Fred Holt, Architect, Partner, 3XN, Denmark

What attributes of steelwork lend themselves to the design of the Heliostat and the cantilever extending from One Central Park’s East tower?

The Sky Garden cantilevers 25 metres from the east tower column supports and the reflector frame an additional 20 metres from the Sky Garden, providing a total of 45 metres of cantilever. In order to create an efficient and durable structure that maintained the required architectural profiles, a high strength and efficient material was required. Grade 400 steel was chosen for the majority of the truss and reflector structure, as it had high yield strength, and was also reasonably workable for welding and maintained some ductility.

Some of the high stress components used Bisalloy steel up to grade 800 to work within geometrical requirements. The steel trusses were able to be manufactured off-site (in Adelaide) and match fitted to within millimetre tolerances before being taken to site. Also, steel was able to be profiled to the shapes required to ensure access to connections and ensure safe erection.

How will the developments at One Central Park impact on the use of modern steelwork?

The capacities and efficiencies of structural steel were maximised on One Central Park to produce a structure that defies gravity. Future designers will be able to reflect on the profiles created with the cantilever and through the expressed nature of the structure, know that it was realised through a material with equal tension and compression capacities such as structural steel.

Also, the design pushed the boundaries of software integration to ensure that the every detail was thought through to ensure that the truss was not only an external feature, but also did not compromise the layouts and usability of the apartment space. The structural shop detailer used Tekla to complete all of their 3D shop detailing. The structural engineer, Robert Bird Group, were then able to convert and import the LOD 500 Tekla model into a combined structure Revit and Navisworks model. This was then integrated into the architectural model to complete all clash detection and allow 0mm tolerance for all service coordination and finishes requirements. This project enabled all parties to assist in developing the Tekla/Revit interface that will become critical in all modern steel dominated structures. For instance, Robert Bird group used the lessons learnt to better detail the recently completed Sydney Convention Centre that included some 17,500 tonnes of steel and were able to continue to assist with developing the Tekla/Revit interface.

What technical challenge did PTW team encounter in implementing the hovering heliostat and/or the cantilever? How was it addressed?

Managed by Robert Bird Group, the innovative use of structural steel realised the architectural intent of a cantilevered Sky Garden extending 25 metres beyond the east tower façade as well as supporting the reflector mirror array that extends an additional 20 metres from the Sky Garden, 94 metres above ground floor level. Deep steel truss sections form the base structure of the cantilever. Bridge building techniques were employed to build the cantilever safely at height, avoiding the use of formwork or excessive scaffolding. The 200 tonne cantilevered section of structure was built in three lift able boxes that could be safely pre-fabricated on the ground. The design successfully managed to level-out the expected elastic deformations of the structure as it settled into place following installation.

Cantilelering a further 20 metres from the Sky Garden, the heliostat reflector frame carries 500m² of reflective mirrored panels. Kennovations managed the design and construction of the heliostat panels that attached to the reflector frame. These fixed mirrors are arranged in clusters targeting pre-determined areas at ground and podium levels across One Central Park. 40 sun-tracking motorised heliostats measuring 4m² located on the roof of the west tower are designed to follow the sun’s path throughout the day and reflect the light up onto the fixed mirrors on the underside of the reflector frame. Each mirrored panel on the cantilever is fitted with coloured LED’s (2,880 in total) that play Yann Kersale’s ‘Sea Mirror’ lighting artwork. 16,000 colours are be used in the artwork, produced by a complex system of tri-chromic LED dots especially designed for this project. The result is a “chandelier for the city” according to Ateliers Jean Nouvel’s design.

What role did steel/steelwork play in the One Central Park project or how did itcompliment it?

The efficient use of trusses meant steel was the ideal material of choice with its equal tensile and compressive capacities. Steel allowed the elements to be slender and create the effect of weightlessness in the design of the reflector frame.

The steel perimeter beams were able to be designed as porous/castellated beams to allow wind pressures to be reduced and create a lighter looking structure.

Where do you see the future of vertical gardens/green walls lies with high rise development?

Richly landscaped facades, extending the neighbouring parkland onto the façade and delivering a remarkably green, organic lived experience in a highly urban, high-density context. The world’s tallest vertical gardens, 1,200m² of vertical gardens in 23 panels spanning ground level to rooftop, were designed by artist and botanist Patrick Blanc, using Blanc’s patented hydroponic growing system. In addition, 5.5 linear kilometres of planter boxes wrap both towers providing a living, green ‘veil’ for the building and a softer, organic aspect for residents.

What design details/elements in the One Central Park structure that is often overlooked?

The back span of the steel cantilever trusses through the tower to the eastern face. These two storey/seven metre high trusses are significant pieces of engineering, concealed within apartments, the lift cores and plant areas. Whilst their concealment easily assists the trusses being overlooked, their size and geometry are complex, essentially a bridge-like in proportion.

At the eastern face of the tower, the trusses connect to tension columns to resist the uplift and consequential sail-like reflector frame movement. These are again complex and fit neatly within the façade and apartment interior.

Whilst it is enjoyable to walk through the upper levels of the building tower in a finely finished polished interior, the considerable forces and loadings that also move through the structure in these same floors yet remain ‘invisible’ are possibly as successful as the more expressive easily seen structure.

Also, the tower experiences effects that are more common on buildings twice its height. As there is such a mass cantilevered off the building, almost all of the building weight above level 29 is supported by only four columns. Due to the stress on these four columns, high concrete strengths up to 100MPa were required.  Also, special construction methodology and façade modification was required to deal with the short-term and long-term differential shortening effects of the high stress elements adjacent to the lesser stressed elements.

In your opinion, which design elements played a key role the commercial success of One Central Park and why?

Retail Podium

There is 16,000m² of retail area over five levels within the podium of One Central Park. A large atrium with skylight above occupies the centre of the podium levels with heliostat sunlight directed though the skylight and atrium from above. A gymnasium on the retail upper level internally links to a garden terrace.

Innovative Façade

With the concept of ‘parkland’ at the heart of the precinct, this biomechanical building showcases extensive vertical and horizontal gardens, hydroponic green walls, modular metal clad polyethylene hydroponic planter boxes with stainless steel vine climbing cables, a 40 metre cantilevering sky garden/heliostat reflector and green roofs make this building’s envelope unparalleled in the international architectural arena.                                                        

Sky Garden

Exclusive to level 29 - 32 Sky apartments at One Central Park is access to the ‘Sky Garden’. Suspended approximately 100 metres above ground floor and cantilevering out 20 metres from the west facade of the east tower, the Sky Garden offers a dramatic green spatial experience at altitude.

Heliostat Reflector Frame and Mirrors

Cantilevering a further 20 metres from the south-west corner of the Sky Garden, the heliostat reflector frame carries approximately 500m² of mirrors. These fixed mirrors are arranged into several clusters designed to specifically target pre-determined ground and podium areas across the precinct.

The reflector frame receives sunlight from the 40 sun tracking heliostat mirrors located on the roof of the west tower. These heliostat mirrors follow the suns path and reflect light up to the fixed mirrors of the reflector frame.

Green Building Council Green Star Rating

Incorporating both Central Thermal Plant and Water Recycling Plant into the development, One Central Park successfully achieves a 5 Star Green Star design rating under both the Multi Unit Residential Design v1 and Retail Centre Design v1 Green Building Council of Australia assessment systems.

Which innovations at One Central Park have since gained momentum and what projects in the future will we expect to see more of those innovations?

Blackwater treatment is a major credential contributor with a precinct close to achieving water neutrality. The heliostat and reflectors dramatically carries sunlight to spaces that would never receive direct sunlight. Specialised glazing provides clear coloured natural light into the building without excessive tinting and performance quality allows for maximum glazed façade extent for natural light. Green walls are the largest extent to any building world-wide and other planting systems allow for extensive green elements over and around the building face.  

Thank You

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