Image Credit: Darren Pateman (Newcastle Herald)

The 1989 Newcastle earthquake in NSW, Australia, serves as a stark reminder that even regions with seemingly stable geological conditions can experience significant tremors. This highlights a crucial point: seismic design is not just for those living in traditionally earthquake-prone zones. It’s a vital consideration for ensuring long-term structural resilience, no matter where a building stands.

“Earthquakes such as Newcastle in 1989 show that nowhere in the world is immune from seismic events. While generally the major seismic events occur on the tectonic plates, intraplate earthquakes are real.” – Gregg Klopp

Traditional structural design often focuses on keeping a building elastic under loads like wind or the weight of the structure itself. This means the building bends but bounces back to its original shape once the load is removed. Seismic design, however, acknowledges that during an earthquake, structures might be pushed beyond their elastic limits. Instead of preventing all damage, the focus shifts to preventing collapse and ensuring the safety of occupants. This acceptance of potential damage – cracking concrete, yielding steel – is a fundamental difference in the seismic design philosophy. It prioritises life safety over complete structural preservation, acknowledging that in a major earthquake, the building itself might be irreparable.

“Designing for seismic events is different from the way we design for dead, live and wind loads. For the latter, the structure is designed to remain elastic… When designing for seismic events we allow the structure to respond post-elastically. That is the structural elements can yield, the concrete can crack, and the structure may have permanent deformation.” – Gregg Klopp

Failing to incorporate seismic design can have far-reaching consequences. Beyond the immediate tragedy of potential injuries and fatalities, the economic and social impacts can be immense. Disrupted infrastructure, damaged businesses, and the strain on healthcare systems can cripple a community for years following a significant earthquake. The Newcastle earthquake, for example, caused widespread damage, business closures, and significant disruption to the local economy, demonstrating that the repercussions of seismic events extend far beyond the immediate area of impact.

“An earthquake occurring where seismic design has not been considered can lead to death and injury and in the long term significant economic effects due to the disruption to industry as it is not a localised event.” – Gregg Klopp

IL4 Buildings: Maintaining Community Function in Times of Crisis

In the aftermath of a major earthquake, certain buildings become absolutely critical for a community’s survival and recovery. In Australia and New Zealand, these are classified as Importance Level 4 (IL4) structures, and they encompass a range of essential facilities: hospitals providing urgent care, emergency shelters offering refuge, command centres coordinating disaster response, and the infrastructure that keeps vital services like electricity and water running.

“An Importance Level 4 structure is one that is required to be able to be used after a seismic event. For example, a hospital needs to be available for patients, buildings are required to provide shelter, control centres are needed to coordinate disaster recovery, and electrical, water and sewer infrastructure is necessary to provide lifelines to the community.” – Gregg Klopp

What sets IL4 buildings apart is the expectation that they must remain operational even after a significant seismic event. This requires a design approach that goes beyond simply preventing collapse. IL4 structures need to be engineered to withstand strong ground motions while maintaining their functionality. This has significant implications not just for the structural framework of the building but also for its internal systems. Lifelines like power, water, and communication networks must be robust enough to remain intact, ensuring that these critical facilities can continue to serve the community when they are needed most.

“Structures that are designated IL4 need to be designed to withstand a seismic event and remain usable. This does not just apply to the structure but also the services in the building, lighting, electrical, communications, water and sewer.” – Gregg Klopp

Designing for this level of resilience requires careful consideration of both structural and non-structural elements. While robust structural systems are essential to resist seismic forces, the building’s architectural components and services (like lighting, ventilation, and medical equipment in a hospital) also need to be designed to accommodate the potential movements and stresses of an earthquake. This often necessitates specialised design strategies, such as flexible connections for services and bracing for ceiling systems, to ensure that everything within the building can withstand the demands of a seismic event.

“These structures basically need to remain elastic, and the structure and the non-structural elements need to be able to accommodate the large displacements the structure is subjected to during a seismic event.” – Gregg Klopp

Seismic Retrofitting: Strengthening Existing Buildings

Retrofitting existing structures presents a unique set of complexities. One of the most significant challenges is determining the appropriate level of seismic resistance to target. For a new building, codes and standards provide clear guidelines based on a typical 50-year design life, but when assessing an older building, factors like its remaining lifespan, the feasibility of upgrades, and the overall cost-benefit need careful consideration.

The physical constraints of working within an existing structure pose further difficulties. Upgrading foundations, for example, can be incredibly complex. If the building’s footprint doesn’t allow for external reinforcement, engineers might need to devise intricate solutions to strengthen foundations from within, often working in confined spaces and around existing services. This requires ingenuity, careful planning, and close collaboration with other disciplines involved in the retrofitting process.

“The first challenge when upgrading an existing structure for seismic events is what seismic event to design for. The second challenge is the practical problem of being able to physically retrofit an existing structure. Upgrading footing systems are particularly difficult. It is necessary to be practical as to what is reasonable.” – Gregg Klopp

The complexities of seismic design and retrofitting are considerable, but the benefits are invaluable. By understanding the risks and implementing appropriate measures, we can significantly reduce the devastating impact of earthquakes on our communities. Whether it’s designing new buildings to withstand seismic forces or retrofitting existing structures to improve their resilience, the goal remains the same: to save lives, minimise property damage, and ensure the protection of essential services. As the Newcastle earthquake demonstrated, seismic resilience should not be a luxury, but a necessity, regardless of geographic location.