World’s First Double Helix Bridge Opens in Sydney
Although no ceremony could be hosted due to COVID-19 health and safety restrictions, a world-first double helix dilating diameter bridge opened in Sydney, Australia, just last week.
The 178-meter-long (584-foot) helical Lachlan’s Line bridge connects the local Riverside Corporate Park with North Ryde Metro Station.
“The bridge dilates on the basis of its moment diagram with the helix being the tubular truss and the deck sits inside and doesn’t do any spanning work,” said Arup project director Andrew Johnson.
“The truss dilates based on its structural demand so it is deeper over the supports where the bending moments of the beam are higher and shallower at mid span where the moments are lower. The diameter of the truss varies from 5.5m to 7.8m.”
Lachlan Line Bridge
Breaking ground in October 2018, the $40 million project crosses over the Hills M2 motorway and Delhi Road in Sydney, connecting Macquarie Park, a railway station and Lachlan’s Line residential development.
The NSW government land and development organization Landcom was reported to be the entity behind the project, in collaboration with bridge architect KI Studio and structural engineering firm Arup. Construction, however, was carried out by the Arenco Daracon joint venture under a design and build contract.
“The helix-shaped bridge design was chosen to mark the gateway to Landcom’s Lachlan’s Line urban precinct with an inspiring, unique structure,” said Landcom Chief John Brogden at the time. “Thousands of pedestrians, cyclists and motorists will interact with this engineering marvel each day.”
Bridge spans were originally scheduled to be in place by late 2019.
According to Johnson, designing the bridge was done digitally from start to finish, which aided the engineering-led approach that made the design possible. From there, teams utilized Grasshopper to undertake the parametric design with Rhinoceros modelling software so that changes could be quickly analyzed both in the design and temporary states.
“The need for the bridge to span the shortest point and accessibility, along with the small pieces of land available at the station and for the central pier, drove the geometry of the bridge,” Johnson reports.
Once a final design was approved, the bridge was fabricated in four sections in Western Sydney, where they underwent a trial assembly prior to being taken to the site. There, the bridge was lifted into position using a single crane, with its heaviest section (125 tons) being set into place by a 500-ton crawler crane. Some road closures were required when positioning the bridge sections into place.
According to Johnson, the final section of the project was lifted prior to the COVID-19 outbreak and lockdown, so developed restrictions were not an issue for the project. Final tasks were completed following social distancing guidelines.
Bridges in the Works
Back in August 2016, architectural studio Penda and engineering firm Arup announced a partnership for a bridge concept featuring the iconic Olympic rings for the 2022 Winter Olympics, slated to be hosted in Beijing.
The structure will span the Gui River and act as a gateway between the city center and the town of Zhangjiakou, where all major outdoor competitions at the Beijing Olympics are being held. In essence, competitors and visitors will be “entering the rings” on their way to the games, the studio explains.
The studio took its inspiration for the design from the undulating hills surrounding the bridge site (the name San Shan/Three Mountains describes both its form and setting) as well as the symbolism behind the Olympic logo—five interlaced rings of equal dimensions representing a union of five continents.
The main structure of the bridge will feature a series of 300-foot-tall steel rings connecting at their highest and lowest points, the studio says. These rings will serves as the main structural element from which high-strength steel cables will suspend the 452-meter-long bridge deck.
Each of the rings, which tilt at 45-degree angles toward each other in pairs, serves as the rim in Precht’s bicycle-wheel comparison, connecting 100 cables to the four-lane bridge deck. In turn, the deck and cables serve as the hub and spokes of the wheel. “That’s what really stabilizes the bridge,” he said.
Three sets of cross-connected arch-like steel structures, with a maximum span of 95 meters, will form a “pre-stressed double helix,” Penda says, intersecting and providing support for each other at the bottom and top. “The helix is designed and engineered to be as slender as possible and positioned to offer the best structural performance,” it says.
The steel cables supporting the bridge deck are designed to connect to the arches in a cross-weave pattern.
According to reports, the infrastructure is slated to be completed in time for the games.
In 2017, the Florida State Department of Transportation announced that it had chosen a joint venture team led by Archer Western and The de Moya Group for an $800 million reconstruction project of Interstate 395 in Miami—replete with the design of its so-called “signature bridge.”
Archer Western’s plan for a new bridge over Biscayne Boulevard features six support arches of varying heights that rise from the center of the elevated span and reach toward its outer edges, a design meant to replicate a fountain. According to the contractor, when complete, it will be the largest segmented concrete arch in North America. According to the technical proposal submitted by the firm, the deck will be connected to the arches by steel cable suspenders, with each strand covered with a corrosion-inhibiting wax.
Construction on the five-year project was slated to begin by the end of 2017. A rebuild of the 836/I-95 interchange and reconstruction of 836 between I-95 and Northwest 17th Avenue are other components of the plan.