While the 2010 and 2011 Canterbury earthquakes were not New Zealand’s largest seismic events in recent years, they resulted in the most damage and loss of life. The Canterbury sequence also put a renewed community focus on earthquake engineering and the strengthening of structures.
Metropol Build talks to Mainmark New Zealand technical manager Theo Hnat about the new approaches to structural strengthening and what they mean for building owners.
The Building (Earthquake-prone Buildings) Amendment Act 2016 (EPBA) was introduced on 1 July 2017, coinciding with the updated New Zealand Society for Earthquake Engineering assessment guidelines.
The EPBA provides leadership and direction regarding risks to public safety posed by existing buildings, and brought major changes to the way earthquake-prone buildings are identified and managed under the Building Act. It also educates building owners and tenants about seismic performance and risk.
NEW BUILDING STANDARD
The EPBA categorises New Zealand into three seismic risk categories under the New Building Standard (NBS) and sets timeframes for building owners to identify and take action to strengthen or remove earthquake-prone buildings.
The lowest performing structures (33 percent NBS or less) are deemed earthquake prone and will most likely be demolished. For intermediate risk (33 percent – 67 percent NBS), building owners are required to strengthen the building to meet 67 percent NBS.
The challenge for building owners undertaking earthquake strengthening works to existing buildings is to consider both ground strength and building behaviour so that the most cost effective and long-lasting solution can be identified to improve the %NBS.
The past two years has seen growing awareness about the interaction between building structures and soil during an earthquake.
While the EPBA and %NBS has created a growing market for earthquake strengthening, encompassing architects, engineers, builders and developers, earthquake strengthening of existing buildings has historically focused only on bracing or stiffening the visible structure, rather than modifying the ground or footings.
The design methods typically considered include FRP wrapping, concrete jacketing, additional bracing or damping, selective weakening, base isolation, foundation widening or stiffening, piling, and ground improvement.
EARTHQUAKE STRENGTHENING OF BUILDINGS HISTORICALLY FOCUSED ON THE VISIBLE STRUCTURE
Apart from ground improvement, the strengthening solutions are structural approaches that seek to either increase flexibility or strength, or decrease deformation and displacement of the building, without addressing the behaviour of the soil. For building owners seeking to achieve a higher %NBS, it is recommended to engage both geotechnical and structural engineers when considering the best approach.
Photos from the 1964 Niigata earthquake, with buildings lying on their sides, demonstrated structures strong enough to withstand the impact of an earthquake on poor ground. It is important to remember that much of the damage observed in Canterbury was due to poor performance of the ground rather than the strength of the buildings.
Since the Canterbury earthquakes, ground improvement research trials have shown that resin injection can demonstrably improve the density and stiffness of earthquake affected ground and increase the resistance of soils to liquefaction. The full research report is now available on the New Zealand Geotechnical Society online library, and is included in MBIE Module 5: Ground Improvement of Soils Prone to Liquefaction.