King Street Bridge (Melbourne)

The King Street Bridge was designed in 1959 by Hardcastle and Richards, consulting engineers for the main contractor, Utah Australia for the Country Roads Board, and constructed over the next two years,[1][2] being opened on 12 April 1961.[3][4] It was of a welded, deck-girder, suspended-span construction with spans up to 160 feet (49 m) long.

Soon after completion, on 10 July 1962, one span collapsed under the weight of a 47-ton semi-trailer, though the weight was within the bridge limits. The subsequent Royal Commission found the bridge failed with a brittle fracture on a very cold Melbourne winter day. The commission identified the cause of girder failure as cracked welds that started at welded-on doubler plates which were used to thicken the flange. The cracks were due to a combination of hydrogen embrittlement and poor design. The doubler plate was welded on all 4 sides which counter-intuitively, greatly reduces the fatigue strength when compared to a 3-sided weld. This could have been prevented by either taking measures to prevent hydrogen embrittlement by taking actions such as pre-heating or by using a 3 sided weld. Also, a thicker flange I beam could have been used.[5][6] The south end of the bridge has been undergoing differential settlement between the approach ramps resting on fill, and the suspended section supported by bored piles.[1][7]

Reconstruction.

As authority in charge of the bridge at time of failure, MMBW was responsible for the design of the reconstruction method.

MMBW Highways Division carried out this work. The engineering staff were William (Bill) Burren, Chief Engineer, Highways Division, Stan Long 2nd in Charge, Bruce Day Chief Structural Engineer, Tom Dobson Senior Structural Engineer, and Graham Ebbage Assistant Structural Engineer. All are deceased (2020) bar Ebbage who went on to design bridges in Melbourne, Brisbane and Hong Kong.

It had to be assumed that all girders contained cracks, whether presently detected or not. It was therefore decided to post tension all girders so as to leave no parts in tension.

As each span consisted of four girders, it was decided to construct large heavily reinforced concrete blocks at the ends of each pair of girders. These were to house the anchor blocks of the post tensioning cables. These blocks were held between the girders by heavy high tensile rods passing through each block from girder to girder, averaging about 19 rods each.

The cables were of the Freyssinet type with cast iron wedges fastening each cable into a mating block. Each cable was made up of a number of strands each containing seven high tensile wires. As an example, on span No 11, 110 feet long, the total prestressing force employed amounted to about 11000 tonnes.

To protect the cables from corrosion, the cables were enclosed in fibre cement ducts and then high pressure grouted. These are what some people now see as pipes under the bridge.

The reconstruction was carried out by John Holland Constructions. Bruce Day was MMBW Site Engineer for the contract.

As originally constructed, the bridge had eight lanes across the Yarra River, two through lanes in each direction connecting King Street to Kings Way, in addition to two lanes on each side that connected to Yarra Bank Road. At the south end north facing on and off ramps connected to Whiteman Street, and the running lanes from the viaduct descend to ground level, with tram route 58 emerging from City Road to the median strip.[8] In the 1990s, the development of Crown Casino closed Yarra Bank Road, and the bridge ramps were connected to the basement carpark of the complex.[9][10]

• 
  Viewed from the west
• 
  The elevated section of Kingsway headed north through Crown Casino
• 
  Looking south to the end of the elevated section of Kingsway

Media related to King Street Bridge, Melbourne at Wikimedia Commons

• Transporting of girder through South Melbourne