This week’s magnitude 7.8 earthquake in the Philippines came with scenes familiar to New Zealanders: collapsed buildings, shattered facades and streets strewn with rubble.
Earthquakes of such force test buildings to their extreme limits. As occurred this week, and in Christchurch in 2011 , some ultimately fail with tragic consequences.
But, for structural engineers, preventing collapse is only part of the challenge. Increasingly, we are also asking what happens to buildings that survive major earthquakes.
Many modern buildings are designed to protect lives, but often at the cost of damage that can take years and millions of dollars to repair. Some quake-damaged buildings have ultimately been demolished despite never having come close to collapse.
On top of this, the construction sector is under growing pressure to shrink its substantial share of global greenhouse emissions . This is raising the need for building systems that are sustainable and resilient.
Last month, in one of the country’s most demanding full-scale earthquake tests, we assessed an emerging timber-based technology to find that it can meet all these requirements.
Buildings that bounce back
Over the past decade, many people will have heard growing talk about timber as a low-carbon alternative to concrete and steel. While we might picture traditional timber-framed houses, modern mass timber construction is very different.
One of the most promising products – cross-laminated timber (CLT) – is made by bonding layers of timber boards together at right angles, creating large structural panels that can be used to construct multi-storey buildings.
As a renewable material, it stores carbon absorbed during tree growth and can reduce the embodied emissions of buildings compared with concrete and steel. It is also well suited to prefabrication, with entire building components or modules manufactured off-site and assembled later, reducing construction time, waste and disruption.
During earthquake shaking, engineered timber structures have been found to perform extremely well. But less understood is how these new modular mass timber buildings accommodate movement.
To model this, we developed a system that allows storeys to move relative to one another during an earthquake, rather than forcing the entire building to act as a rigid unit.
This controlled movement reduces the strain placed on the building during an earthquake. Once the shaking stops, the system helps the structure return to its original position, reducing damage and improving the chances it can be used again quickly.
Putting timber to the test
To understand how our system performs under realistic earthquake conditions, we built a full-scale, modular CLT building and tested it on the University of Auckland’s ” shake table ” simulator.
While the test building was physically two storeys high, additional weight was added at roof level to replicate the forces experienced by a typical three-storey building – one of the most common forms of medium-density housing in New Zealand.
The simulation itself subjected the building to a series of increasingly demanding earthquake shaking, reflecting what would be experienced from both distant and near-source events.
The building performed as hoped, with the connection system allowing each storey to move in a controlled way during the simulated earthquakes. This helped absorb and dissipate energy while protecting the main timber structure from damage.
Perhaps most importantly, the building returned to its original position after the shaking stopped, rather than being left permanently tilted or displaced. Engineers call this ” self-centring ” – a key feature of buildings designed not just to survive earthquakes, but to recover from them.
And while the building moved about during the shaking, the main timber structure remained undamaged. In a real-world earthquake, that could mean lower repair costs, less disruption and a faster return to normal use.
There were, however, questions our test could not answer. For instance, it did not assess how non-structural elements such as wall linings, services and interior finishes – which are often damaged during earthquakes – would perform in a real building.
Nevertheless, the results provide encouraging evidence that modular timber buildings can be designed not only to withstand major earthquakes, but also to recover from them with minimal damage.
The next step is to incorporate the technology into complete building systems and assess its long-term performance, practicality and commercial viability.
If those hurdles can be overcome, it could help support a new generation of low-carbon buildings that are safer, more resilient and quicker to return to service after major earthquakes.
As countries such as New Zealand continue to grapple with both seismic risk and the need to reduce construction emissions, innovations like these may help show that resilience and sustainability do not have to come at the expense of one another.
