Melbourne bridge repair and strengthening

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					Melbourne bridge repair and strengthening
      elbourne City Council is performing
M     repair work on the Stock Subway
Bridge in Melbourne. The repair, which
will cost about $250,000, is based on a
risk assessment by consulting engineer
Pitt and Sherry. Victorian contractor
Morverk is undertaking the repair work
which is scheduled for completion by the
end of the month.
   The repair is part of a progressive
strengthening of bridges in Melbourne
city on all 12 bridges under its care as part
of its bridge management plan for 2005-
2010. These include Princes Bridge, Morell
Bridge, Arden Street Bridge and Macaulay
Road Bridge.
   As part of the plan, the council has
decided that all its structures are to be
upgraded from current load capacity to
fit a new requirement for 46.5t. The Stock        The Stock Subway bridge in Melbourne was recently assessed for deterioration.
Subway Bridge, which was designed in
1913 and carries about 15,000 vehicles per      database and over time, it became a bridge          causing failure.
day, was identified as having inadequate         management system.                                      It examines the overall condition of
strength for this mass limit.                      The tool produces two ratings, a condi-          the structure and the material a particular
   “When the Stock Subway Bridge was            tion rating and a load rating. The condi-           component is made of. For example, if a
designed, it would have been required           tion rating looks at the deterioration of           component is made of timber, the profile
to hold a steamroller,” said David Coe,         the structure and the risk of the structure         for rapid deterioration is greater than if
director of Pitt and Sherry. “But now it        deteriorating between inspection periods.           it is reinforced concrete.
must be strong enough to hold articulated       It also determines when maintenance                     The load capacity of the structure is
trucks.”                                        work should be undertaken at certain                assessed to see how close to the target it is.
   Inspections have been performed and          points so that the money to be spent on             Finally, the tool considers the importance
strengthening work is under way. This           maintenance is optimised.                           of the bridge to the road network. If it is
mostly involves cutting slots in the sides         “We don’t let the bridge deteriorate             on a more critical route, it will be assigned
of the bridge’s in situ concrete beams          to the point where we have to do major              a higher risk factor.
and placing steel bars inside for extra         reconstruction work to get the bridge up                The company is currently incorporat-
shear reinforcement. Holes are then cut         to a reasonable standard. We aim for the            ing environmental factors into the tool
in the concrete deck of the bridge to run       reasonable point of intervention so only            so it can consider deterioration due to
the reinforcements through the deck.            relatively minor work is needed to bring            local conditions such as extreme heat or
The bridge was temporarily closed at the        the condition up to a desired level,” Coe           humidity or corrosion due to salt water.
beginning of the month to allow a new           said.                                                   According to Coe, the main challenge
concrete deck to be installed.                     On a scale of 1 to 5, the Stock Subway           was coming up with a design which could
   Other works include repairing cracks         bridge scored about 3.4, just above the             meet the heritage values of the structure
in the concrete beams and concrete cross        rating of 3 which is deemed “fair” but              in a busy urban location. “With the work
heads and repairing the bridge’s abut-          bordering on the intervention level. As             done on existing bridge structures, you’re
ments.                                          part of the council’s bridge management             never quite sure what’s there until you
   To calculate the risk of the bridge          plan, no bridge shall have a rating of less         start work.
deteriorating, Pitt and Sherry used its         than 3 without actions taken to reduce                  “For example, the contractors were
in-house tool BridgeAsyst. This is a            the risk.                                           required to identify where the existing
database that stores the conditions and            The tool uses a preset formula which             reinforcement was in the concrete which
asset information of the structure. It was      assesses the risk profile of a structure and         would serve as a guide to where we had to
developed when the company was doing            the consequences of failure. It considers           cut the holes. After some time in the field
bridge inspection work for the Tasmanian        the probability of failure as the potential         we found they were in different places
Rail Authority ten years ago. It began as a     for deterioration of the structure and              from what we expected, from the design

                                                                                                   ENGINEERS AUSTRALIA JANUARY 2008            43

      drawings. Since the bridge is so old, things   steel inside the beams, rather than using      will cause an outage on the cable alone,
      were in slightly different places than         external reinforcing, observers cannot tell    with no loss of transformer capacity at
      assumed. So we had to add a bit more           that the bridge has been strengthened.         the zone substation. By comparison, a
      reinforcement in a few more locations to       The contractor also had to work around         66kV cable fault in Melbourne has the
      achieve the strengthening objectives.”         a water main attached to the bridge by         potential to result in the loss of up to four
         Since the bridge is heritage-listed, the    temporarily detaching the pipes’ sup-          zone transformers. Most commonly two
      overall appearance of the structure could      ports, performing the strengthening work       transformers will be lost.
      not be altered. By placing the reinforcing     and then reattaching them.                         One of SKM’s key recommendations
                                                                                                    was to eliminate the multiple transform-
      Planning for electricity security                                                             er-ended feeder configurations by adding
                                                                                                    new switchgear at several critical sites.

      A    draft decision by Victoria’s Essential
           Services Commission (ESC) has ap-
      proved a plan by CitiPower to improve
                                                         The study suggested that if the CBD
                                                     66kV network had been designed to the
                                                     higher security standard of N-1 Secure,
                                                                                                        With the existing configuration, a
                                                                                                    single 66kV circuit supplies a number
                                                                                                    of transformers at different substations
      the security of its electricity supply in      the adverse effects of the two events          but with only one circuit breaker – at the
      Melbourne’s CBD.                               could have been avoided or reduced in          terminal station source.
          The project will initially involve de-     severity.                                          All switching and isolation is currently
      veloping a 220kV/66kV transmission                 Under the proposed N-1 Secure              achieved by means of manually operated
      connection facility at Brunswick terminal      standard, the network would be able to         isolators. A forced outage would take out
      station and associated works.                  withstand the loss of any element and          a number of 66kV/11kV transformers,
          The estimated initial capital cost is      maintain supply to all customers.              albeit at different locations.
      $125.4 million with terminal station costs         The network would then be able to be           The design has usually proved ad-
      of $36.2 million, zone substation costs        reconfigured to withstand a further out-        equate in the past. However in the event of
      $36.8 million and security enhancement         age without loss of supply to customers.       a prior transformer outage, a subsequent
      costs of $52.4 million. The initial capital    During the time taken to reconfigure,           fault elsewhere in the network could cause
      costs exclude overheads and include civil      which is targeted to be 30 minutes, there      tripping of an additional 66kV/11kV
      works.                                         would still be a risk of supply interruption   transformer and result in total loss of
          “When completed, the upgrade will          if a second contingency occured.               a substation (due to overloading of the
      give Melbourne a robust and secure 66kV            SKM’s analysis also found that com-        remaining transformer).
      network, able to withstand rare, but po-       pared to similar CBDs in Auckland, Wel-            SKM also recommended that addi-
      tentially widespread and critical, double      lington (New Zealand), Brisbane, Cape          tional 66kV ties between 220kV/66kV
      contingency network outages,” said Neil        Town (South Africa), Glasgow (Scotland),       terminal stations be created to provide
      Watt, the manager of network reliability       Johannesburg (South Africa), and Sydney,       improved load transfer capability.
      for the company.                               Melbourne had the second lowest security           These ties would ensure supply reli-
          The construction of the project and the    of supply, even though it had the highest      ability in the event of a catastrophic
      regulatory conditions to be imposed are        CBD load at 700MW.                             failure of a 220kV/66kV transformer and
      currently the subject of a public consulta-        In addition, the configuration of the       would support the move to the recom-
      tion process by the ESC.                       66kV subtransmission network supplying         mended higher security standard.
          The project, which aims to enhance         the CBD of Melbourne is an old design              The improved 66kV switching capabil-
      the security of electricity supply to the      based on a multiple transformer-ended          ity could only be achieved by rebuilding
      CBD, was originally part of CitiPower’s        feeder configuration.                           or refurbishing existing substations at
      price service offering for the 2006-10             Unlike a fully switched subtransmis-       considerable expense because of space
      regulatory period.                             sion system configuration adopted in           limitations.
          It was based on the findings of an         most developed cities, a greater amount            SKM then undertook a probabilistic
      analysis by Sinclair Knight Merz (SKM)         of capital cost is invested in underground     assessment of the benefits of security
      of two outages in 2001 affecting electricity   cable and transformer capacity in Mel-         enhancement to the CBD network for the
      supply to the Melbourne CBD.                   bourne.                                        period 2009 to 2032.
          These events disrupted electricity sup-        By comparison, a proportionally                The total benefit of security enhance-
      plies to tens of thousands of customers,       higher amount of capital cost is invested      ment to the CBD was predicted to in-
      and directly affected a further 100,000 to     in switchgear such as circuit breakers in      crease from $2.27 million in 2011 to $7.67
      200,000 people within and surrounding          other cities to ensure that the impact of      million in 2032.
      the CBD.                                       any single contingency and subsequent              “In net present value terms, the total
          Energy was lost for about 30 minutes       fault is limited to the faulted element        security benefits outweighed the total
      in November and one hour in January.           only.                                          costs of the security upgrade works, con-
      The total cost to the community of both            For example, a 33kV-132kV sub-             firming a positive outcome for electricity
      incidents was around $13 million.              transmission cable fault in most cities        consumers,” Watt said.