![[PukiWiki]](image/pukiwiki.png "[PukiWiki]")
*Polk County Br. [#w89175bb] **1.Field of application [#g98af300] >Des Moines (Polk County) Bridge over Chicago Rock Island and Pacific railroad route and East Four Mile Creek of Rt. 163 near Des Moines, Iowa. **2.Circumstances of repair [#ff00db1f] >Fatigue cracks were found in the girder web gaps at the girder-floorbeam connections in 1979 after 16 years of service. **3.Types of structure [#v157fcd4] >The structure is five span continuous skew bridge and is composed of two longitudinal plate girders, which were welded members, floorbeam and stringers. The framing plan and cross section are shown in [[figure 1>#fig1]],[[2>#fig2]],[[3>#fig3]]. The web depth of two longitudinal girders is 1.88m and the web thickness is 11mm. The floorbeam support two stringers and has stiffeners on both sides of the web. The floorbeam web plates are 1.22m by 8mm. The floorbeams at the piers were bolt connected to the transverse connection stiffeners which were located away 229mm from the end stiffener. The reinforced concrete deck was of 184mm thickness and was supported by two stringers and two longitudinal girders. All the steel members are fabricated from A36 steel. &br; CENTER:&aname(fig1);&attachref(PolkCo9.gif);&br; &br; CENTER:Fig.1 Plan view. &br; CENTER:&aname(fig2);&attachref(PolkCo10.gif);&br; &br; CENTER:Fig.2 Typical cross section. &br; CENTER:&aname(fig2);&attachref(PolkCo11.gif);&br; CENTER:&aname(fig3);&attachref(PolkCo11.gif);&br; &br; CENTER:Fig.3 Framing of the floorbeam. &br; **4.Details of loading [#lda8705c] >Fluctuating loads due to vehicles live load. **5.Description of damage [#pede9452] >Fatigue cracks were found in the girder web gaps at the girder-floorbeam connections near the pier locations of the negative moment regions. >Fatigue cracks were classified into three types as follows ([[Fig.4>#fig4]]). >1. Horizontal fatigue cracking along the girder web-flange weld toe ([[Fig.5>#fig5]]). >2. Fatigue cracking which initiated from the girder outside and inside web faces at 50mm to 75mm locations under the girder web-flange weld ([[Fig.6>#fig6]]). >3. Vertical fatigue cracking which occurred in the top ends of the floorbeam connection plates. After having propagated along the stiffener weld toe, this crack intersected the web crack ([[Fig.6>#fig6]]). >Cause of fatigue cracks was as follows: -Out-of-plane displacement of the girder web gap at the floorbeam connection plate end which resulted from the rotation of the transverse floorbeam end. &br; CENTER:&aname(fig4);&attachref(PolkCo12.gif);&br; &br; CENTER:Fig.4 Outline of the cracks. &br; &br; CENTER:&aname(fig5);&attachref(PolkCo13.gif);&br; &br; CENTER:Fig.5 Horizontal fatigue crack along the girder web-flange weld. &br; &br; CENTER:&aname(fig6);&attachref(PolkCo28.gif);&br; &br; CENTER:Fig.6 Fatigue cracking initiated from the outside and inside girder web face. &br; **6.Repair method applied [#uaa084c9] >Repair of parts subjected to cracking was performed as shown in [[figure 7>#fig7]]. The retrofit procedure was as follow. >1. At the negative moment regions apart from the piers, upper parts of the floorbeam connection stiffeners were frame-cut ([[Fig.8>#fig8]]) and the original weld areas remained were ground out ([[Fig.9>#fig9]]). >2. The 19mm holes were drilled at the crack tips to prevent the crack propagation ([[Fig.10>#fig10]]). >3. At the pier locations of the negative moment regions, L-type plates were bolt connected to the girder top flanges and the end stiffeners. The concrete deck upon the stiffener were chipped out in a oblong ([[Fig.11>#fig11]]). The holes were drilled in the girder top flanges and the L-type plates were installed ([[Fig.11>#fig11]]). &br; CENTER:&aname(fig7);&attachref(PolkCo29.gif);&br; &br; CENTER:Fig.7 Outline of the repair method. &br; &br; CENTER:&aname(fig8);&attachref(PolkCo30.gif);&br; &br; CENTER:Fig.8 Partial removing of the connection plate. &br; &br; CENTER:&aname(fig9);&attachref(PolkCo31.gif);&br; &br; CENTER:Fig.9 Finish by grinding out. &br; &br; CENTER:&aname(fig10);&attachref(PolkCo32.gif);&br; &br; CENTER:Fig.10 Stop hole at the crack tips. &br; &br; CENTER:&aname(fig11); &attachref(PolkCo33.gif); &attachref(PolkCo34.gif); &br; &br; CENTER:Fig.11 Bolt connection and clipping. &br;
