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AS3600 2017 Draft

AS3600 2017 Draft

RE: AS3600 2017 Draft

Thanks Trenno. I see they've switched the shear design to the modified compression field theory (MCFT) as in the Canadian code (A23.3). Being Canadian I know the MCFT is generally good but has a few murky corners (e.g. crack spacing parameter). I'll be interested to dig in and see Australia's interpretation. Do you know of any other major changes?

RE: AS3600 2017 Draft

(OP)
There seems to be many changes from the 2009 version.

It would seem our designs just got 5% better, given most of the reduction factors have increased by 5% (for example shear phi = 0.75 instead of 0.7).

RE: AS3600 2017 Draft

Trenno,

No, the materials got 5% more reliable than they were 40 years ago apparently!

RE: AS3600 2017 Draft

cooperDBM - the latest Australian Bridge Code (AS 5100) already has MCF based shear design rules (but if you want to take a look, the draft AS 3600 is available at no cost during the comments phase).

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: AS3600 2017 Draft

The section capacity reduction factor in the current AS 3600 for shear is 0.7 and it has only gone up slightly to 0.75 in the draft. This value is 0.7 in AS 5100.5-2017. The equation for shear in the current AS 3600 is empirical (from test data and fitting an equation to get a lower bound) which explains the use of a low reduction factor. The shear formulation uses the Compression Field Method which is expected to be more accurate and the factor should be larger than 0.7. Unless someone carries out a reliability study to determine a more accurate value to use, using 0.75 in the interim is conservative.

RE: AS3600 2017 Draft

It seems like the code no longer allows the option to use a more conservative value of the strut angle Theta V going up to 60 degrees? Does anyone know why this was removed?

RE: AS3600 2017 Draft

nonplussed,

Where does it say that?

RE: AS3600 2017 Draft

Clause 8.2.10 in AS3600-2009. The theta angle in the old method is just an assumption, usually 45 degrees. In the new draft the MFCT calculates the specific theta angle that satisfies equilibrium given the actual state of stress on the section. It's usually less than 45 degrees.

RE: AS3600 2017 Draft

CooperDBM,

I meant in the Draft! 8.2.4.2 will allow theta up to 50 degrees based on a strain of .003. And it does allow the designer to use a value > than the calculated equilibrium value but no greater than 50 degrees. As you suggest, the model is based on the Canadian MCFT and the theta chosen is more logical than the old method. RAPT has been using the minimum value depending on level of shear compared to Min and Max for many years, so normally significantly less than 45 degrees and often as low as 30 degrees.

RE: AS3600 2017 Draft

In 8.2.4.2 I don't see how the designer can control theta other than controlling the level of reinforcing and prestress (e.g. the long. strain). Is that what you mean?

RE: AS3600 2017 Draft

cooperDBM,

The last paragraph of 8.2.4.3

"kv and thetav may be determined from Clause 8.2.4.2 using a value of εx that is greater than that
calculated from the equation in this clause. The mid-depth strain parameter εx shall not be
greater than 3.0 * 10-3."

RE: AS3600 2017 Draft

Rapt, yes I see it now. They really should have added that clause with the Theta V equation instead of inside a separate clause. Or better yet, just simply stated an optional maximum of 50 degrees. Even now I occasionally use 60 degrees when I have longitudinal steel development issues.

RE: AS3600 2017 Draft

That is then section on the calculation of ex, so it is logical to put the upper limit on it there.

It is not technically a limit on theta, it is a limit on ex, which is used to calculate theta and then kv

RE: AS3600 2017 Draft

Bit late to the table on this one. Public Comment period closed but...

Does anyone know the intent of <Table2.2.2> (j) "Bending, shear and axial force in singly reinforced walls forming part of a primary lateral load resisting system" resulting in theta=0.65.

I interpret the "primary" to mean walls subject to in-plane shear/structural wall action and NOT out-of-plane actions (i.e. face loading of slender walls)

Cheers
Toby

RE: AS3600 2017 Draft

Toby43

I think "primary lateral load resisting system" might give it away!

RE: AS3600 2017 Draft

rapt
True, pure semantics - but in my book out-of-plane resistance is of primary concern in resisting lateral loads.
I assume the reduction for singly reinforced walls relates to the lack of confinement in the "boundary elements".
What of seismic 100% in-plane loads coupled with 30% out-of-plane and vice-versa, which would seem reasonable in that out-of-plane action of walls must contribute to action perpendicular to them, which they may have demand placed upon them from buildings torsional response. Also "primary lateral load resisting" elements located at a building corner will receive in-plane demand and out-of-plane demand form wind loads.
So my take on this would be to reduce in-plane response mechanisms by theta=0.65 (when acting alone)
and to reduce out-of-plane response mechanisms by theta=0.8 (Class N reo) - out-of-plane buckling a consideration for effective struts from in-plane action coupled with direct out-of-plane actions.
Example
A typical rectangular tilt building with large opening in wall on "short" side.
The wall "pier" will be subject to in-plane demand from actions parallel to "short" direction of building and out-of-plane actions from "header" over in a tributary sense....
Got called away - may give it more thought and get back later

Cheers
Toby

RE: AS3600 2017 Draft

Back...

What I was getting at essentially was that are we to "blanket" all singly reinforced walls with theta=0.65, or only when the bending, shear and axial forces are considered as "in-plane" actions.
Back to tilt up buildings - it would seem unreasonable for a wall subject to out-of-plane bending, with N-class reinforcement that meets the minimum ratios and demands to be assigned theta =0.65. Essentially a wall designed utilizing the findings of the slender wall task committee and SEAOC bluebook recommendations should be sufficiently reliable with theta =0.8-0.9.
Maybe too many tilt panels have been designed ignoring minimum steel quantities, with L class reinforcement and neglect of P-Delta effects, that theta=0.65 is warranted. In my opinion such poorly designed walls would need more than theta=0.65 to make them sufficiently reliable.

Toby

RE: AS3600 2017 Draft

(OP)
I think the low theta for singly reinforced walls came from the observed performance of these walls in the Christchurch earthquake.


RE: AS3600 2017 Draft

Trenno - Granted, yet theta=0.65 for a out-of-plane would not seem reasonable.

If this blanket theta applies to out-of-plane actions then 2 identical walls in every way, except one has L class and the other N class, would be "rated" the same.

RE: AS3600 2017 Draft

Hi
Apparently the draft is no longer available. Does anyone have a pdf copy to upload on here?

Thank you

RE: AS3600 2017 Draft

Any thoughts on the new restrictions placed on the use of the simplified wall capacity method? I know plenty of buildings over 12m tall using this clause to design blade columns.

RE: AS3600 2017 Draft

No use wondering about it now. That was a draft. if you had wondered earlier you might have been able to have some input to improving it.

Now you will have to wait until the next version of the draft is released to begin wondering again.

If you are registered as an AS3600 owner/user with standards they will probably tell you when it is available.

RE: AS3600 2017 Draft

Are you suggesting that this might not make it into the final code? If anything, I think that makes it even more worth wondering about, and important to discuss, regardless if the next drafting is released or not.

RE: AS3600 2017 Draft

It has been modified in the next draft to more specifically target singly reinforced walls.

But the next draft is already written, so it is too late to change except by comment on the draft when it is released for comment in the next couple of months.

RE: AS3600 2017 Draft

I dislike using the simplified capacity for walls reinforced with a central layer of reinforcement. I almost exclusively use 2 layers of reinforcement for any wall that forms part of lateral system. For low-rise and mid-rise buildings this is generally governed by Earthquake loads.

RE: AS3600 2017 Draft

Agreed.

I also do not think the simplified wall method was ever meant to be used for blade columns. That was part of the reason the "wall definition" of length > 4 width was removed from the code many years ago. Unfortunately is was reintroduced in the fire design section and now many consider it to apply to design of columns/walls in all situations.

It was introduced for fire as a longer column will have less heating of bars as the corner bars, which are affected more by heat, are less dominant.

RE: AS3600 2017 Draft

I was hoping this clause would potentially help clarify when it is/isn't appropriate to use chapter 11, however if it does only place the new limitations on singly reinforced "walls" then I guess the ambiguity will remain, and we'll continue to see blade columns designed as walls in multi-level high rises.

RE: AS3600 2017 Draft

Quote (rcassar)

20 Jan 18 04:23

I dislike using the simplified capacity for walls reinforced with a central layer of reinforcement. I almost exclusively use 2 layers of reinforcement for any wall that forms part of lateral system. For low-rise and mid-rise buildings this is generally governed by Earthquake loads.
This goes back to tilt up and precast panels which are mostly singularly reinforced.

There has been testing over the years to prove that they are more robust than you may think.

Not all buildings are affected by the earthquake provisions.

RE: AS3600 2017 Draft

FYI in NZ they are also coming down hard on singularly reinforced walls, basically previously you could use the standard 0.85 strength reduction factor, now you need to use 0.7. You are also not allowed to use singularly reinforced walls where walls are face loaded and part of the lateral system (as there is no confined concrete). Lots of other requirements/restrictions as a result of findings from the Royal Commission into poor performance in these systems in the Christchurch earthquakes.

Part of the reason is that concrete is much stronger over time than the design value, so rather than forming distributed cracking to dissipate energy you get a single crack (with lower reinforcement ratios at least) and hence reinforced capacity can actually be lower than the uncracked concrete capacity. This generally results in bar fracture due to low cycle fatigue after a low number of cycles, as might occur in a seismic event.

As I believe it the intent of the recently updated NZ provisions are essentially making sure that the walls behave almost in an elastic manner up to the Maximum Credible Earthquake (MCE, 2500 yr return period event).


There might be laboratory testing to show they are robust, but real events in NZ recently generally showed in certain circumstances very poor performance.

RE: AS3600 2017 Draft

Thanks a lot aspix for sharing the 2017 draft.
Apparently there is a new draft out for comment. Does anyone have a copy to share? Thanks in advance

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