Earthquakes can be frighteningly destructive. On average globally, a major earthquake strikes somewhere each year and at least 10 000 people die in earthquakes annually, urban life is disrupted, and parts of cities are destroyed.
With more than half the world’s population now living in urban areas, building resilience and post-disaster functionality of urban infrastructure proves more relevant than ever.
However, prompted by an earthquake which reportedly measured 3.1 on the Richter scale in Durban, South Africa, in early February this year, it is worth asking why most of our developers rarely consider seismic effects in the design of South African or other African projects.
“In regions of moderate seismic hazard, we design most structures according to what is called ‘life safety’ performance criteria,” says Aurecon’s Johann van der Merwe, Structural Engineer in the company’s Tshwane office. “The aim of this approach is to accept localised damage, but to prevent disproportionate structural damage which could lead to building collapse and loss of life. Structures in these regions which have specific post-disaster functionality (such as hospitals, police stations, fire stations, etc.) are designed with the aim of limiting structural damage as much as possible to ensure that the structure remains operational after a seismic event. Many areas in Africa, and indeed South Africa, are classified as being regions of moderate seismic hazard. Aurecon has experience in the seismic design of building structures in many of these African countries, including Mozambique, Tanzania, Uganda, Rwanda, Kenya and Ghana.
Following the revision of the South African National Standard on seismic actions (SANS 10160-4:2011), South Africa is divided into regions of natural seismic hazard (including Cape Town) and regions of mining induced seismic hazard (including Johannesburg) with well-defined design procedures in terms of loading and conceptual design requirements for each region. Seismic design requirements for other African countries are based on local loading standards.
“The additional cost of designing a building structure for seismic induced actions can be limited significantly by engaging the structural engineer during the early stages of conceptual design to ensure well-defined load paths and optimum response to dynamic loads,” comments Van der Merwe.
Code specified seismic design criteria are based on economically viable probability of recurrence. However, seismic events that exceed design specifications are possible and do occur, leading to significant structural damage and possible collapse. One such example is the devastating earthquakes in Christchurch, New Zealand. Research and development of resilient structural systems in regions of significant seismic hazard are focussing on more affordable techniques to limit structural damage and ensure improved post-disaster functionality. It is important to note that not only the structural system, but also fixing details of non-structural elements such as non-load bearing masonry, shopfronts, air conditioning ducts and other building services should be considered.
The risk of disproportionate structural damage during a seismic event can be minimised by implementing sound earthquake engineering techniques, such as ductile design. Traditional ductile design allows the primary building structure to undergo a series of controlled failures that dissipate the seismic energy. This maintains building stability and, most importantly, protects life. The drawback is that buildings designed in this way, although robust and safe, are susceptible to irreparable damage during major earthquakes and often have to be demolished. In regions of significant seismic hazard where seismic events occur regularly this technique may prove undesirable from an economical perspective.
The choice of building form and lateral force resisting system could have a significant effect on the overall cost of the structure. In regions of significant seismic hazard, however, it might be desirable to opt for an earthquake resilient building capable of being occupied immediately after the design level earthquake.
To avoid saying: ‘If only I had protected my investment and occupants better’, it is important to involve experienced engineers, who are not only able to offer the latest earthquake resilient building design, but will also make an appropriate business case for a cost-effective investment.
“Typically, engineers react to an architectural design and proceed to design around it, whereas by working closer with architects during the early stages of conceptual design, a more economical seismic design is possible. We cannot avoid structural damage under all earthquake conditions, but considerable future-proofing by ensuring optimum building performance during design level seismic events is possible and should be taken into account,” concludes Van der Merwe.