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Contractor (June 2011) PRESSS the Right Concrete Buttons

01 Jun 2011

Rob Gaimster
Cement & Concrete Association of New Zealand (CCANZ)

As engineers begin to develop a fuller understanding of why buildings behaved the way they did during the Canterbury earthquakes they will inevitably begin to consider how new building technologies can contribute to rebuilding plans.

Chief amongst such technologies for low-rise and multi-storey buildings will no doubt be the concrete PREcast Seismic Structural System or PRESSS.

A development in damage resisting seismic structural design, PRESSS helps buildings accommodate the rocking motion that accompanies an earthquake via the use of tensioned cables threaded through walls, beams and columns.  A simple comparison for PRESSS can be drawn with a child’s wooden doll whose jointed limbs are held firm but moveable with elastic thread.

As fanciful a concept as this seems, it is one which, in the form of the Southern Cross Endoscopy Hospital in Christchurch, has been proven in practice.

At the Pacific Conference on Earthquake Engineering held recently in Auckland, renowned structural engineering Professor Nigel Priestley presented a concise paper on how seismic design philosophy is continually undergoing development.  A short summary of part of the paper helps place PRESSS at the cutting edge of damage resisting seismic structural design.

Professor Priestley illustrated how during the decades following the 1920’s, elastic design became prevalent.  This was superseded during the 1960s by strength design, which in turn was replaced by multi-mode assessment with its recognition of ductility’s beneficial influence.  The concept of capacity design, heavily influenced by Professors Park and Paulay at Canterbury University and the (then) Ministry of Works, followed shortly after, and dominated structural design principles for several decades.  These approaches have collectively become known as Force Based Design (FBD).

Professor Priestley went on to outline that throughout the 1990’s and into the current millennium, the adoption of performance based seismic design grew, along with an awareness that while strength and performance were not closely related, displacement and performance could be.  This approach has become known as Displacement Base Design (DBD).

With its revolutionary advancement of earthquake engineering, based around concrete structural elements, PRESSS pushes DBD into new territory.  This ground-breaking design approach, developed at the University of San Diego in the 1990s under the leadership of Professor Priestley, has undergone further development in New Zealand through the work of Dr Stefano Pampanin at Canterbury University.

Key to PRESSS is its use of un-bonded post tensioning cables within the members of a precast wall, beam and column structure, which along with specially designed ductile joints, allow for a controlled rocking mechanism that returns the building to upright without significant structural damage, even after a major seismic event.

Specifically, the PRESSS system reduces movements under small earthquakes through its pre-rocking stiffness, while for larger seismic events the yield action of its ductile joints combine with the un-bonded post tensioning cables to spring the building back to its original vertical position.

An innovative building solution, PRESSS offers enhanced performance during a seismic event and in turn minimal repair requirements, is efficient and economical to construct using precast concrete, and provides cost benefits to building owners and occupiers.

As mentioned previously, the Southern Cross Hospital Endoscopy Building in Christchurch, designed by Structex and peer reviewed by Dr Pampanin, is an example of PRESSS in practice.

This five storey building includes a half depth basement, suspended car parking deck, theatre suites and a roof level plant room. The theoretical principles of PRESSS performance during a seismic event were verified by the September 2010 and February 2011 Canterbury earthquakes, with the building emerging structurally unscathed and available for immediate reoccupation.

Another, more recently completed example of PRESSS is the Alan MacDiarmid Building at Victoria University, Wellington.  Completed in 2010, the building was designed by Dunning Thornton Consultants, and once again peer reviewed by Dr Pampanin.

As a long-term property owner Victoria University selected PRESSS for this multi-storey building to not only ensure minimal structural damage and need for subsequent repair in the event of an earthquake, but also superior protection for its immensely valuable content, along with the flexibility offered in terms of services distribution.

Different approaches to seismic structural design, and the application of various structural framing systems, will continue to evolve as a result of research and in-use experience.  Within this context, and at the same time complementing more conventional concrete seismic structural design, PRESSS constitutes a viable option for damage resisting, reinforced concrete buildings in any plan for the reconstruction of Christchurch.

Article appeared in Contractor magazine.