AS/NZS 1170.2:2021 Clarification

[aussiestruc]

Hi All,

Given there is yet to be a commentary released for the above code, seeking clarification for Clause 6.1 a) "whenever significant coupling is evident in the first three modes of vibration":
1. Assuming "significant coupling" pertains to some sort of torsional mass participation and/or torsionally-involved mode shape?
2. What would be the extent for "significant coupling" to be evident in a mode according to the code? Say 5% MPF? Modes with torsion having the highest MPF (vs the two sway cases)? i.e. what would be the MPF cut-off to start calling it "significant"? Or is there some other metric to better go by?
3. Does the phrase itself pertain to ALL 3x modes having coupling, or just in ANY of them (seems very open to interpretation)?

Many thanks in advance!!

 

[Agent666]

Believe coupling simply means a response in one direction for example also generates a response (over some threshold) in another orthogonal direction. Basically the dynamic parts of the standard assume a random and independent response for x/y/z responses (this is noted somewhere in one of the other clauses).

'Significant' is open to interpretation I guess, agree it doesn't seem to be well defined.

My reading is if coupling exists in any of the responses then section 6 cannot be applied, you're into more specialist advice or modelling at that point. It's not all, less likely for example to have vertical and horizontal coupling, so unlikely to mean in all three directions at once with coupling.

https://engineervsheep.com

 

[aussiestruc]

Thanks, Agent666,

Hmm, certainly no chance in this instance (wind-governed 20-storey RC building with 300mm thick lift/stair cores and use of some perimeter shear walls) of getting a clear split between sway x / sway y and torsion modes across the first three fundamental modes without any other sort of mass participation involvement from any of the others (probably just the inherent geometry and stiffness of the building).

Understood wind tunnel testing would be advised in these sorts of instances; unfortunately (and understandably) the client doesn't want to go down this route. If you were to compare the costs of the testing vs the extra stiffness required through thicker/larger number of walls to achieve the "pure" split then obviously testing would end up much cheaper.

I guess I would just find it hard to find a building with the "pure" MPF split between the firs three modes without any other direction having at least a small MPF involvement, which if this is indeed the code's intent then you'd be wind tunnel testing essentially every tall-ish wind-governed building?

I'd be curious what experiences all have had in terms of typical heights where you'd start wind tunnel testing vs. being able to rationalise the above issues?

As an aside example, say the first three x/y/z split is as follows: 6/38/12, 38/0/4, 2/19/11 - clear, but not necessarily (?) significant involvement from other x/y/z responses?

 

[rowingengineer]

Note there is a commentary as such to 2021 by the awes, doubt there is going to a better one

https://www.awes.org/product/wind-l...zs-1170-2-wind-actions-wind-loading-handbook/

 

[Agent666]

Understood wind tunnel testing would be advised in these sorts of instances; unfortunately (and understandably) the client doesn't want to go down this route. If you were to compare the costs of the testing vs the extra stiffness required through thicker/larger number of walls to achieve the "pure" split then obviously testing would end up much cheaper.

I guess just demonstrate this cost aspect and let them decide that they made poor life choices trying to cheap out on appropriate design advice. Seeking wind engineering advice doesn't always end up in needing wind testing, they have lots of means of coming up with more refined answers or recommendations. Only way you'll know is by seeking out that advice.

I'd maybe be asking the standards committee what 'significant' is supposed to mean in this context and also confirm that the coupling they are referring to is actually as I noted. Code writers always seem to make vague statements if there is either no research basis or some disagreement among the committee, and end up leaving it to the engineer to make an interpretation and judgement on the application based on that. As engineers, we want black and white, but some things end up in the grey and we have to make judgment-based decisions. Hard to do with no experience in this case if you're not a wind expert and other is no context for what is and is not significant ... I'm not even sure significant refers to mass participation, perhaps it is the force generated, or some other measure.

I mostly work in New Zealand so seismic really dominates and wind is really an afterthought for cladding/envelope design, almost never governing the lateral force-resisting system. My experiences are limited to about 10 storeys though.

But if you don't meet the requirements in the standard for it to be applicable that's when you need further expert advice, really has nothing to do with height sometimes. I've seen some pretty unusual building forms that have had wind tunnel testing undertaken which are only the equivalent of a few storeys because there is just no way to apply the code to them based on the idealised "rectangular building form" they are basing the code provisions on in most cases.

https://engineervsheep.com

 

[PersonalProfile]

The following may be of interest, despite does not directly address the enquiry:

There are several dynamic excitation mechanisms for structures all of which are dependent
on the external geometry and structural dynamic properties. Section 6.1 of the Standard
defines cases that are not covered, primarily due to the lack of research and/or sensitivity of
the geometry to dynamic excitation. These cases require more detailed consideration on the
fundamental loading mechanisms with additional analysis, and may require more advanced
modelling techniques.

Ex: The AWES handbook linked earlier.

Regards.