10 years have passed since the magnitude 7.1 Darfield earthquake shook the ground beneath Canterbury, kicking off what has been a decade of significant earthquake activity in New Zealand. Non-structural seismic specialist Dr Andrew Baird reflects on the growing importance of ensuring non-structural elements are considered in earthquakes, given his first hand experience.
When the 2010 Darfield earthquake struck, I had just started a postgrad research project at the University of Canterbury, looking at ways to reduce earthquake damage to non-structural elements in buildings. The experiments I had planned in the university lab that summer were intended to replicate earthquake damage to buildings.
The quake itself closed the lab and my tests were scuppered. Instead, laid out across the city of Christchurch were thousands of real-world examples of earthquake damage to learn from. Unbeknown to me at the time, the Darfield quake would shape my career. It would lead me to obtain a PhD in earthquake engineering in 2014, and to working as a consulting engineer for Beca, where I’ve spent much of my time improving the seismic performance of non-structural elements in buildings - so we don’t see as much damage next time there’s a decent shake.
What are non-structural elements?
Over the past decade, there has been a shift in the way that engineers approach the design of a building. The damage to buildings that occurred in Christchurch and later in Wellington during the 2016 Kaikoura earthquake, highlighted that getting a structure through an earthquake is not enough. It is also necessary to get a building’s fitout, fabric and contents through the earthquake too. Such items are often grouped together in what we call ‘non-structural elements’ – these are all the parts of a building that are essential to its functioning, but are not there to hold the building up; such as ceilings, partitions, HVAC systems, electrical systems, plumbing and drainage etc.
Why do we care about non-structural elements?
Non-structural elements comprise up to 70% of a building’s capital value. It’s not surprising then that the damage to these elements in an earthquake has resulted in many buildings being declared economic losses, even when the structure itself was not badly damaged. Damage to non-structural elements also presents a significant risk to building occupants and affects whether the building can be reoccupied following an earthquake. Recent research suggests there is a need to focus on reducing the damage to non-structural elements.
- Non-structural element damage is the main contributor to overall NZ earthquake injuries
- Non-structural element damage is the main contributor to building related property loss and business disruption following an earthquake
What can we do to improve?
You might assume that the poor performance of non-structural elements in recent earthquakes points to shortcomings in the building code and design standards. While improvements to the design standards are necessary to bring them up to date and make them easier to implement, the biggest hurdle remains having appropriate seismic design undertaken and this design being properly implemented. A survey conducted in 2016 for MBIE found that typically between 50-80% of ceilings, partitions and services are inadequately braced in commercial buildings.
The design of seismic restraints for non-structural elements lies at a messy junction between structural engineers, building services engineers, architects and contractors. Experience has taught us that unless someone takes responsibility for the design and coordination of the seismic restraints, it will likely fall through the cracks. The role of a specialist seismic restraint designer is now emerging in New Zealand. This specialist role has existed in the United States for many years, after they went through a similar learning curve following the devastating 1994 Northridge earthquake. The role is effectively an entirely new discipline combining structural, building services and architectural knowledge and experience.
The multi-disciplinary demands on this role make it more suited to a multi-disciplinary team of specialists who can collaborate and develop solutions that achieve the seismic restraint requirements, while not compromising the functionality of the element being restrained. For example, the seismic restraint design of a pipe requires a structural engineer to design for the seismic demands upon the pipe, but also requires input from an industrial engineer to understand the pipe’s thermal expansion and contraction demands, as well as any thrust force considerations.
A better way
Part of the solution involves getting a seat at the design table earlier. Historically the design of seismic restraints was not undertaken until the construction phase of a project. This led to poor design outcomes, poor cost-certainty and very little opportunity to coordinate the seismic restraint requirements of different disciplines.
If seismic restraint design is undertaken earlier in the design programme, the approach can be coordinated between disciplines. This improves construction efficiency and also ensures a compliant design is completed and adequately costed. However, early design is not without its challenges. Unlike traditional building services and structural engineering disciplines, seismic restraint does not naturally lend itself to hand over from Designer to Contractor at the end of the design process. This is because many seismic restraint design outputs are dependent upon the Contractor’s final equipment selections, as well as construction phase set out and coordination. It is therefore essential that an early design procurement model includes continued input between Designer and Contractor.
The final piece that has enabled a step change in the approach to seismic restraint is the advancement of 3D BIM modelling tools. Modelling of seismic restraints at the same time as other disciplines means that coordination can take place easily during design. This is essential for complex buildings like hospitals and laboratories, to ensure that buildable solutions are achieved. With these tools, the lessons learnt from recent quakes and the knowledge of what has been successful in the United States, there is no reason why we can’t build better buildings.