Elizabeth Quay Pedestrian Bridge, Perth - The jewel of the Quay
The Elizabeth Quay development began in Perth in January of 2016. It is graced by a distinctive, dual-arched bridge for pedestrians and cyclists. An artfully meandering structure, designed to delight those who use it, the bridge spans a newly-created inlet of the Swan River with a curvaceous shape that visually links the city with the water.
The bridge completes a waterfront redevelopment that has transformed the river foreshore into a busy promenade, lined with bars, restaurants and public spaces, and is set to be surrounded by a vibrant mix of offices, apartment buildings, hotels and shops. The bridge promotes continuous movement around the Quay and links the island to a popular recreational circuit around the Swan River. Visitors to the precinct can now walk, run and cycle across the bridge, enjoying spectacular panoramic views.
Accessibility For All
A key objective for the Elizabeth Quay is to have the inlet used by both public ferries and private crafts ensuring that the waterway is accessible to all. The bridge is designed to be accessible to pedestrians and cyclists alike, providing continuity of the riverside cycle path and of the pedestrian circuit around the inlet.
Ferries & River Vessels
The navigation clearance is critical to the bridge’s geometry and form; for each 100 millimeters increase in clearance, the bridge length would increase by up to 7 meters. The height of the public ferry was topped with a 1.5 meter communication aerial.
Arup successfully negotiated for the relocation of the ferry’s aerial. Arup also challenged the design brief drawing attention to the probability of concurrent storm surges, flooding, waves and rising sea levels. The subsequent changes allowed for increased cost efficiency and a significantly shorter bridge. It also created opportunity for a simple yet interesting bridge geometry. For example, the smooth curves of the final geometry were not possible without the reduced soffit level. The lowered bridge deck also enhances the user’s experience due to the closer proximity to the water, interesting views, reduced height climb, a shorter path length, and a safer cycling speed limit.
Pedestrians and Cyclists
The ‘S’ shape of the bridge deck provides the 110-meter length required to clear the navigation channel at acceptable gradients whilst providing dynamic and changing viewpoints for pedestrians and cyclists using the bridge. This shape creates natural vantage points to the Swan River and back towards the heart of Elizabeth Quay with the Perth CBD as a backdrop. Feature lighting creates a relaxing sophisticated ambience when night falls.
The two 22-meter high arches, that support the deck lean away from each other, acting as complementary foils to the bridge’s deck shape. This simple arrangement of curved structures provides a series of different visual experiences, depending on whether the individual is on the bridge, in the precinct or looking at the bridge from the city.
The bridge’s arches echo the ‘S’ shape of the deck with steel arches and cable hangers that enhance the experience of movement throughout the structure and across the bridge. This reinforces the sensations created by the curved deck constantly changing in height, and always curving in plan.
Parametric modelling software was used to optimize the geometry of the pedestrian pathway and overcome design constraints. The solution to providing a very slender deck was two leaning arches with 45-meter spans, sweeping down towards the water to rest on the concrete piers. The arches are arranged on opposite sides of the deck and span to offset the deck plan curvature. The support cables are all on one side of the deck which allows for unhindered views from the bridge in different directions on each span.
The arch cross-section changes along the arch length and is generally comprised of a five-sided steel element welded from flat plate and internal stiffeners. The major axis dimensions vary from 2.1 meters at the pier connections to 1.2 meters at the crown. The angle and height of the arches was in part determined by available equipment that could be used to inspect and maintain the cable-torch connection. Detailing of the stiffeners and diaphragms were selected to facilitate the fabrication sequence where the last side of the arch section could be placed onto the initial four sides and then welded from the outside to form a sealed box section.
The box section deck is an appropriate response to torsion in the eccentrically hung deck. The deck measures 650 millimeters at the box center and 250 millimeters at the deck edges. The slender shaped deck was tested for its susceptibility to wind-induced vibration and vortex shedding as well as human induced vibration.
The piers rest on foundations that are 900 millimeters in diameter concrete piles with the pile cap located below bed level such that they are not an obstruction to the 2.5-meter deep navigation zone. This design offers a striking pier shape that mirrors the force lines from the arches and deck. The concrete outline extends so that the arch creates an elegant transition, but still leads to a heavily reinforced and complex pier. This transition had to accommodate significant reinforcement for each arm of the pier as well as the large diameter stress bars used to stress the arch onto the concrete substructure. This section was 3D modelled in full and strict individual and cumulative tolerance limits were specified for each element and sequence of the construction.
Presented with a windy, tidal, and saline riverside location as well as the sometimes harsh Western Australian climate, the materials used to build the bridge had to be carefully selected. They needed to be able to withstand to the area while also complementing the bridge’s structural form and maintaining the highest quality standards. The materials were chosen for aesthetic appeal, their ability to reduce wind impacts and vibrations, and for ease of ongoing maintenance.
The digital design workflow was a highly collaborative process between Arup’s architects and engineers that proved to be a pivotal factor in the successful delivery of this project.
The architect initially used Rhino and Grasshopper as a parametric form generating tool to solve the geometrical constraints set by the client, allowing them to quickly converge on the architecturally desired and conforming ‘S’-shaped bridge concept. Preferred sculptural forms of the bridge were further developed parametrically by the architects using Grasshopper in the subsequent design phases, creating the opportunity for Arup’s engineers to link into the architect’s parametric scripts to integrate the design workflow.
Working from the same shared Grasshopper canvas, the engineering team generated analysis models to assess the bridge’s structural performance as it developed as a concept. 1D finite element models were parametrically generated directly from Grasshopper to GSA using Arup’s Salamander plug-in. These direct digital links allowed the design team to optimize and rationalize the profiles of the complex bridge form in a very short amount of time.
Engineers also extended the architects’ initial Grasshopper scripts to generate the internal steelwork necessary to define the structure. This geometry was then referenced into Revit to populate the BIM model and produce structural documentation. A fully detailed Strand7 analysis model consisting of 2D shell elements was also translated from the Grasshopper surface geometry which was used for final structural verification.
The holistic workflow approach meant that direct relationships were made to the shared geometry between architect and engineer via their linked scripts. This association meant that architectural and structural refinements could occur in parallel without losing element-to-element connectivity or resulting in the separation of modelling parts which avoided rework at each design update.
The plane geometry of the bridge was set out parametrically from a series of arcs, subdivided by equally spaced planes, perpendicularly aligned with their varying radii. Each of these planes was then set out vertically via an unrolled section, taken through the bridge centerline. A cross-section of the bridge was projected onto each plane, and then linked together from plane to plane. This in turn set out all the various 3D elements of the bridge: the structural section, the fascia panels, the timber decking, and the balustrade.
Fabrication and Erection
Perth-based steel manufacturers were selected by the bridge contractor to build the structural steelwork for the bridge’s arches and deck. This permitted close collaboration between the design team, contractor and fabricator. The required construction tolerances were necessarily onerous to achieve the structural adequacy and aesthetic aspirations, as compounded construction tolerances were unacceptable.
Collaboration with the manufacturers commenced with a briefing during which Arup shared the parametric scripts used to create the bridge geometry. This allowed for architectural, structural, and fabrication 3D models to be created from the same base parameters, to optimize the coordination.
The arches were built offsite in three different lengths, and the deck in twelve different lengths. The modules were built adjacent to each other and fit together in the manufacturer’s yard to ensure construction tolerances were achievable. The high level of attention given to coordination and tolerances throughout the process directly improved the constructability of the bridge, and majorly contributed to the success of its delivery.
The prefabricated arches were delivered to site in three parts which were then aligned and welded together. The standing of the two arches was done in a single operation that lasted about two hours. The arch was fixed into the ground with hinges at the base connections and a central crane. The arch was never lifted free of the ground. The crane lift trailed a central strut along a temporary track. This approach allowed for the arch to be securely locked into place and the site evacuated quickly and safely at any time if needed. The planning and constructability assessments enabled the arches to be erected within 2 millimeters of the documented position.
The deck was brought to site in segments that were placed onto a temporary support system and then welded together. The cables from the arches were attached to the deck, the abutments were installed and the deck weight transferred to the cables as the temporary support system was gradually lowered below the deck box.
Project Outcomes and Conclusion
The Elizabeth Quay pedestrian bridge was completed in 2016, on time and within budget. It has been very positively received locally, regionally and globally. The bridge has received numerous accolades and awards, including Engineers Australia WA Excellence in Engineering 2016 and the prestigious global IStructE Structural Awards 2016 for in the ‘Pedestrian Bridge’ category.
The authors wish to acknowledge the bridge project team:
Arup Client: CPB (formerly Leighton Broad). Ultimate Client: Perth Metropolitan Redevelopment Authority (MRA), Government of Western Australia. Managing Contractor: CPB (formerly Leighton) and Broad. Bridge Contractor: DASSH (Decmil Australia, Structural Systems, Hawkins Engineering). Steel manufacturers: CivMec (Arches), Phillips Engineering (Deck)