PRESSS

PRESSS Technology Seminar gets Enthusiastic Support in New Zealand

Photo below: (From left) Len McSaveney, Professor Nigel Priestley (the coordinator of the technology), from the University of California, San Diego and the ROSE School in Pavia, Italy; and Dr Stefano Pampanin from the University of Canterbury.

Professor Nigel Priestley, in his typically modest way, claims that because he proposed the best acronym to describe the development of PREcast Seismic Structural Systems, he was picked to lead the multi-million dollar research programme shared between many of the top research facilities in the US and co-operating with Japanese researchers. The programme goals were simple: develop a low-cost precast concrete building method that could meet the intent of Building Codes, for structures that could survive major earthquakes with minimal damage.

For Nigel, this was a fantastic opportunity to test theories that had evolved over several years of discussions with his Seismic Engineering colleagues, including Professor Tom Paulay, from Canterbury University. The use of capacity design principles to design lateral load resisting walls or frames that emulated cast-in-place construction, though dependable, was not providing value to the building industry: while the method undoubtedly saved lives, the Northridge and Kobe Earthquakes of the 1990’s showed that relying on ductile members to dissipate energy resulted in high damage and expensive downtime to owners or tenants as structures were repaired, or demolished and rebuilt. Residual drift (a leaning building, after the earthquake) was a serious irreparable problem.  Observations of many structures that had survived catastrophic earthquakes (including some of the ancient Greek and Roman buildings) showed Nigel that there was a better method.

PRESSS Technology makes use of precast walls, beams and columns; jointed in a ductile manner, and combines un-bonded post-tensioning cable within the members to restore the structure to its original position after the shaking has stopped. The principles are similar to a car’s suspension, where the designer has carefully tuned the combination of springs and shock absorbers to give the driver and passengers a smooth ride. In PRESSS buildings, the springs are un-bonded post-tensioned cables and the shock absorbers are yielding reinforcing bars. By balancing these two lateral load resisting mechanisms, the Structural Engineer can design a structure that will survive a major shake with minimal post-earthquake damage. Attendees at the New Zealand Concrete Society seminars (4) saw how simple these buildings are to construct, using techniques that have been widely used in New Zealand for many years, but by combining them in a unique way.

Introducing PRESSS to the New Zealand market has been assisted by its inclusion in the new draft concrete design standard (DZ 3101) and by the introduction of more severe seismic actions implicit in the earthquake loadings standard (NZS 1170.5: 2004). We have also been able to take advantage of the acceptance of PRESSS principles in publications of the American Concrete Institute (1) and The International Federation for Structural Concrete (2 & 3). Many earthquake-prone countries are now commercialising this technology, but while the public domain research is readily available, proprietary research to prove an application in a particular niche market is closely guarded. This secrecy gives the early adapters a commercial edge for a limited time, but like post-tensioning, or capacity design, a system that offers such significant benefits cannot be restrained for too long.  

More than 170 attendees at the Concrete Society seminars in Christchurch, Wellington and Auckland(including one enthusiast from ) were treated to an overview of the design method by Nigel, explaining the background theory and the experimental validation. Nigel’s presentation was followed by Dr Stefano Pampanin, a Senior Lecturer at the University of, who has conducted extensive research on PRESSS-type structures. Stefano explained how to use the design methodology for Hybrid Jointed Ductile Structures, that is included in the draft of the new Concrete Design Standard. Through his active involvement in both the US and European applications of PRESSS Technology, Stefano was the ideal candidate to write this section of the Standard for the DZ 3101 Drafting Committee. Stefano’s suggestion that Architects could make a feature of exposed, replaceable dampers, which would be purchased from seismic design shops in earthquake-prone cities, drew a response from audiences. Worked examples, to DZ 3101, Appendix B will be posted on the NZCS website, when Stefano has completed them.

Nigel’s second presentation was a plea for a more rational seismic design method – Performance-Based Design (with “Displacement” being the performance criteria most suited for the seismic design of buildings). This displacement-based technique ideally suited the design of PRESSS-type structures, and gave designers a much better appreciation of how their design would perform in a major earthquake. The argument was very persuasive for the expert designers in the audiences, and there was no doubting the clarity of thought that Nigel has on this complex topic. Simplified assumptions in our current loadings and materials design standards can lead designers to incorrect conclusions and poor design details.

The seminars were wrapped up by Len McSaveney and Stefano Pampanin showing examples of real buildings being constructed in Italy and in California. The audiences were eclectic mixes of designers, academics, contractors, precasters and council representatives; they asked questions related to the dispelling of old myths: how dependable were modern un-bonded cables and their anchorages; could precast hollow-core floors cope with such displacements; and how are typical NZ construction tolerances allowed for? The concept that these rocking buildings are actually stiffer than conventional ductile-frame buildings within the serviceability range, due to the absence of cracks, was obvious when it was explained. Nigel also confirmed that the temporary gapping that occurs in the floors of these structures when the Big Earthquake hits, is actually less than the permanent gaps that we can expect with conventional ductile frame buildings (the post-tensioning eliminates progressive beam growth). Fears of damage to cladding and building services at serviceability design earthquakes, or severe wind loads, are completely unfounded, with correctly designed hybrid structures.

To be successful, a new building system must be cheaper, or faster to construct, than existing systems: PRESSS Technology, or Hybrid Ductile Jointed Systems to give it a generic name, have the advantage of being both faster and cheaper, based on US, NZ and Italian experience. The technology has also arrived on the world construction scene at a time when designers are struggling to come to grips with: Performance-Based Design; more demanding seismic design parameters; and while contractors are facing global shortages of skilled labour and dramatically shortened construction programmes.

Nigel Priestley and his research colleagues have presented the precast concrete and construction industries with a powerful solution to limit earthquake damage to buildings: it is now up to us to rise to the challenge and to use this technology to keep precast construction as New Zealand’s preferred construction material.

• ACI T1.2-03. Special Hybrid Moment Frames Composed of Discretely jointed Precast and Post-Tensioned Concrete Members.
• Displacement-based seismic design of reinforced concrete buildings. fib May 2003.
• Seismic design of precast concrete buildings. Fib October 2003.
• New Zealand Concrete Society. PRESSS Technology. An Introduction to the Design of Ductile Jointed Precast Concrete Structures.

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