Until recently, the only protection building owners had from the risk of significant earthquake damage was base isolation design structures. New Zealand’s deadly 2011 Christchurch earthquakes have promoted an increased focus on and strong uptake of earthquake-resilient building technologies, in response to a better understanding of seismic risk versus building investment.
Earthquake-resilient building design is often referred to as ‘low damage design’ or ‘damage avoidance design’ because it enables buildings to respond to significant seismic events, without causing irreparable damage to primary structural elements. The design process is aimed at minimising the level of seismic damage to structures; so that buildings may be rapidly reinstated for their purpose after a major earthquake.
“The reality is that even code compliant buildings are far from indestructible and, when a significant earthquake event occurs, are often damaged beyond repair,” explains Stephen Hogg, a Technical Director at Aurecon’s Christchurch office. “Consequently, designers need to make their structures ‘tougher’ to withstand overstressing and collapse. This requires a sound knowledge of earthquake-resilient building technologies.”
Based on his experiences in Christchurch, Hogg outlines traditional earthquake-resilient design, including base- and non-base-isolation-resilient building techniques. He also explains the benefits and versatility of leading-edge, innovative earthquake-resilient building design and its cost implications.
Providing assistance in the aftermath of recent earthquakes and, in particular, the devastation in Christchurch provided a more all-embracing appreciation of the need for architects and engineers to pursue earthquake-resilient building design. The benefits can be summarised as:
“When a building is shaken by a large earthquake, the building’s immediate response is to deflect sideways, displacing its own weight,” explains Hogg. “This displacement sets in motion large inertia forces that the building’s primary bracing structure is designed to resist. Large buildings, however, generate extremely large inertial forces which are often much greater than the designed structural strength of the building.”
To overcome this, a traditional method of ductile design is used, which allows the building structure to yield to the earthquake forces and dissipate the seismic energy. This is achieved by designing the primary structure for a series of controlled failures, absorbing the earthquake induced inertial forces and maintaining building stability, as well as, most crucially, life safety. While this is a robust and safe design method, experience has shown that traditional ductile buildings are highly susceptible to irreparable damage following a large earthquake, often necessitating building demolition.
In Christchurch, there has been a significant interest in the application of base isolation to offset seismic stress. This is a well-understood technology which typically uses lead rubber bearings or concaved friction bearings. The design allows a much reduced transfer of seismic force into the primary structure, compared with a more traditional ductile design.
“Base isolation is currently the best method for dealing with high forces from earthquake induced horizontal ground shaking by providing separation between the structure and the ground, simultaneously dissipating seismic energy,” says Hogg.
While New Zealand designers have traditionally used lead rubber bearings as a base isolation device, Christchurch has seen the country’s first introduction of concaved friction slider bearings. Concaved friction slider base isolation devices are commonly referred to as friction pendulum bearings and characteristically transmit lower seismic forces into the structure and fit-out of buildings compared with lead rubber bearings. This is due to a lower inertial force being required from the building before the bearings start sliding, although the displacements are proportionally larger.
Ideally, the best-performing resilient designs also incorporate systems that allow re-centring after a large earthquake. This is achieved by simply providing a restoring force. In a wall braced building, this is easily achieved by post-tensioning a wall vertically on its centre line and allowing the wall to rock back and forth in an earthquake. The post-tensioning acts elastically through elongation and contraction, restoring the wall back to a vertical position.
Energy dissipation is included in the overall system with replaceable ductile energy friction pendulum bearings that yield in tension and compression at the point of rocking, usually at the base of a wall.
“These act like the shock absorbers in a car,” says Hogg. “The ductile energy dissipaters are often termed a ‘plug-and-play’ device, meaning it is replaceable after it has yielded.”
Following a large earthquake, the intention is for the ductile energy dissipaters to be easily removed and replaced. This rapid retrofit process allows the integrity of the primary structure to be reinstated quickly. Resilient systems are designed to protect the primary structure from irreparable damage, allowing the building to be quickly repaired and put back into normal service again.
“Resilient building design accepts a greater level of seismic damage than base isolation at a lower application cost. It is not possible to design and build structures which are totally damage-resistant under all earthquake conditions, so the term ’resilient building design’ should be used carefully,” asserts Hogg.
Resilient building design simply means that there should be less damage than in conventional construction during design-level earthquake shaking. An earthquake-resilient building should be ‘occupy-able’ immediately after experiencing large shaking (design level), and might be ‘occupy-able’ in a short time frame after very large shaking.
“Earthquake-resilient building designs in New Zealand to date have generally been bespoke and require an innovative application to be incorporated into the architectural fabric of a building. There are alternative solutions not found in regular design codes and can be applied to concrete, steel and timber,” says Hogg.
Architects and drivers of building function will determine the type of earthquake-resilient building designed. Earthquake-resilient buildings have the same primary structure as any conventionally designed building, i.e. shear walls, frames or braced frames. The key difference is the resilient components are integrated into the conventional structural form, usually as an accessible replaceable component with sufficient ductility to dissipate seismic energy.
“In general, any structural form is capable of resilient detailing so architectural and building functionality drivers are not compromised,” explains Hogg.
Each approach has an associated cost premium linked to relative performance. The perceived expense associated with base isolation often limits its use to high importance and high-budget buildings. However, in most cases, base isolation of structures encompasses only a small percentage of the total build cost (5% - 7%). This increase in construction cost needs to be considered in the context of significantly improved potential business continuity following a large seismic event and the aftershock sequence that follows.
The cost premium for earthquake-resilient building design (additional techniques over and above base isolation) applied to a regular building is typically between 1% and 3% of the total construction cost.
Unfortunately, the cost of earthquake-resilient design is not well understood in the construction market. This can add cost and uncertainty to new developments that, in turn, impacts on the financial feasibility of earthquake-resilient buildings, and is often the deciding factor in determining the degree of resilience applied into a building design.
“To avoid the need for that all too familiar expression in future years: ‘If only I had protected my investment and people better’, it is important to involve an experienced consultant able to both incorporate the latest earthquake-resilient building design and develop a suitable business case for a cost-effective investment,” concludes Hogg. “No, we can’t guarantee a building design that is proof against all earthquakes, but considerable investment future-proofing is available.”