The new 265 m long, 17 m wide bridge across the lower reach of Hatea River, a rolling bascule bridge, opened late last month in New Zealand. Patrick Reynolds
The new crossing of the lower reach of New Zealand’s Hatea River that opened late last month pays homage to Maori cultural traditions while offering a sleek, earthquake-resistant design.
August 13, 2013—From a distance, the new crossing of the lower reach of the Hatea River, on New Zealand’s North Island looks nothing like a bascule bridge. The 11-span, 265 m long crossing is just that, however. Its sleek, minimalist design pays homage to local indigenous cultural traditions while offering an earthquake-resistant design—no mean feat when one considers the poor ground conditions at the site, the region’s seismicity, and the difficult road access to the crossing’s location.
The Whangarei District Council commissioned the Transfield/McConnell Dowell Joint Venture to design and build the bridge, which is part of a larger highway scheme on Pohe Island, according to Duncan Peters, a director of Peters & Cheung Ltd, the Auckland, New Zealand, firm that handled the structural and geotechnical engineering for the bridge. Peters wrote in response to written questions posed by Civil Engineering online.
The 265 m long, 17 m wide bridge is aligned perpendicular to the tidal river’s navigation channel and contains nine 25 m long spans and two 20 m long end spans. The bridge carries two 4.1 m wide traffic lanes inclusive of shoulders. One side of the bridge has a 2.5 m wide path for pedestrians, and the other has a 3 m path for cyclists. The bascule span provides 7.5 m of vertical clearance and 24 m of horizontal clearance over the central navigation channel when closed.
The bascule span is composed of two structural steel J beams
that support a fabricated steel orthotropic highway deck; the span
is opened and closed by hydraulic rams housed in a reinforced
pier. Patrick Reynolds
The bridge is founded on V-shaped cast-in-place concrete piers that are supported by groups of driven steel tube piles. The piers are topped with longitudinal fabricated steel beams and transverse steel beams that form a “ladder-type” arrangement, according to the bridge’s architect, Martin Knight, RIBA. Knight also wrote in response to written questions posed by Civil Engineering online. His firm, Knight Architects, based in High Wycombe, United Kingdom, designed the bridge in conjunction with Peters & Cheung and Eadon Consulting, the latter based in Rotherham, United Kingdom, and serving as the mechanical engineer on the project.
“The various structural systems were selected primarily on grounds of economy and constructability, mindful that access to the site was not easy due to the relatively shallow water and tidal conditions—preventing [a] large floating plant—and constrained road access,” Knight wrote. “This supported the use of prefabricated and precast modular elements that could be shipped and installed using [an] available plant.”
The bridge’s deck is formed from deck units of precast concrete grouted into position with shear studs, Knight explained, along with precast deck edge units that have been installed to form an attractive finished edge and base for the parapet. “The edge units follow a relatively complex 3-D curve [that] rises and falls and moves in and out, yet the completed installation is the most beautifully finished I have seen in twenty years of building bridges,” Knight said. “The care and attention to detail of McConnell Dowell, the main contractor, and its subcontractors were exceptional and uniformly of the very highest quality—I think this comes across in the photographs but is certainly worth noting.”
Engineers had to contend with the site’s varying soils, which
comprise soft to firm silt and clay alluvium underlain by mudstone.
The eastern abutment rests on a 12 m thick layer of refuse fill that
is experiencing ongoing settlement. Patrick Reynolds
The bascule span is composed of two structural steel J beams that support an orthotropic highway deck of fabricated steel and two cantilever footpaths with an aluminum decking system. The hydraulic rams that open and close the bascule span are housed in a reinforced pier that has been strengthened with large posttensioned prestressing tendons extending down both sides of the V-shaped columns, Peters said.
“The lightweight orthotropic steel deck is unique in New Zealand and reflects international state-of-the-art design,” Peters said. “The counterweights at the top of the J beams balance the weight of the orthotropic deck and minimize the power needed to raise and lower the bascule span.” The bascule span has been designed to operate in gale-force winds, he said.
The bascule is a rolling bascule and is similar in style to a Scherzer rolling lift bridge, a type that is fairly uncommon outside of the United States, explained Michael Thorogood, CEng, a director of Eadon Consulting. Thorogood, who also wrote in response to written questions posed by Civil Engineering online, was the principal designer of the bridge operating equipment. “The rolling bridge design was selected because it achieves the required clearance quicker due to the deck moving backwards as well as rotating,” Thorogood said.
This quick movement decreases the wait times for travelers on the bridge, which was a key concern of the client, Knight noted.
“Most previous designs have used a large frame to operate via a gear rack, [but] this design is lifted by two large-diameter hydraulic cylinders located at the edge of the deck. This means that the arms, which are usually quite visually cluttered, are very minimal,” Thorogood said. The counterweights are located at the ends of the masts and within the hooks. According to Knight, this works to keep the system “nose-heavy.”
The bridge is features cast-in-place concrete V-shaped piers atop
groups of driven steel tube piles. The piers are topped with
longitudinal fabricated steel beams and transverse steel beams
that form a “ladder-type” arrangement. Patrick Reynolds
“Due to the design of the bridge, the curve of the rolling track was both architectural and integral to the operation,” Thorogood said. “The larger the rolling track, the smaller the angle the bridge needs to rotate through to achieve the clearance, but this also affects the angle of the masts and how the bridge looks in the raised position, the stroke of the hydraulic cylinders, and the length of the rolling track.” Knight and Thorogood created a three-dimensional parametric model of the geometry so that each variable, as well as slight design changes, could be assessed relatively quickly, Thorogood explained.
“The Council wanted to have an iconic bridge [that] will become known throughout the country, showcase the culture of Whangarei, and give the city identity,” Peters said. The design does just that. However, while the “rolling bascule span is the focal point of the architectural and structural design of the bridge,” Peters said, “the deep, soft alluvial sediments overlying the site and the ongoing settlement of the landfill on Pohe Island have had a critical role in the design and construction of the bridge.”
The site features soft to firm silt and clay alluvium underlain by Northland Allochthon mudstone, Peters explained. Bedrock begins at a depth of approximately 14 m at the western bridge abutment and at more than 30 m at the eastern abutment, on Pohe Island. Complicating matters, the eastern abutment is located on a 12 m thick layer of refuse fill that is experiencing settlement, Peters said. “To limit the amount of settlement in the future, the [eastern] embankment has been made of lightweight expanded polystyrene and capped with a thin concrete slab,” he said.
New Zealand is known for its earthquakes. “During an earthquake the eastern and western halves of the bridge will undergo out-of-phase longitudinal and lateral movements [that] have to be accommodated by the bascule span,” Peters said. “A critical aspect of the design has been to limit these relative movements to a level that can be accommodated by the bascule span.” As a result, small-diameter driven piles and a pile cap at each abutment and pier were used to limit the lateral sway of the bridge. Using fixed connections between the bridge deck and the piers and making the bridge monolithic with the abutments had the additional effect of increasing the stiffness of each side of the bridge, Peters noted.
The earthquake design offers the additional benefit of helping to protect the bridge from most impact loads. “The majority of vessels using the river are light, and the collision loads from them would be less severe than the effects of the design earthquake,” Peters noted. However, there is the possibility of a 350-metric-ton barge operating in the river at some point in the future, he noted, so the piers have been designed to resist a “very large impact load from an errant barge.”
The bridge has been named Te Matau ā Pohe, which, according to Knight, means “the fish hook of Pohe,” the Maori chief who welcomed the first English settlers to Whangarei. “The structural form of the rolling bascule was developed into a contemporary interpretation of the fish hook form—Hei Matau—that is central to Maori culture and is seen in ceremonial, sculptural, and artistic as well as functional form,” Knight said. “In doing so, the counterweight function of the J beam is perfectly integrated with its architectural form.”
“There are very few new bridges anywhere in the world that are of this design [rolling bascule] and even fewer that are hydraulically actuated using cylinders,” Thorogood said. “In New Zealand there are very few moving bridges, and hence it has been a real privilege to contribute to their infrastructure.”
The bridge is expected to carry up to 8,000 vehicles a day and will offer quicker access to Whangarei’s airport.