Steel Railway Bridge in Norway のバックアップ(No.2)

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  • 1 (2008-03-08 (土) 10:19:25)
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King Street Bridge †

1.Field of application †

King street Bridge across the River Yarra at Melbourne between two earlier crossing points, Spencer Bridge and Queen Bridge, Australia (Fig1,2).

2.Circumstances of repair †

The main bridge was completed on 12 April 1961. On the morning of 10 July, 1962, that is to say one of the coldest days of the Melbourne, winter-the temperature was 30 F-, one span collapsed.

3.Types of structure †

The bridge consists of two 2,300-ft-long and is of welded, deck-girder, suspended-span construction, with spans up to 160 ft. The cross section is shown in figure 3. The superstructure consisted of reinforced concrete decking cast in site. The steel used conforms closely with British Standard Specification 968. ( Steel conforming to BSS 968 has a minimum yield point of 55,000 psi and an allowable basic design stress of 26,000 psi in normally used in Australia.)

4.Details of loading †

Fluctuating loads due to vehicles live load.

One span collapsed under the load of s semi-trailer vehicle with a total weight of 47tons; a load within the permissible limits for the bridge.

5.Description of damage †

The collapsed span was made up of four 100 ft long suspended girders, and each girder fractured at a point 16 ft from the northern end of the span. The structure sagged 18 in. Inch-wide cracks spread acrossed the deck.

The cause of the failure was given as brittle fracture, at a point on each girder where a change in flange section occurred. The night temperature contributed to the collapse. The flanges of the girders were thickened locally by welded-on cover plates (Fig.4), and each fracture was at the point of termination of one of these cover plates (Fig.5). The cover plates were welded manually.

Under bead cracking was observed in welds at ends of tension-flange cover plates. It was suggested that the cause of the cracks might have been due to a lack of preheating at these points or to the presence of excess hydrogen or inadequate precautions for the difficulties of welding.

6.Repair method applied †

The recommended repair was to pre-stress the girders with steel cables or other devices within the depth of the present structure so that flanges liable to cracking under tension would be subjected to compressive stresses.

Reference †

Engineering News Record,Sept., 20, 1962.

Engineering, Sept., 21, 1962.

Nishimura T. and Miki C., Fracture of Steel Bridges Caused by Tensile Stress, J. Japanese SCE, Nov., 1975. (In Japanese)

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