University of Auckland Centre for Earthquake Engineering Research

Structural seismic validation of self-centering precast concrete wall details

Investigating issues involved with the design of unbonded post-tensioned precast concrete walls. The research extended studies previously conducted at The University of Auckland into the behaviour of such wall systems.

Until recently the prevailing seismic design philosophy worldwide has been ductile design, whereby a structure undergoes large plastic deformations in order to safely dissipate the energy applied to it during an earthquake. This often leaves a structure with severe and sometimes irreparable damage. However, there had been a shift towards a damage avoidance design approach involving the safe dissipation of the earthquake's energy with minimal or no residual damage. One approach to this damage avoidance design philosophy is the use of an unbonded post-tensioning system.

Unbonded post-tensioning involves the placement of prestressing tendons into ducts that are cast in to concrete walls or frames. In a precast construction system unbonded post-tensioning can be used to tie all the components together. When subjected to seismic loads these walls or frames can rotate about the joints that connect the separate elements, with the pre-stressing tendon providing a restoring force. Through this motion the seismic loads can be safely absorbed while ensuring minimal damage to the structure. Research into the use of this innovative technique is limited, with previous studied having primarily focused on a "next generation" solution rather than dealing with practical design implications.

Several areas were identified as requiring further attention. Firstly, the magnitude of the tendon stresses has traditionally posed a problem. A new equation has been developed at the University through the investigation of a masonry wall system. Research continued the validation of this equation for a concrete wall system as well as looking closely at the concrete strain profile at the rocking interface. This equation for the tendon stresses was also to be compared to a similar equation developed by Iowa State University in the hope that combining the two will result in a uniformly agreed equation that once published will form the basis of a redeveloped design procedure. Additionally, further investigation into the influence of the interface/bedding material was also conducted. The main focus was the effect of this contact material on both the wall behaviour and tendon stress levels.

This research involved a strong collaboration with Iowa State University and the National Centre for Research on Earthquake Engineering (NCREE) in Taiwan. It enabled the use of high tech laboratories with superior testing facilities as well as well as ensuring the research had a greater international impact.