SOUE News Issue 2

Northridge and Kobe - Earthquakes That Are Revolutionising Engineering Practice

Tony Blakeborough

Every year, something like five thousand to ten thousand people die in earthquakes worldwide. The worst earthquake in recent times, in 1976 at Tangshan in China, killed over 500,000. By this measure, the moderately large 1976 earthquake at Northridge in Los Angeles (magnitude 6.9), which killed 57 people, and the magnitude 7.1 earthquake that hit Kobe, the principal port of Japan, the following year were relatively insignificant. Although five thousand died at Kobe, most of the victims were killed in fires that took hold in the lightly framed wooden houses in residential districts; the number of deaths in engineered structures was in the tens. Nevertheless, these two earthquakes have changed the direction of earthquake engineering research throughout the world.

The reason that these two earthquakes were notable lies not in the number of dead, but in their economic cost. Each event was a direct hit by a moderate earthquake on a built-up area. In Northridge, around 15,000 buildings had to be demolished, resulting in a total loss of around $15bn; in Kobe, the repair costs alone were estimated to lie in the range of $90bn to $147bn and 180,000 buildings were destroyed or seriously damaged. Each earthquake set a record loss for natural disasters.

Both the US and Japan had well developed seismic codes directed primarily at saving human life, and in this they were successful. But in practice, both codes were too limited and missed a wider purpose - that of saving resources. Withdrawing $15bn from an economy kills more than 57 people in terms of reduced provision for healthcare or road safety improvements.

Following these earthquakes, it was immediately apparent to earthquake engineers that the guiding principles behind the codes had to change. Whereas previous measures had been designed primarily to guard buildings against collapse, the new and much more arduous aim is to keep the main load-bearing structure undamaged, thus removing expensive demolition and rebuilding costs. In consequence, it was no longer acceptable to use the primary structure to absorb energy (and thus become damaged); new ways of damping the structure had to be found. In addition, old structures - now recognised as a risk - had to be made more robust.

Two ways of meeting this rigorous challenge are currently being researched and adopted. In the first, base isolation, the main structure is positioned on rubber springs so that it remains static as the ground moves. This technique had been in occasional use for some years before the earthquakes but the events at Northridge and Kobe have given the idea new impetus, although it can only be incorporated economically for new buildings. A more radical development is the addition of supplementary damping systems to both old and new structures. These can take two forms: elements that deform plastically in response to the earthquake; and shock-absorbing viscous dampers which damp the building to reduce overall response. Both kinds are attached to bracing elements within the structure.

The potential use of viscous devices in this application has been highlighted by the availability of a design of viscous damping devices originally intended to isolate and protect the propulsion motors in nuclear submarines, now brought into the public domain by the ending of the Cold War. The installation of these dampers into buildings has demonstrated the concept's viability, but their expensiveness has prompted a search for similar, cheaper ways of doing the same thing. A number of new avenues of research have now opened up.

At Oxford, Dr Martin Williams and I have developed a test method that allows us to simulate and explore in a uniquely realistic manner how damping devices in a building respond to an earthquake. The "real-time hybrid testing" method simulates the total performance of a structure by splitting it into a physical part (the damping device) and a computational part (the remainder of the structure). These two models interact at fixed points (the connecting nodes) in the structure. Recordings of real or invented earthquakes are fed into the computational model and the displacements of the connecting nodes are calculated. Hydraulic actuators then apply these displacements to the physical model and the resulting forces are measured. These forces are in turn applied with the next part of the earthquake to the computational model and the new displacements calculated. This loop is repeated until the earthquake is over.

The significant advantage of our system is that we can perform the calculations fast enough for the system to run in real time, reproducing the velocities as well as the displacements of an earthquake response. This is important because the performance of these damping devices is rate dependent. For a truly accurate result, loading must take place at the velocities the devices would actually experience and our method successfully achieves this.

We have also investigated knee-braced frames - a plastic dissipative system that absorbs energy by yielding the web of a steel beam. These are made from standard beam sections and are supplied with a small number of fixed strengths. Our research has shown how drilling holes in the web can allow the designer to vary the strength of the beam to provide a wider range of design options without impairing fatigue endurance to levels unacceptable in an earthquake.

We are now embarking on a programme to test a new plastic energy absorber designed by Professor Uwe Dorka of Kassel University in Germany. The simple design of this device would make it cheap to produce, but real time testing has not previously been possible and it is important to ascertain whether the heat generated by the dissipating energy significantly affects the performance of the device. We shall also be testing some Jarret devices, recently developed viscous devices whose properties depend in a very complex way on the loading rate and the extension of the device.

Research of this nature will both enable us to check the performance of the kinds of damping systems now being installed in buildings and also to develop numerical models that can be confidently used in purely computational models for the design of earthquake-resistant buildings. Producing economic dissipative designs for buildings in earthquake regions will, it is to be hoped, ensure that the revised more challenging goals of seismic design are achieved.

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