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Rankine interview: Beyond the conventional

gazetas george

George Gazetas will deliver the Rankine Lecture in March where he hopes to excite the profession to be a little more daring when designing foundations to resist earthquakes.

“I jumped up and I’m not sure I have come down yet,” laughs George Gazetas on recalling his reaction when told by the British Geotechnical Association that he was to deliver the 59th Rankine Lecture on 20 March at Imperial College London.

Gazetas, who has been professor of geotechnical engineering at the National Technical University of Athens for 30 years, will deliver a lecture entitled Benefits of Unconventional Seismic Foundation Design.

“I have been working on soil dynamics and foundation dynamics for many years, but in the last 20 years it has become apparent that some radical ideas can be extremely helpful,” explains Gazetas. “This is especially useful to avoid oversized foundations, the role of which may be negative for the structure they support.”

It is not prudent to unnecessarily increase the size of foundations to resist earthquake, as this may not only be uneconomical and difficult to implement

Gazetas believes that current seismic geotechnical practice has historically embraced concepts influenced by pseudo-static thinking and force-based methodologies ― and this needs to change.

“Ensuring safety is the prime objective for us geotechnical engineers and, following the static traditional way of thinking, we want to make sure that our systems don’t approach failure,” says Gazetas. “In static design for permanent loads we want adequate margins of safety. But in earthquake dynamics things are different.”

“Engineers have been used to treating seismic loading pseudo-statically as if the forces on the foundation are both, predetermined and permanently acting,” he says. “But in an earthquake, they are neither.

“Due to the interaction with the soil, the loads depend on the foundation itself, while the cyclic nature of the motion makes the loads oscillatory.

“The exceedance of a threshold lasts only momentarily to cause damage and, equally important, if one component reaches a threshold and fails, then it cannot transmit the larger forces on top of it. You might say, the system is self-healing.”

“It is not prudent to unnecessarily increase the size of foundations to resist earthquake, as this may not only be uneconomical and difficult to implement,” Gazetas will argue in his lecture. “Unexpectedly it may also lead to poor technical performance of the engineered system.”

Gazetas refers to Nathan Newmark’s seminal Rankine Lecture on the effects of earthquakes on dams and embankments delivered in 1965.

We have numerous joint projects all over Europe, and the contribution of the UK’s academics and engineers is extremely important

“Newmark showed that we could design slopes for much larger accelerations than what we can compute they can withstand pseudo-statically,” says Gazetas. “We have applied a similar idea to foundations and proved that we do not need to worry about pseudo-statically imposed limitations. We can exceed them.

“Instead of constructing relatively-huge foundations for earthquakes, we can make them smaller and let them move. This movement, whether sliding or rocking of the foundations, can be quite beneficial. You reduce the dimensions and safety isn’t compromised. You allow it to exceed the safe limits and things become much better.”

But Gazetas believes there is a penalty to pay.

“Sometimes these deformations are too much for the structure,” he says. “Geotechnical engineering solutions and unavoidable compromises are needed to secure the benefits of an unconventional rocking foundation without the damage of large rotations or displacements.”

Using actual case histories of bridge and building performance in earthquakes, Gazetas will prove the merits of his theory and illustrate the mechanics behind the success of the unconventional design.

“I won’t say what the case examples are now, let’s make it a surprise on the night,” laughs Gazetas.

“A lot of people have already embraced the unconventional idea. Although the seismic codes are still conservative, I want to change the static way of thinking for seismic problems.”

And it is the power of the Rankine Lecture that Gazetas wants to harness.

“A year or two ago, people would ask me why I hadn’t been chosen to deliver Rankine yet,” he says. “The reply was easy: with so many other great thinkers and pioneers in our profession who were or deserved to be Rankine lecturers before me, I really felt humble and I thought I was too young and too little for that great honour!”

Gazetas recalls when he was asked to deliver the lecture.

“I did really jump up in the air and I haven’t come down yet. It seems as if I have defied gravity for more than a year. But I hope that even if the UK leaves the EU, Newton’s law of gravity will still be allowed to apply in Europe, in order to pull me down,” laughs Gazetas.

With Rankine, you need to excite people, mark a change in the paradigm and not just document what someone has done

On a more serious note, Gazetas is concerned about the UK’s imminent departure from the European Union. “We have numerous joint projects all over Europe, and the contribution of the UK’s academics and engineers is extremely important,” he explains. “In particular for earthquakes, despite the fact that this is not the hottest subject in Britain. For instance, the contribution from the UK to the Seismic eurocode is very significant.”

With only a few weeks of preparation left before the lecture, does Gazetas feel he is ready to step in to the shoes of the great and the good?

“With Rankine, you need to excite people, mark a change in the paradigm and not just document what someone has done,” says Gazetas.

“We don’t want another ‘here is my work, I did this, the analysis is this, here is the formula, and here are my findings’. That does not satisfy me.

“I want to make my lecture exciting and change the way of thinking, to reduce the fear of daring to exceed the well-established conventional limits. The profession should not be afraid.

“If I can excite people, then I think I will have been successful,” adds Gazetas.

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