New Zealand is maintaining its reputation for being at the forefront of seismic technology advances, with new buildings debuting some exciting new features.
OVER THE LAST 5 years, the focus of seismic design has changed from preserving lives and accepting building damage to also preserving the building. The recent Canterbury and Kaikoura earthquakes highlighted the wide-spread structural damage earthquakes can cause – even to buildings that appear to come through the shaking unscathed.
Low damage or damage avoidance
The resulting time, cost and disruption of repair has an impact on the building industry and our communities. Seismic design is now taking advantage of new systems and advanced analysis and testing capability to minimise the post-earthquake repair burden.
As in the 1970s when New Zealanders were leaders in the initial phase of modern seismic design, local engineers are again at the forefront of this new era of low-damage and damage-avoidant systems. Here are examples of solutions that are being developed, tried and delivered in our newest buildings.
The 3-storey Kaikoura District Council building survived the 2016 Kaikoura earthquake unscathed – and indeed was used as the emergency response centre – thanks to resistance provided by its cross-laminated timber (CLT) wall panels. CLT walls are becoming increasingly popular worldwide as they can be designed to provide sufficient strength and seismic resistance to be used in open-plan medium-rise structures, while providing the aesthetics and feel of timber.
The building’s structural system used 15 CLT and laminated veneer lumber walls each 13 m high and 3.4 m wide, post-tensioned to the foundations, with seismic fuses running along the outer face of the wall panels.
Fuses include energy-absorbing metals or yielding components that dissipate seismic energy forces that would otherwise damage the building’s primary structure. They are designed to be physically damaged and easily replaced after a major event.
On inspection of the building after the earthquake, there was no damage to the walls, and the expected yielding of the fuse rods did not occur.
Precast seismic structural system
The precast seismic structural system (PRESSS) comprises unbonded post-tensioned steel cables running through the frame of a building, allowing controlled rocking of the building without damage to the frame itself. PRESSS technology was developed in the 1990s by New Zealand engineer Nigel Priestley.
After an earthquake, the steel cables pull the frame back to square. The $55 million Forté Health hospital constructed in Christchurch in 2014 marked the country’s first ever use of PRESSS within a steel-framed building.
Replaceable shear links
The Three35 commercial building in Lincoln Road, Christchurch, built in 2013 has the first New Zealand use of replaceable shear links within an eccentrically braced frame structure. Acting like a fuse, these links deform in an earthquake, then are unbolted and replaced.
Consulting s tructura l engineers Ruamoko Solutions developed innovative shear link design solutions, particularly around link endplates and column base-plates. They overcame challenges within the local market regarding fabrication and material sourcing, given the first-time use of this solution.
The project has helped pave the way for future use of these devices, with Steel Construction New Zealand publishing design guidelines for shear link systems partially based on learnings from this project. Ruamoko Solutions won a 2014 ACENZ Innovate NZ Award of Excellence for the building’s structural system.
Post-tensioned laminated veneer lumber
The $18 million Trimble Navigation building built in 2012 in Christchurch is one of the first commercial buildings in New Zealand to use a post-tensioned laminated veneer lumber (LVL) structure with mild-steel energy dissipators as a seismic-resistant system.
The building is formed by a large prefabricated LVL frame and wall units strung together by post-tensioned steel cable. This system allows the building to rock and joints to open, with the seismic energy absorbed in the dissipating devices. This system provides high levels of damping, and once the shaking stops, elastic cables return the building to its original position.
Trimble and building designers Opus collaborated on a building monitoring system. This uses sensors to monitor structural components, collect structural data to inform safe occupancy after an earthquake and advise on the replacement of the dissipators.
The building won the Commendation Award for Commercial or Retail Structures in the prestigious global IStructE Structural Design Awards in 2014.
Compact lead extrusion damper innovation
While lead extrusion dampers have been around for over 30 years, their large size has meant they have only been viable in limited situations. University of Canterbury (UC) Associate Professor Geoff Rodgers has developed a compact size damper that achieves a higher internal pressure, which is able to provide the seismic resistance of full-size dampers.
The smaller size gives greater flexibility for placement within both steel and concrete building frames and broader use in other structures such as bridges. These compact dampers are used in the Forté Health hospital mentioned earlier.
Taking UC innovations to the world, Associate Professor Rodgers was invited to incorporate these dampers into a low-damage community housing project in San Francisco. He is also part of a team testing a 2-storey reinforced concrete rocking-wall structure.
Resilient slip friction joint
The ongoing advancement of seismic resistance systems continues with the resilient slip friction joint (RSFJ) device. This has recently been developed at the University of Auckland and will receive its first application in the new Nelson Airport Terminal under construction from the end of 2017.
RSFJ devices use friction plates with angled grooves pressed together by high-strength bolts and disc springs. They are bolted in place at structural joints such as beam-column joints.
In an earthquake, the devices accommodate very large lateral forces as the plates slide across each other under the clamping force of the bolts. After dissipating earthquake energy through friction, the bolts restore the plates to the starting position.
RSFJ devices can be scaled from small to large sizes, can be used with concrete, timber and steel and can be retrofitted to existing structures, meaning very broad potential use.
The RSFJ device goes beyond the concept of fuses as it does not deform or suffer damage while dissipating energy. This means that, after an earthquake, there is zero building-use downtime for component replacement, so the building is always safe to use and remains at full strength throughout any aftershocks.
A new building going up in the University of Canterbury’s science precinct is a multi-storey, all-timber moment frame. While buildings already exist that use some of this technology, it will push the boundaries of multi-storey timber-framed construction. Low-damage timber moment frames were developed by the New Zealand research consortium Structural Timber Innovation Company (STIC). STIC technology was also used in the Trimble and Kaikoura District Council buildings.
A moment frame is a two-dimensional structural assembly typically consisting of beams and columns, where the beam-column connections are designed to be rigid such that they can resist bending moments. It can resist lateral and overturning forces.
Scheduled for completion in 2019, the design for the new building was developed by a team of academic researchers from UC, Beca and architects Jasmax and uses laminated veneer lumber.
Articles are correct at the time of publication but may have since become outdated.