Something spectacular

“Something spectacular” was the design brief, and Manukau got it in the form of the country’s first cable-stayed road bridge.   BY HUGH DE LACY

They could have used more conventional technology – such as a suspension bridge – but the demand for something iconic persuaded Beca Infrastructure, in conjunction with Moller Architects, to adopt the cable-stayed system for a high-profile road-bridge for the Manukau City Council.

The city wanted a bridge designed to “touch the ground lightly” and provide a soaring gateway over 70 metres of parkland to the new township of Flat Bush which, over the next 20 years, is estimated to grow to a population of 40,000.

It was the requirement for a gateway appearance that prompted the designers to opt for cable-stayed construction, the first New Zealand examples of which were at that time being built in the form of two footbridges over State Highway 20 in Manukau for NZ Transport Agency (see last month’s issue of Contractor). Cable-stayed construction is usually dictated by long spans, so the choice in this case was largely based on aesthetics.

The bridge, completed in July this year by Fulton Hogan Northern Civil under project manager Alastair Blackler, owes its spectacular appearance to the 15 degree longitudinal and five degree inwards angling of the two pylons, which are topped by 12 metre lattice spires made of stainless steel and glass, and internally lit at night to give an appearance similar to Auckland City’s Skytower. Including the spires, the pylons are 45.5 metres high, beginning with a 28 metre section of reinforced concrete tapered from 1.8 metre diameter at the base to 1.3 metres at the top.

Between the base section and the spires sits the 5.5 metre structural steel box which anchors the stay cables. The 20 cables all connect to this box, creating a fan appearance – as distinct from the harp appearance of some cable-stayed bridges where the cables are parallel to each other and join the pylons at different heights.

The two deck carriageways, to which the cables are attached at seven metre intervals, each comprise two 3.2 metre lanes, a 1.4 metre cycle lane, and a footpath. The carriage-ways, cast in situ, are only 170mm thick to meet the design requirement for a slender-looking deck, and are separated by a 3.5 metre void down the centre of the bridge, giving an overall width of 27 metres, and allowing natural light through to the parkland beneath. 

The pylons are stayed back to an abutment on the western side of the bridge which, like the eastern end abutment, is supported by bored piles socketed into the Waitemata mudstone. The western end piles are post-tensioned to counter the uplift forces generated by the asymmetrical design of the pylons.

A dog-bone-shaped horizontal steel portal beam connects the two pylons at the anchorage box to resist the horizontal axial load generated from the cable stays.

And if all of this sounds complicated – and it’s certainly unprecedented for this country – it didn’t stop Fulton Hogan making the project an almost entirely local effort. The one overseas contribution was that of supplying the cables. The installation was done by BBR Construction Techniques (NZ), a member of Switzerland’s BBR network of companies. That company also supplied some technical advice at the design stages, and carried out some modelling in Switzerland, but it was local engineers and tradespeople who did the work.

Fulton Hogan regional manager Peter Wissel told Contractor the tight tolerances of the cable-stay design, the product of a competition run by the Manukau City Council, presented challenges to the builders, mostly relating to the position of the pylon and the associated length of the cable stays. These ranged from 20 metres to 53 metres with a load capacity of up to 780 tonnes.

BBR’s DINA system was specified by the designers because of its fatigue performance. It’s a refinement of the standard BBRv wire post tensioning system used on many concrete bridges in New Zealand between 1960 and 1990, and consists of 7mm diameter wire arranged in a compact parallel bundle within a thick-walled high density polyethylene sheath filled with a flexible corrosion-inhibiting compound.

The individual wires are terminated at each end with a cold-formed button head that transfers the load to the anchor heads.

Such was the precision needed in manufacturing the stays that it was decided to set up a 60 metres long wire cutting and assembly facility on-site to avoid the complications of either coiling the stays onto large-diameter reels or transporting them as excessively long loads by road. Nearly 70 kilometre of wire had to be measured and cut to within a tolerance of +/- 5mm. Once a set of between 78 and 144 wires had been cut they had to be inserted through a pre-determined component at the ends, and then through the sheath to ensure they remained free of twists. Any mistakes and the full set of wires would have to be replaced.

Installing the cables was also a delicate operation because of the flexibility of the sheath. A number of options were explored before it was decided to install them complete with the clevis, using two cranes and a mechanical excavator.

The fitted cables had to be tensioned in pairs, one on each side of the bridge, to avoid inflicting torsional forces on either the pylons or the deck. Tensioning was done with 250 tonne centre-hole hydraulic jacks working through a re-locatable 450 tonne pull-rod that was 75mm in diameter.

Wissel says that while manufacturing and fitting the cable stays was an intricate job, the earlier building of the pylons and the curved concrete cross-beam was no less complicated. The pylon forms were made from 8mm thick by 1.2 metre long steel plates rolled with a 22mm taper, and the design specified an F5 finish with no visible joints on the pylons themselves.

To get that sort of a finish it was decided to base-pour the pylons with self-compacting concrete, though Wissel says there was some doubt at the time as to whether this was possible, given the pressures involved. A six metre high trial pour was carried out to determine pumping pressures and the best method of discharging and pressurising the form to minimise bubbles in the finish. Eventually the pylons were base-poured in two sections with the first one up to footpath level, a head of about nine metres, and a second pour up to the full height of the pylon. Both pours were problem-free and resulted in an excellent finish.

The cross-beam was built after the pouring of the bottom sections of the pylons. Because it had to be raked to 15 degrees in harmony with the pylons, and had to incorporate a curved soffit, this proved to be another tricky job. Reinforcing the connections to the pylons was achieved using Reid Bar and couplers.

At peak there was a Fulton Hogan workforce of 25, plus another 20-or-so subcontractors’ staff, on the bridge site. The bridge comprised about half of a wider contract that included converting a roundabout at the Ormiston Road-Chapel Street junction into a full intersection, with all the attendant road works and services installation.

The bridge itself was structurally completed in July but it won’t be opened to traffic until October when the road network surrounding the project is completed.  And is it beautiful?

“You can argue whether or not it’s beautiful – that’s in the eye of the beholder,” Wissel says. “But I can tell you it’s a fascinating piece of structural engineering, and our whole team is very proud of the finished product.”