener, was ground smooth. &br; CENTER:&aname(fig10);&attachref(I-93_h78.gif);&br; &br; CENTER:Fig.10 1984 retrofit procedure for all of the longitudinal girder bottom web gaps.&br; >At the locations where the crack propagated in the girder web, Two 32mm diameter holes were drilled at the angle from the web opposite faces through the girder web thickness ([[Fig.5>#fig5]]). &br; CENTER:&aname(fig11);&attachref(I-93_h79.gif);&br; CENTER:&aname(fig3);&attachref(Lafaye89.gif);&br; &br; CENTER:Fig.11 1984 retrofit of a longitudinal girder bottom web gap viewed from girder interior. CENTER:Fig.3 The retrofit procedures. &br; &br; CENTER:&aname(fig12);&attachref(I-93_h80.gif);&br; CENTER:&aname(fig4);&attachref(Lafaye90.gif);&br; &br; CENTER:Fig.12 1984 retrofit of a longitudinal girder bottom web gap viewed from girder exterior. CENTER:Fig.4 Retrofit hole at the gusset plate. &br; -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>#fig13]],[[14>#fig14]]). &br; CENTER:&aname(fig13);&attachref(I-93_h81.gif);&br; CENTER:&aname(fig5);&attachref(Lafaye91.gif);&br; &br; CENTER:Fig.13 1987 retrofit procedure for I-section of beam-column connection.&br; CENTER:Fig.5 Retrofit hole through the girder web. &br; CENTER:&aname(fig14);&attachref(I-93_h82.gif);&br; &br; CENTER:Fig.14 1987 retrofit of beam-column connection showing flange of wide-flange bolted to web column. &br; &br;
