Shaking in Darwin: an EqEng Perspective

On the 26th of June, AEES member Helen Goldsworthy from the University of Melbourne did an interview with ABC Radio about the effects of the June 24 earthquake in the Banda Sea on buildings in Darwin. The audio of that interview is available here:

Helen had some additional commentary to share with the AEES community, as follows:

I had wanted to talk further in the interview about the general need for ductility in structures if they are to withstand large ground motions without collapse, but we ran out of time. As I said in the interview, the question for engineers (and regulators) in Australia is how far they should go in providing (or requiring the provision of) ductility in the structures that they’re designing. Given the uncertainty associated with predictions of earthquakes in Australia, due mostly to lack of data, I think a prudent engineer will give serious consideration to detailing their structure to achieve some ductility. Many of the engineers I’ve spoken to in the AEES are aware of the need for this, especially those who’ve been educated across the ditch (or in advanced courses on Earthquake Engineering in Australia).

In 2017, I was part of a group that made some limited progress on improving our design standards for concrete structures in Australia, under the leadership of Gil Brock. This resulted in some changes to the concrete structures standard AS3600. I believe these changes have been a step in the right direction, but more work is still needed. I think we should aim to have a minimum level of detailing at least as good as that required for structures of nominal ductility in the equivalent New Zealand standard (NZS 3101). Unfortunately, we’re still not there yet. In the NZS 3101 standard, this level of detailing is said to correspond to a ductility factor of only 1.25, which is a low level of ductility. Please note that the Australian standards allow designers to use a ductility factor of 2 for an even poorer quality of detailing (clearly a less conservative approach). I am hopeful that further changes to AS3600 will be possible (in a political sense) when the standard is next revised. I also think that a similar review should be conducted of the other material standards in Australia.

Ideally it would be better to achieve an even higher level of ductility than 1.25 since that would make it more likely that the building would withstand a very rare event for Australia. A very rare event is often defined internationally as one with a 2% chance of exceedance in 50 years at the site in question. The intensity of the ground motions corresponding to an event with this probability of exceedance would, of course, be higher in Christchurch than in Australia (and higher again in Wellington). Even so, the New Zealanders now aim for any given building to remain operational under such an event. They previously aimed for collapse prevention for this level of event, but the more recent approach would ensure much less overall business disruption (we’re all aware of the widespread disruption that can be caused by an earthquake). The Australian building code does not give any guidance about what the performance objective should be for a very rare event, and hopefully in the future this will be rectified.

One other thing I would have liked to mention in the interview is that there is more education needed about what people should do if they find themselves caught in an earthquake. I found the information from SES Victoria useful, and another worthwhile link from the University of Washington tells people what not to do in the event of an earthquake.

I think many members of the public would not be aware of the potential danger of objects falling both inside the building (such as ceiling tiles, unsecured bookshelves etc) and outside of the building (eg. falling glass) if the ground motions were more substantial. Is there something that the AEES can do to alert people to these guidelines?