Steel Railway Bridge in Norway
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*Steel Railway Bridge in Norway [#l658d09f]
**1.Field of application [#q32f651d]
>The "Iron Ore" railway line between Lulea, Sweden, and Narvik, Norway.
**2.Circumstances of repair [#be166874]
>Completed in 1902, this bridge was renewed at the beginning of the 1960s and several of the old riveted bridges were replaced by welded plate girder bridges. Fatigue cracks were found in the webs of some plate girders at the ends of the vertical stiffeners after a few years of service.
>A total five bridges of the same structural type exhibited 100-200 mm long fatigue cracks at the ends of the vertical stiffeners.
**3.Types of structure [#q5cc1116]
>Welded plate girder bridge.
>Five bridge of same structural type were constructed of two parallel, simply supported welded steel I-beams, with the sleepers set directly on the top flange ([[Fig. 1>#fig1]]) One bridge is over the Maunojokk River, built in 1961 and has a span length of 11.6 m. The track lies in a horizontal curve with a radius of 600 m. The two main girders, 1900mm apart, are straight, 1000 mm in depth and connected by cross-framing approximately every 2.0m. The web plate of both girders is strengthened on the inside by a vertical stiffener at every cross frame. These stiffeners are not directly attached to the top and bottom flanges. Instead, they are in contact with the flanges through a steel plate fitted at their ends. This steel plate is welded to the stiffener only, and not to the flange. The bridge is stabilized by a horizontal truss at level of the top flange.
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CENTER:&aname(fig1);&attachref(Norway1.gif);&br;
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CENTER:Fig.1 Bridge type where fatigue cracks occurred.
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**4.Details of loading [#yeab6531]
>Fluctuating loads due to railway live load.
**5.Description of damage [#x57ecd95]
>Fatigue cracks occurred in the webs of some plate girders at the ends of the vertical stiffeners. These cracks had propagated first horizontally and then gradually towards the neutral axis ([[Fig.2>#fig2]]). The orientation of the fatigue cracks were almost horizontal and parallel to the main girder stresses.
>Cause of fatigue cracks is as follow.
-Out-of-plane displacement of the gap between the flange and the end of the weld of the vertical stiffener due to train pass ([[Fig.3>#fig3]]).
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CENTER:&aname(fig2);&attachref(Norway2.gif);&br;
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CENTER:Fig.2 Typical fatigue cracks; propagation arrested by holes drilled at crack tips.
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CENTER:&aname(fig3);&attachref(Norway3.gif);&br;
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CENTER:Fig.3 The effect of displacement on the transverse bending stresses in the web plate.
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**6.Repair method applied [#qec07795]
>The cracks were arrested by drilling holes at each crack tip. No cracks so treated continued to propagate.
>To improve structural behavior, authors suggested some alternative bracing systems due to finite element analysis. The design changes which had the beneficial effects is as follows. Alternative 1 : The horizontal bracing of the as-built bridge consists of six horizontal diagonal truss bars. Alternative 2 : Removing all the vertical cross-framing diagonal bars between the supports. Alternative 3 : All vertical web stiffeners are attached to the bottom flange. To minimize any reduction in fatigue strength of the tension flange, the connection could use high strength friction grip bolts.
**Reference [#ze343660]
>Akesson B., Edlund B., Shen D., Fatigue Cracking in a steel Railway Bridge, '''Structural Engineering International,''' Vol. 7, No. 2, 1997.
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終了行:
*Steel Railway Bridge in Norway [#l658d09f]
**1.Field of application [#q32f651d]
>The "Iron Ore" railway line between Lulea, Sweden, and Narvik, Norway.
**2.Circumstances of repair [#be166874]
>Completed in 1902, this bridge was renewed at the beginning of the 1960s and several of the old riveted bridges were replaced by welded plate girder bridges. Fatigue cracks were found in the webs of some plate girders at the ends of the vertical stiffeners after a few years of service.
>A total five bridges of the same structural type exhibited 100-200 mm long fatigue cracks at the ends of the vertical stiffeners.
**3.Types of structure [#q5cc1116]
>Welded plate girder bridge.
>Five bridge of same structural type were constructed of two parallel, simply supported welded steel I-beams, with the sleepers set directly on the top flange ([[Fig. 1>#fig1]]) One bridge is over the Maunojokk River, built in 1961 and has a span length of 11.6 m. The track lies in a horizontal curve with a radius of 600 m. The two main girders, 1900mm apart, are straight, 1000 mm in depth and connected by cross-framing approximately every 2.0m. The web plate of both girders is strengthened on the inside by a vertical stiffener at every cross frame. These stiffeners are not directly attached to the top and bottom flanges. Instead, they are in contact with the flanges through a steel plate fitted at their ends. This steel plate is welded to the stiffener only, and not to the flange. The bridge is stabilized by a horizontal truss at level of the top flange.
&br;
CENTER:&aname(fig1);&attachref(Norway1.gif);&br;
&br;
CENTER:Fig.1 Bridge type where fatigue cracks occurred.
&br;
**4.Details of loading [#yeab6531]
>Fluctuating loads due to railway live load.
**5.Description of damage [#x57ecd95]
>Fatigue cracks occurred in the webs of some plate girders at the ends of the vertical stiffeners. These cracks had propagated first horizontally and then gradually towards the neutral axis ([[Fig.2>#fig2]]). The orientation of the fatigue cracks were almost horizontal and parallel to the main girder stresses.
>Cause of fatigue cracks is as follow.
-Out-of-plane displacement of the gap between the flange and the end of the weld of the vertical stiffener due to train pass ([[Fig.3>#fig3]]).
&br;
CENTER:&aname(fig2);&attachref(Norway2.gif);&br;
&br;
CENTER:Fig.2 Typical fatigue cracks; propagation arrested by holes drilled at crack tips.
&br;
&br;
CENTER:&aname(fig3);&attachref(Norway3.gif);&br;
&br;
CENTER:Fig.3 The effect of displacement on the transverse bending stresses in the web plate.
&br;
**6.Repair method applied [#qec07795]
>The cracks were arrested by drilling holes at each crack tip. No cracks so treated continued to propagate.
>To improve structural behavior, authors suggested some alternative bracing systems due to finite element analysis. The design changes which had the beneficial effects is as follows. Alternative 1 : The horizontal bracing of the as-built bridge consists of six horizontal diagonal truss bars. Alternative 2 : Removing all the vertical cross-framing diagonal bars between the supports. Alternative 3 : All vertical web stiffeners are attached to the bottom flange. To minimize any reduction in fatigue strength of the tension flange, the connection could use high strength friction grip bolts.
**Reference [#ze343660]
>Akesson B., Edlund B., Shen D., Fatigue Cracking in a steel Railway Bridge, '''Structural Engineering International,''' Vol. 7, No. 2, 1997.
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