Belle Fourche River Br. の変更点

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Belle Fourche River Br. へ行く。
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*Belle Fourche River Br. [#kff96f66]

**1.Field of application [#b2341a2f]

>Belle Fourche River Bridge of U.S. 212 over Belle Fourche River in the suburb of Belle Fourche, South Dakota.

**2.Circumstances of repair [#wb02e2ee]

>Fatigue cracks were found in the girder web gaps of the girder-diaphragm connections near the girder compression flanges of the positive moment regions in 1976 after 18 years of service.

**3.Types of structure [#df6ad511]

>The structure is composed of three spans welded plate girders, which are hunched at the intermediate supports, and four simple composite girders ([[Fig.1>#fig1]]). Each structure consist of seven main girders and X-type diaphragms within the girders ([[Fig.2>#fig2]],[[3>#fig3]]). X-type diaphragms (cross bracings) are weld connected to the vertical stiffeners which are fillet weld connected to the girder webs.
The reinforced concrete deck behave composite action with the seven main girders. All the main girders contain vertical stiffeners, 150mm by 9.5mm (width by thickness), which had 19mm scallops at the inside corners. These vertical stiffeners are fitted to the girder top and bottom flanges and are weld connected to the girder webs. The structural steel of the superstructure is A373.

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CENTER:Fig.1 Plan and elevation views.
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CENTER:Fig.2 Cross section and internal diaphragms.
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CENTER:Fig.3 General view.
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**4.Details of loading [#p40d2669]

>Fluctuating loads due to vehicles live load.

**5.Description of damage [#kac24995]

>Fatigue cracks were found in the girder web gaps of the girder-diaphragm connections near the girder compression flanges of the positive moment regions ([[Fig.4>#fig4]]). 

>Fatigue cracks observed were classified in three types as follows

>Ⅰ．Fatigue cracking which propagated along the girder web-flange weld toe of the girder web side.

  
>Ⅱ．Fatigue cracking which initiated from the stiffener fillet weld toe and propagated down at 45 degree angle from the stiffener end.

>Ⅲ．Vertical fatigue cracking which initiated from the scallop end of the vertical stiffener.
These cracks cut off the stiffener fillet weld from the girder web.

>Cause of fatigue cracks was as follows:
-Out-of-plane displacement of the girder web of the girder-diaphragm connection which resulted from the relative vertical displacement between girders.

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CENTER:Fig.4 Fatigue cracking at the diaphragm connection plate end.
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**6.Repair method applied [#fb2c3af6]

>The 19mm retrofit holes were drilled at the crack tips in order to prevent the crack propagation and the brittle fracture of the girder web ([[Fig.5>#fig5]]). The top ends of the diaphragm connection plates were also fillet weld , 100mm length and 8mm leg length, connected to the girder top flanges at both sides of the vertical connection stiffeners.

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CENTER:Fig.5 Stop hole repair. 
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