In a world first for structural engineering, a Western Australian firm has employed what has been referred to as the "piggy-back" construction method to add multiple storeys to an existing high-rise building.

Consultancy Pritchard Francis used customized engineering methods to add 18 more storeys to an existing 10-storey building on St Georges Terrace in Perth.

The abandoned Oakleigh Building was thus transformed into the Condor Tower, which stands almost three times higher than its foundational predecessor.

One of the most vexing difficulties of the project was reconciling the older structure, which had changed significantly since construction, with the new structure which was erected on top of it.

The key to overcoming this challenge was the precision measurement of the strains placed on the building structure as construction progressed – a task for which the University of Western Australia provided pivotal assistance.

“We had load sensors installed in the existing building so we knew load levels in advance, and had data for assessment once we began the construction of additional storeys,” said Anthony Sims, Pritchard Francis strategic development manager and project leader.

Another challenge was the settlement of the building following the introduction of new loads. Pritchard Francis solved this problem via the use of an innovative new technique called the partially piled raft, which had only previously been applied once in Western Australia.

Sims said the pioneering methods employed in the construction of the Condor Tower use traditional techniques already in widespread use amongst builders, adding that piggy-back construction has major potential for urban centres around the world.

“Perth is such a small market, and there are probably hundreds of building in major cities like New York where this engineering technique can be applied,” he said.

Key advantages of the use of the piggy-back method include a more rapid construction process, since new buildings are essentially erected on pre-existing structures; and major savings in terms of landfill and greenhouse gas emissions, as the costly process of demolition is averted completely.

The Condor Tower project was the overall winner of the WA Engineering Excellence Awards for its application of pioneering construction techniques and has also resulted in four UWA thesis projects and a number of papers which have been presented to conferences both at home and abroad.