AS1170.4 - Amdt 2:2018 - Min Z values 2

[blihpandgeorge]

Hi All

The latest amdt 2 has introduced a new minimum hazard design factor of 0.08 (ie min value of z=0.08). This amdt has included new map updates (fig 3.2a to Fig 3.2G) that appear to be the same as previous revision with the contours less than and including z=0.08 now deleted. The lowest contour on the maps is now z=0.09. The table of cities has also been updated, and frustratingly some key cities have been removed (ie Brisbane / Sydney / Melbourne / Canberra) in this amendment.

My query is what Z value do you use for Brisbane? It was previously Z=0.05, and is some distance from the 0.09 contour in fig 3.2f

Same line of questioning would apply to Sydney as it sits evenly between two 0.09 contours and was listed in the previous version as z=0.08. Does this stay as 0.08 as it meets the minimum of the amendment or does it get lifted to 0.09 as there is no contours less than 0.09?

My thought is to use z=0.08 for both, and the following seems to support this:
- Master Builders Qld commentary on Earthquake design Link
- Table 1 of the following hazard assessment published by Geoscience Australia (record 2018/33 eCat 123028) Link

But a strict reading of the standard seems to read as z=0.09

What do others think?

Cheers

 

[Retrograde]

We use 0.08 for Brisbane. I share your frustration that key capital cities are not in the table - who's stupid idea was that? Also, I fail to see why they raised it from 0.05.

 

[Settingsun]

I've taken the missing capital cities as 0.08 as that tallies with the old version iirc, ie they were 0.08 or less. I agree it's strange to leave them out of the table. I mean, why would anyone want to cover two-thirds of the population with just five entries???

Pure speculation: the higher minimum is part of the general trend towards robustness and maybe to force retrofit of more older buildings as they are renovated, as it will be harder to justify leaving them alone if the requirements have significantly increased.

 

[Agent666]

In NZ we have a similar minimum (but set at 0.13), it means in some regions the design loads are higher than what a site specific study would indicate.

But the minimum is adopted to increase the robustness and allow for things that aren't necessarily known in areas of low seismicity. In a way this imparts robustness as steveh49 noted, as we are ultimately designing for a higher load than our current knowledge might indicate is required. Under any return period earthquake there should be achieved an acceptable margin against collapse, not just under the design basis earthquake, the minimum prescribed values are intended to achieve this aspect.

For example in Auckland z=0.13, but site specific studies indicate as low as z=0.08 is the real value. It must also be remembered that these design basis events are all determined via probabilistic methods, and that the design earthquake is just that, an arbitrary line in the sand for design at which the risks are deemed acceptable.

Real earthquakes events can of course be larger or smaller than the design basis earthquake, certainly recent NZ seismic events have been greater than 2 times the design earthquake in certain regions of the spectrum in both Christchurch (2011) and Kaikoura (2016) events.

It seems Australia is up in arms about having to finally design and detail for higher loads and seismic events, perhaps you need a decent earthquake to show you the value of doing this in terms of maintaining life safety.

You only have to look to NZ for recent examples of where our historically far more strict seismic provisions and sometimes blase attitude towards detailing and design practices have ultimately failed us, both in terms of loss of life and the economic reality of rebuilding after such an event.

 

[Agent666]

If you have access to the NZS1170.5 commentary it notes a lot of the reasoning behind the minimum level adopted. As I understand it you don't have a commentary to AS1170.4 to explain the reasoning.

 

[rscassar]

Brisbane & Sydney 0.08

 

[blihpandgeorge]

Thanks all for comments