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Innovative Structural Cantilever safely de-propped

5 Martin Place North-east view

North-east view of 5 Martin Place

26 November 2014 - A significant project milestone was achieved last Sunday the 9th November on the site of the 5 Martin Place project in Sydney.

The ‘Moneybox’ building on the corner of Martin Place and Pitt Street constructed in 1916 is one of Sydney’s most iconic buildings, and one of the first tall buildings to be erected in Australia.

Originally constructed to be the new headquarters of the Commonwealth Bank of Australia, the building’s architecture was featured in the design of the bank’s money boxes for many years after.

The building was later extended in 1933 and 1960. The 5 Martin Place redevelopment, currently nearing completion, consists of the demolition of the 1968’s office building, refurbishment of the 1916 and 1933 heritage buildings, and construction of a new 20-storey office tower.

Fig1: Section showing how light can still penetrate into the re-established atriumThe redevelopment project, a joint-venture between Dexus and CBUS, comprises a large central atrium with large floor plates, totalling approximately 33 860m2 of net lettable area. Floor plates from the ground level to level 10 provide approximately 2 400m2, while new levels 11 to 20 each provide approximately 1,090m2

These upper levels feature large, column free space with primary beams and secondary beams spanning 14.3 and 14.0 M respectively.

A key feature of the project is the design and construction of a unique, ten level 22m cantilever projecting toward the west over the historic buildings, carrying the top ten floors of the new office tower.

This cantilever scheme was developed by the project team to address the requirement to not impose any permanent loads on the existing structure of this historically significant building; and the re-establishment of the 1933 light-well.

The final design, jointly developed by JPW and TKD and constructed by Grocon, has provided a solution that satisfies the main design issues, realising the embodied floor space ratio (fsr), yet still retaining the integrity of the heritage fabric and the planning controls.

The design consists of the following features:

  • A unique structural scheme, which includes the conservation of the existing steel structural system within the significant portions of the 1916-1933 building and does not require additional structural support through the significant spaces of the building, with the new building rising from the south-east corner and cantilevering over the 1933 building.

  • Re-establishment of the 1933 light-well in its entirety, including highlighting this feature for heritage significance. The light-well provide substantial environmentally sustainable design benefits, such as additional ambient daylight, in addition to the heritage recovery and highlighting.

Fig 2: View of the structural model from the south-west Aurecon were engaged to provide the structural, civil, fire and hydraulics engineering services. 

The structural solution developed was a cantilever structure in which all the building structures located above the existing 10th level of the heritage building are supported using hanging columns on the western facade, forming an inverted King Post truss.

This truss delivers the load of this section of the building to the northern and southern elevations, which rely on a strut and tie arrangement to transfer the load from the hanging columns to the main building concrete cores and down to the foundations. 

The construction methodology for the building necessitated the temporary support of the building during construction of the main structure and some levels of fit out.

The structure was temporarily supported on hydraulic jacks located over selected strengthened columns of the heritage structure.

Fig 3:  View of the structural model from the north-east In a major operation involving some fifty project team members from Aurecon, Grocon and key sub-contractors, these jacks were recently lowered and the cantilever structure was engaged. 

The vertical movement after the lowering of the jacks was measured and compared to predicted values based on Aurecon’s calculations.

Aurecon Project Director, Tony Lavorato, commented on the de-propping: “This is an outstanding result for the entire project team and we are proud that Aurecon was able to play a key role, together with the contractor Grocon and the Architect JPW, to be able to safely deliver this project milestone. The final result was that this innovative structural scheme was able to unlock the development potential of this site.”

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