Seismic Anchoring in New Zealand
New Zealand is one of the most seismically active countries in the world, located on the Pacific Ring of Fire. The entire country is affected by earthquakes leading to significant peak ground acceleration (PGA), in parts over 1.0g. As proven more recently the town of Kaikoura had a 7.8 magnitude event that resulted in one fatality and major damage to critical infrastructure. This earthquake had the highest PGA ever recorded in New Zealand at 3g. And more famously, the Christchurch earthquakes of 2010/2011 resulted in 186 fatalities (115 of whom died in the Canterbury Television building collapse), which were measured at 7.1 and 6.2 magnitude respectively.
Although smaller in magnitude than previous earthquakes, the resulting devastation from the Christchurch event was caused by other factors. Most obviously, the epicentre was closer to Christchurch CBD and it was shallower at just 5km underground. On top of that it was a busy weekday lunchtime, with the CBD full of people.
From an engineering and geotechnical point of view, the buildings that collapsed were said to have been already weakened by previous tremors and the PGA in some locations was extremely high (at 2.2g, i.e. 2.2 times the acceleration of gravity) – simultaneous vertical and horizontal movement made it almost impossible for many buildings to stay intact, especially for non-seismically-designed heritage ones. In addition, there was a massive liquefaction occurrence—around 400,000 tonnes of silt—which created a degree of ground instability rarely seen anywhere in the world.
For building design and construction, these earthquake risk factors create the need for a very stringent building code across New Zealand, which has been implemented for many decades for structures. However, connections with post-installed anchors into concrete did not have a standard that considers reduction in strength of fasteners in cracked concrete during an earthquake. More recently both ICC-ES (USA) and ETA (Europe) anchor qualifications have been referenced in NSZ 3101 for seismic anchor design. Unfortunately, many buildings in Christchurch, and many other places in New Zealand, were built before implementation of modern standards.
Delving deeper and more specifically into this code, it is compulsory to use only seismically-tested and approved construction anchors in New Zealand, especially in concrete connections and structural components. Because of the stricter guidelines and hence more time required for seismic design, structural engineering fees are high in New Zealand – often double that of a country like Australia whose PGA for an event with 500-year return period is roughly just one tenth of that in New Zealand, which still poses the potential for high-damage earthquakes as it is.
Costs aside, the reasons for using seismically-approved construction anchors are developed as a result of how they behave under earthquake conditions. However, many existing anchors from reputable suppliers are already seismically rated and will not necessarily impose an increase in cost.
And the choice of whether to use a strictly mechanical or chemically-embedded style of anchor depends on load transfer principles at play. Within the mechanical realm, it is then a matter of choosing torque-controlled expansion anchors, screw anchors or displacement-controlled expansion anchors.
Of course, the complexity lies in how the anchor reacts and performs during a seismic event. In general, displacement-controlled expansion anchors are not suitable for seismic load, whilst torque-controlled expansion anchors, screw anchors and chemical anchors are. The reasons for this lie within the research carried out by engineers who help to inform the national standards for the fasteners’ use.
Moving forward, it is important that only seismic-approved anchors be used in construction in New Zealand—whether it be buildings, public infrastructure, or bridge projects.