Structural Wall" definition AS3600-2018 3

[QSIIN]

Hi all,

Just want to get some opinions on the changes to "structural walls" in new 2018 AS3600.

Section 14.2.10 defines a structural Wall as:
"Wall (either load bearing or non-loadbearing) connected to floor diaphragms that attracts horizontal earthquake and wind design actions".

At what point is a wall considered to "attract" EQ and wind actions? If we're talking about "gravity only" walls in a structure that has a stiff core and other lateral resisting shear walls, these elements might attract just a fraction of lateral actions - but they still attract some as they are connected to the diaphragm, but can you argue they aren't part of the lateral system and therefore do not need to comply with section 14, save for drift consideration? Or does pretty much any vertical compression element that is not explicitly a "column" now count as a "structural Wall" and must comply with 14.4.4.3?

Subject to the above, if you have "gravity only" walls, to section 11 in lieu of columns, but a ductile core, is the whole structure ductile or non-ductile? Even if the walls aren't part of the primary lateral system?

Section 11.5.2(b) limitations for the simplified wall design:
"Not to be constructed on sites with soil classification of De or Ee, AND where subjected to earthquake design actions"

Are those two limits mutually exclusive? Can you have walls that are constructed on De soil but not subject to EQ? Is that even possible? Can you have walls on Ce soil classification that are subjected EQ actions?



I think both these points are too vague and not explicit enough in their intent, and will be exploited to get away with using simplified wall deisgn in excess and to continue ignoring seismic design.

I understand the code is not meant to be a detailed how-to guide and their must be engineering judgement, but from my experience unless the code says "can do A, cannot do B", option b will be a viable solution for some..

 

[Gishin1]

Don't forget however that in Australia, many high-rises are making use of "blade columns" designed to Section 11, a Section which might as well not even have slenderness limits in it as they are significantly more relaxed than Section 10 (no moment magnifier for walls..)

This means many buildings have 1000x200 or similar "walls" that are slender and have no confinement, but are considered too small to "attract" significant lateral loads, and are therefore "gravity only".

This is how engineers will continue to get away with these designs.

The only thing in the new code that limits this is the requirement for confinement in walls with 65MPa or more. But as this references Section 14 EQ section, I've already seen designs that say the confinement isn't necessary as it doesn't attract lateral loads..

Small "columns" that don't attract lateral loads...4-1 ratio, therefore wall, therefore Section 11 capacity...no lateral load therefore no confinement...easy peasy

So nothing has changed...

 

[Retrograde]

are considered too small to "attract" significant lateral loads, and are therefore "gravity only".

But the 2018 code now states that all walls connected to floor diaphragms attract horizontal loads, so if you consider them gravity only you violate the code.

Lots has changed...

 

[Retrograde]

Section which might as well not even have slenderness limits in it as they are significantly more relaxed than Section 10 (no moment magnifier for walls..)

Just picking up on something else you said, the slenderness limits under section 11 are actually more stringent than section 10 (He/tw < 30 versus Le/r < 120). Also, the accidental eccentricity term in 11.5.3 provides a moment magnifier.

 

[Gishin1]

Retrograde,

The code doesn't explicitly state that. That's what this whole thread was initially about.

I think the 14.2.10 definition of a structural Wall is vague.

Is it:
Wall (either load bearing or non-loadbearing) (connected to floor diaphragms) that attracts horizontal earthquake and wind design actions.

Or:
Wall (either load bearing or non-loadbearing) connected to floor diaphragms that attracts horizontal earthquake and wind design actions


Is it the floor diaphragm or the wall that is attracting the horizontal actions? By defining that an element can attract horizontal actions, it must mean that the opposite can also happen. Otherwise why not just define it as "all walls (and columns) connected to diaphragms".
I'm just stating interpretations I've already seen to get around the new EQ restrictions.



Regarding slenderness, Section 10 only requires a minimum moment if L/r<25. Beyond this a moment magnifier is required, which can be in the order of 4-5x the minimum moment, sometimes more.

Section 11 is just H/t<30. Which most typical walls are, and the eccentricities are just minimum values, far less than potential magnifiers from Section 10. Since radius of gyration, r, is 0.3D, more "columns" need magnification beyond min moments than "walls" would.

This means lightly loaded columns of fail due to slenderness, when their equivalent Section 11 capacities pass with flying colours. So either the moment magnifier is far too conservative, or the wall code is unconservative, or both.

 

[Settingsun]

Jishin1, 'wall' is singular so 'attracts' refers to that. 'Diaphragms' is plural so 'attract' would be used if that were the intent. Probably a fortunate escape from modern under-use of commas but clear nonetheless to a reader who isn't trying to 'get around' the code.

Why should code writers bother trying to make the code ironclad against such designers? It's a fine line between endangering safety because it's profitable by relying on ambiguity and endangering safety simply because it's profitable.

 

[Gishin1]

steveh49,

That's exactly my interpretation of the Grammer of the definition, lack of commas and all. So if that is the correct grammatical interpretation, then the code isn't saying that all walls connected to the diaphragm attract horizontal loads, as is retrograde's interpretation.

Which, again, allows engineers to bypass some of the strict section 14 detailing for "blade columns" that don't "attract" horizontal actions.

I'm not trying to find a loop hole for myself, I'm just pointing out the loop holes practicing engineers have already found and are already exploiting.

 

[Settingsun]

Unless I'm missing something, you'd have to provide some specific detailing to isolate the wall from attracting loading. Unless thee main load-resisting elements are infinitely stiff (not possible) or the wall has zero stiffness (not possible), the relative movement of upper and lower floors will cause the wall to 'attract' some load. AKA no such thing as gravity-only as Agent666 said earlier.

Again, the code can't stop designers ignoring it.

 

[Retrograde]

The code doesn't explicitly state that.

I would agree with you that the wording in places leaves a lot to be desired.

 

[QSIIN]

Thanks for all the responses

I guess there's still no real conclusion though, as any element connected to a diaphragm, no matter how small, will attract horizontal loads, even if it's just a fraction.

Strange that the new code doesn't have similar restrictions for the column section though, as by definition they will attract horizontal actions too by being connected to the diaphragm, unless the lateral system is IMRF.

If this is the case, Section 11 can ONLY be used for blade columns, shear walls, and even interconnected core walls if a mu of 1 is adopted.

And if a mu of 2 is used, ALL walls must be proportioned to be at 0.2f'c, even if designed as a column using Section 10??

 

[Retrograde]

Strange that the new code doesn't have similar restrictions for the column section though

My thoughts on this are that columns are inherently more ductile than walls given the closed ties and cross ties required by 10.7.4. And therefore the restrictions applied to walls are not required.

 

[rapt]

QSIN

That is the conclusion.

Any wall connected to a diaphragm will attract horizontal loads and must be designed for the loads attracted.

So must columns according to the Drift requirements in 14.4.2. But there is no simplified column design method to rule out so the previous discussions do not apply. I assume columns are not as badly affected by the high pre-compression effect so do not require the .2f'c limit.

The simplified method can only be used for a limited range of walls, based on loading and stress state.

There are other things to consider that some will not realize, or will ignore. e.g. In the analysis, you cannot just assume that all columns/walls are fully cracked and use a single % of I in the analysis for all columns and walls. You have to check the level of cracking in each element. If a wall is fully in compression, Ig is used. If in tension I reduces depending on the stress state. There is not a single % of I for all.

As I keep saying, AS3600 is not a text book on the analysis and design for seismic effects. No-one should be designing to AS3600-2018 seismic rules without first reading some good text books on the analysis and design logic. The whole logic is foreign to anything they have had to deal with before and you should understand the logic before even reading the code. Maybe then designers will understand what the code is trying to achieve and stop trying to find illogical ways around it. It is nothing like designing for wind and cannot be approached like that.

Agent666 might be able to point people in the right direction on good text books.

 

[Drapes]

rapt,

just to clarify, you mentioned earlier that the 0.2fc' axial load limit check does apply to columns - "The logic is meant to get you to increase the column dimensions until the clause is satisfied."

But now you appear to be saying the opposite - "I assume columns are not as badly affected by the high pre-compression effect so do not require the 0.2f'c limit."

 

[rapt]

Drapes,

Sorry, I meant "walls" back on the 20th. I will edit that post.

 

[rapt]

Except I cannot edit back that far!

 

[Drapes]

Thanks rapt, all your subsequent comments make a lot more sense now.

So if the axial load limit does not apply to columns designed strictly to section 10, what about for walls designed as columns but with ligs avoided as per cl 11.7.4? Would the axial load limit apply in this case given no ligs will be present? I imagine it would apply in this instance.

Also, with regards to clause 14.4.2 - Inter-storey drift where it mentions "all vertical load-bearing elements shall be designed for the calculated horizontal drift..." (which includes columns), how do you design something for the "horizontal drift"? Sorry I might be reading into it, but is it simply referring to the horizontal forces that the columns will attract under equake loading based on the ductility assumptions? Are there any other considerations or checks (apart from say P-delta effects) that would need to be undertaken when designing for the "horizontal drift" or is it as simple as designing for the horizontal forces that the cols attract?

 

[Gishin1]

If that's the conclusion, then why does the code insist on defining something so vaguely? Why not just say "ALL walls and columns MUST comply with the following" ? Implying that some walls attract horizontal actions implies that some don't.

After 10 years, these vague definitions are the best they could come up with?

The code isn't a how-to guide, sure, but the code is pretty explicit, and very near a how-to guide when it comes to slabs for example. Just look at the detailing rules. You can layout Reo without even knowing what you're doing by following the diagram.

Look at the slab design section, or the slab deflection equations. Has anyone even used them since the computer was invented? Pages and pages of simplified slab calculations and tables, explicit minimum requirements for slabs and beams, but for earthquake, which they are clearly trying to emphasise more and bring more attention to, is still so vague.

Maybe the code doesn't consider earthquakes, or the consequence of error, as important as slab detailing. Afterall, I don't remember the last time an ultimate earthquake event occurred in Australia, so it's just a gamble.

Or maybe the code committee just doesn't want to upset the industry too much, lest the contractors get on their backs about it, so they keep it as vague as possible. These new requirements drastically change the way buildings are being built right now. God forbid the day Australia gets an actual earthquake, the fires will just be a blip by comparison.

 

[rapt]

Drapes,

The design method used to calculate the wall capacity (chapter 10 or 11) does not affect it. Any wall that is partly in tension has to be designed using column design logic or strut/tie if it is short.

ACI apparently suggests a 3:1 ratio to be a wall. Some parts of AS3600 suggest 4:1 (Fire). Personally I do not like definitions like that as there is not a sudden step in the logic, a 799*400 column is a column while a 800*200 column is a wall!

In a multi-storey building, if you had some very stiff walls, then 4:1 blade columns would probably act more like columns than walls in this regard as the very stiff wall would be in single curvature and brace the relatively very un-stiff walls which would be in double curvature.

If you only had all 4:1 blade columns and no stiffer walls, then they would more likely act like walls in this regard.

I will see if we can get more discussion into the commentary.

Jishin1
Maybe you are too young to have experienced it. Newcastle in 1989, 13 dead, 160 injured and I felt it very strongly in Sydney. Sufficiently strong for people to get out of the building very quickly!

I will not dignify the remainder of your comments with replies!

 

[Gishin1]

And it's only taken 31 years to get an earthquake Section that is obviously still too vague for readers to draw clear conclusions.

And so engineers will continue to ignore earthquake.

 

[rapt]

There has been an earthquake section in AS3600 for the last 31 years. It was not very good as it was created from the ACI logic in the early 1980's and never upgraded as earthquake logic evolved.

The problem before about 1993 was that there were limited areas where earthquake had to be considered, mainly some areas around Adelaide and south of the Harbour in Sydney, controlled by AS1170.4, not AS3600.

Seeing Newcastle is north of the harbour, this was changed after the earthquake with a new version of AS1170.4 in 1993 (ps it had nothing to do with AS3600!).

But unfortunately most Australian designers assumed Wind Loads were worse so they did not have to worry about earthquake anyway.

Other than that hopefully most engineers in Australia will attempt to learn more about earthquake design and once they understand the logic apply the "vague" rules intelligently and in the intent in which they were written.

 

[Agent666]

It seems like (based on the replies of many in this post and other earlier posts) that in Australia there's a persistent base of engineers simply looking for a way to bend the rules/interpret things in a way that is more like what the've been doing incorrectly for the last 31 odd years... rather than simply getting on with it and giving the design and detailing for earthquake forces the credence it deserves.

Honestly it's the best chance you'll get to kill as many people as possible in your career.... go for gold.