AS3600 - Seismic design loads for foundations (...continue) 2

[li0ngalahad]

Since the previous thread is now closed, I am opening a new one. There was a AS1170.4 seminar organised by CIA last week, held by Dr. John Wilson and Dr Scott Menegon. Very informative seminar in my opinion.
Towards the end of Menegon's presentation there was specific mention on the design of Foundation when the superstructure has a ductility mu>1.

Here's the screenshot of the presentation (hoping not to incur in copyright infringement) :

I think this clears out any confusion on what to do with foundations etc (refer to the discussion we had in the previous thread)

Scott also mentioned that requirements for foundations will be likely included in the new AS1170.4 and/or next amendment of AS3600, which is great news.

 

[Just Some Nerd]

Will definitely be organising a session at my workplace for that seminar...this scenario has even come up in a peer review recently.

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Why yes, I do in fact have no idea what I'm talking about

 

[rapt]

It will be covered in the next release of AS3600 also.

 

[structuree]

Is there a way to access a recording of this presentation or copy of the slides?

Rapt, any word on when the next release will be published?

 

[Just Some Nerd]

https://concreteinstitute.com.au/professional-development/econcrete-online-learning/

The title you're looking for is "Design for Earthquake Wilson Menegon 220224"

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Why yes, I do in fact have no idea what I'm talking about

 

[rapt]

There should be a Draft for Comment later this year. No dates set that I know of so do not start complaining if it is later.

Menegon is on the Seismic sub-committee but there is no guarantee that anything published now will actually be the same as the new code.

 

[steelnz2003]

Thanks for the slide shared
But what is the reference of the Mu flooring>1.2 Mu wall
Most engineering practices commonly employ a value of μ (mu) equal to 1.0. Surprisingly, when I searched for explicit references to this practice in the code, I found none. Instead, the language used is often intricate and vague, rather than straightforwardly stating “use μ of 1

 

[Retrograde]

But what is the reference of the Mu flooring>1.2 Mu wall

I think the value of 1.2 is chosen to align with clause 14.5.6 of AS3600.

 

[Just Some Nerd]

But what is the reference of the Mu flooring>1.2 Mu wall
Most engineering practices commonly employ a value of μ (mu) equal to 1.0. Surprisingly, when I searched for explicit references to this practice in the code, I found none. Instead, the language used is often intricate and vague, rather than straightforwardly stating “use μ of 1

If the footing has a significantly greater unfactored capacity than the wall, then even if the footing is not designed for mu = 1 there is basically no likelihood the footing will fail before the wall and a hinge almost certainly develops at in the wall as intended rather than elsewhere. For this case and Clause 14.5.6, 20% overstrength I think is just the value that Menegon (and the other code writers) consider to be safe enough to ensure this. Designing for Mu = 1 in a footing I think is usually done in practice because it is drastically easier from a workflow standpoint to not be calculating individual unfactored wall capacities and comparing to an unfactored footing strength, and personally I would have said designing to mu = 1 is correct before catching this presentation for the sake of being safe and not assuming reduced loads in a member that isn't detailed for ductility.

 

[li0ngalahad]

personally I would have said designing to mu = 1 is correct before catching this presentation for the sake of being safe and not assuming reduced loads in a member that isn't detailed for ductility.

Most engineers I know will design the foundation in bending and for stability using the same design moment ss the shear wall being supported. It's actually often one of the main reasons many adopt highr ductility on a building, to reduce the foundation size. It goes agains seismic engineering and ductile structure design principles in my opinion however the code being currently completely silent on this has favoured this approach. Glad thsi will likely change in the near future and force engineers to think about failure hierachy etc when designing the foundations.
At least clause 14.6.6 mentions foundations, so at least the shear capacity of foundation will be detailed to resist elastic loads or at least capacity design based on the wall moments, better than nothing I guess.