Australian Timber Awards – Judges’ Innovation Winner

18 May 2015

Winner of the Judges’ Innovation award in the 2014 Australian Timber awards, The Green is currently the tallest lightweight timber frame building in Australia. Constructed by the Australand Property Group, The Green adopted a new construction methodology that achieved an efficient structure while utilising traditional timber industry framing techniques.

Overview of Structural System

The structure consists of a concrete car park below podium level while above the podium, all five levels were constructed completely in timber. The timber frame superstructure was erected using prefabricated timber walls, floors and roof trusses that were manufactured off-site and delivered for installation. This was achieved by collaborating with the concept architects, SJB Architects and Australand to create an efficient grid system and load transfer through the stacking of apartments early in the program. Acoustic and fire proofing concerns also had to be addressed and detailed into the structural design from the start.

Cassette Floor System

The prefabricated floor system was a product of collaboration between Irwinconsult and Australand. A unique floor system was designed to address acoustic, fire proofing, fabrication, transport and lifting challenges. The result was a floor system fabricated as cassettes made out of Tecbeam floor joists that had been constructed into panels with engineered Promat floor sheeting, assembled off-site. These floor cassettes were shop drawn for required accuracy and lifted in by cranes, creating a layered construction. The prefabricated walling systems allowed for installation of the walls to occur concurrently as the floor levels were being completed.

Engineered Walls

Timber buildings exceeding two levels are currently not addressed under AS1684. Therefore, all load bearing walls were fully engineered for both the vertical and wind design loads. Special consideration was given to the detailing of inter-storey wall connections for transfer of design loads. Given the 5 levels of loads, the sizing and spacing of wall studs had to be individually designed. In addition, the acoustic performance was considered in the wall stud size and spacings to achieve acoustic requirements. All the walls were prefabricated using typical timber frame construction methods off site then installed onsite.

Cost Advantages

Approximately 400m3 of concrete was used for the lower basement and ground levels, while only 500m3 of timber used for the remaining five levels of the superstructure. This resulted in significantly lower loads on the footings and foundation system when compared to a full concrete structure. A five storey concrete structure would typically be earthquake governed due to its weight and stiffness. As the majority of The Green was constructed using light weight timber, such reduction in building weight added further advantage as the building was governed by the less onerous wind pressures. This construction methodology incorporated the traditional timber framing domestic labour force and supply chains to construct a multilevel residential timber frame building at a commercial level. With the utilisation of the domestic labour force and building materials, a cost saving of up to 25% of built costs per apartment over conventional concrete was achieved.

Program Advantages

Major program advantages over conventional building techniques became evident during the construction of the first level. Each floor level (averaging 850m2) was completed in about 11 days. This included all the internal walls being installed as part of each level’s construction cycle and no back propping was required as the floors could span from load-bearing wall to load-bearing wall. The actual timber construction time (including roof trusses and floor cassettes) was only 12 weeks out of the 12 month build program. As such, the new construction methodology dramatically reduced the build time for each floor by a week, allowing the building to be completed one month earlier than originally programmed. This clear advantage in both time and cost advocates for a new era in timber multi-residential buildings.

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