Hard graft on Grafton Bridge
Brian Perry Civil uses abseiling in striving to make a 100-year-old structure equal to the weighty demands of the 21st century. BY GAVIN RILEY
When Grafton Bridge was built in Auckland city nearly a century ago, the structure was designed to handle only horse-drawn traffic, early motorcars and pedestrians – and no provision was made for earthquake resistance.
As far back as 1970 vehicles over 13 tonnes were banned from using the bridge. One hundred years old early next year, the bridge clearly has not been up to the traffic demands of the 21st century, which require it to be part of Auckland’s central-connector busway project, able to handle up to 1200 buses a day, vehicles up to 40 tonnes, and (if necessary) a one-in-a-thousand-year earthquake.
So for the past five months, while the bridge has been closed to all traffic except for pedestrians and cyclists, a structural and seismic upgrade has been underway. This has involved strengthening the bridge columns with steel bar reinforcements, strengthening the beams with carbon fibre, and installing reinforcements to resist horizontal earthquake movement. There is also some restoration work – removing algal growth, repairing cracks in the concrete, and replacing joints and bearings.
The Auckland City Council upgrade, designed by Beca, is being carried out by Brian Perry Civil at a cost of just under $7 million. Work began at the end of last October and is due to be finished later this year, well in advance of the bridge’s 100th birthday in early 2010. Brian Perry Civil project manager Ashley Cooper says the project is well on track for this completion.
Grafton Bridge’s central arch spans 98 metres across Grafton Gully and is 43 metres above the gully floor at its maximum. When its two-year-plus construction was completed it was the longest reinforced concrete span bridge in the world (see separate report) and one of the first large concrete structures in New Zealand.
Over the years it has been subject to a number of modifications to extend its life – major reconstruction of approach beams in 1936, replacing joints and repairing cracks in the 1950s, handrail repairs and concrete coating in the 1990s, and the installation of glazed canopies in 2000.
However, all previous work is dwarfed by the current upgrade, which will transform the bridge into being part of an important public-transport link between the central-business district and Newmarket, via key locations such as the city hospital, university and Domain.
A feature of the upgrade has been the use of abseiling to access the bridge structure. Brian Perry’s Ashley Cooper says it was originally intended to use subcontractor Camelspace’s scaffolding throughout. However, it was realised scaffolding would put graves at risk in Symonds St cemetery, which forms the contracting team’s access to the base of the bridge, and would be accessible to homeless people and others who frequented the area.
Twelve staff from New Zealand Industrial Abseilers of Hamilton were employed to carry out the initial bridge cleaning and are being used in repairing cracks and concrete and for applying an anti-carbonation cementitious coating. To enable the required inspections to be done, three staff from Brian Perry and three from consultant engineer Beca were trained in basic abseiling (there was no shortage of volunteers for this task).
Explaining the necessity for the cleaning, which involved three weeks of jet-washing, Cooper says: “You couldn’t see half the defects because of the algal growth and general detritus and grime that had built up over 100 years.
“The algal growth was predominantly in areas where the drainage had failed over the years or drains had been blocked up. It appears that when the structure was built, drainage from the deck was just piped through the deck to discharge into the vegetation below. Which was fine at the time, but it was decided that wasn’t a good idea over the motorway which is now there, so any of the outlets which were anywhere near the motorway were just blocked up with concrete when the motorway was built.
“Drainage water had found its way since through corroded pipes and defects in the structure itself, leading to algal growth where it had run down the face of the structure.”
Since the cleaning and inspection, a team of up to six management staff, 12 Brian Perry workers, and up to 25-30 subcontractors has been engaged on the following tasks:
Strengthening the bridge piers using steel bar reinforcements
Eighteen columns need strengthening and are being extended by around 1-1.5 metres on the inside face of the main beams to give additional support to those beams. The columns are also having additional reinforcement cored down their length to between 12 and 16 metres. The work is being spread over four months and was 90 percent completed by early March, with the additional coring of 12-16 metres about to start.
Strengthening the bridge beams by applying carbon fibre material
The 1936 upgrade included some strengthening, but not the deck structure above the arch section or the two original end spans, at Symonds St and Grafton Rd. So in those three areas a shotcrete layer applied by Brian Perry Civil parent Fletcher Construction in the 1950s is being removed, the concrete cleaned, the surface “prepped”, and fibre-reinforced polymer strips applied in widths of 50 to 100mm. The strips are applied directly to the original concrete and form an external reinforcement to the structure.
“We then overcoat those because the bridge is a listed structure and we’re not allowed to alter its profile. The overcoatings must go back to the original profile so that you can’t see the work’s been done,” Cooper says.
The work is being carried out by subcontractor Contech and will take three and a half months in total.
Installing reinforced concrete shear keys and deck linkage
There are a number of expansion joints down the length of the deck. The ones at each end require additional steel linkage under the footpath section on either side, making a total of four cantilever links to be installed. The 16 shear keys are concrete blocks attached to the structure which bear against the piers to give more lateral restraint. They are part of the seismic upgrade and act to resist horizontal loads in the event of an earthquake.
As at early March the cantilever links were being fabricated and the shear key were on order, with the work scheduled to be done shortly and due to take about one month.
Repairing cracks in the existing concrete
The concrete repairs are a combination of crack injection and spall repairs, the latter being carried out where old concrete has fallen off in a lump.
The crack injection involves making the face of the crack smooth, blowing out any debris from the crack, sealing the outer face of the crack with an epoxy mortar, then injecting about a litre and half of epoxy resin for every metre of crack. After the resin has cured overnight, excess sealant is removed and the crack is ready for overcoating.
Where a concrete lump has fallen away, the concrete is broken back and the rebar cleaned and covered with a protective paint to prevent further corrosion, then filled with a concrete repair mortar. In some cases Beca decides that the existing rebar is so badly corroded that additional rebar is required alongside it.
This work began at the beginning of March and will take about two months.
Replacing joints and bridge bearings
Six expansion joints in the deck must be replaced and a seventh installed at the Symonds St end, where there should have been one but wasn’t. It’s relatively straightforward work. More difficult is the task of replacing the bearings.
“At the Symonds Street end the last remaining original span has a bearing either end of each beam. There are three beams in that section so there are six bearings in total,” Cooper explains.
“We have to jack the structure up by a maximum of 10mm and hydro-demolish the existing bearings, which are just two plates rusted together. We will then install two new metal plates with an elastomeric bearing between them. It should take just over a week for each end of the span.”
The work was scheduled for just after mid-March, immediately after the application of the fibre-reinforced polymer strips to strengthen the bridge beams was completed in that area.
Applying a modified cementitious coating to prevent further corrosion from concrete carbonation
The cementitious anti-carbonation coating will be 2mm thick, will be spread over the entire structure, and will be carried out by Brian Perry Civil and NZ Industrial Abseilers over a period of three months.
“It will be one of the last jobs,” says Cooper, who is from South Wales and joined the Brian Perry ranks after being recruited by Fletchers last August.
“It could be done in about a month and a half if we had access to all areas at once, but we have to wait for all the repairs and everything else to be done first.
“The biggest risk is the weather. But because we have to drop screens to obscure the view of the abseilers from passing traffic, those screens offer a greater degree of protection than we would otherwise have had. They’re primarily to protect drivers from themselves, but they also protect us and the work from the wind and the weather.”
Grafton Bridge has an Historic Places listing, is on the NZ Institute of Professional Engineers’ engineering heritage register, and is a catergory A scheduled item in Auckland’s district plan. A 2006 poll of Auckland University school of engineering alumni placed the bridge third on the list of New Zealand’s engineering achievements, after Manapouri Power Station and the America’s Cup yacht Black Magic.
Last year Auckland City Council was awarded the NZ Concrete Society’s inaugural Enduring Concrete award for Grafton Bridge. The award aims to recognise heritage structures that are both ahead of their time and have stood the test of time.
Contractor Vol.33 No.3 April 2009
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