AS3600 shear wall design

[PeterPl]

I am canvasing opinions of engineers on shear wall design.

I find AS3600 cl 11.5 very limiting and highly illogical (wrong) for tall buildings.
In Australia frame action is all but ignored (because of flat slab construction). Building stability relies 100% on shear walls often precast.

Yet the AS3600-2009 is very primitive for these vital elements.

My biggest issues (with section 11.5 of AS3600) are with wall shear capacity requirements.

a) The blanket use of 0.8Lw
Typical building core is composed of number of walls that meet at many junctions and corners.
The 0.8Lw is meant for isolated wall with uniform reinforcing, not wall that is part of a multi wall core.
If the wall in question is acting as web in H or box shaped core than full Lw can be used - but not according to the code.

b) Total ignorance of axial loads on shear walls when calculating the vital shear capacity.
(No comment needed here except very tall concrete buildings are built in Australia with significant axial load variants in the walls ignored, positive/compression and negative/tension)

c) There is an upper limit in shear strength with no further strength benefit for concrete >f'c=64 MPa for beams but not walls. 80 MPa precast walls have lot to say for them – high axial buckling strength without reinforcing, enabling thinner panels with lower lifting weight. Code allows full use of the 80 MPa for shear! (I have used them but applied the 64 limit on shear even if not required by code)

d) Previous code made distinction between wall total height and wall floor to floor height. This distinction has disappeared in 2009 code. This is un-conservative.

 

[asixth]

It's not much help but I thought that AS3600 could be a bit more informative with the design of walls

 

[Trenno]

The current code commentary elaborates slightly on some of the issues above.

I'm very interested to see the responses to the questions above though.

 

[Retrograde]

My understanding is that Section 11.5 is a simplified method for walls subject to compression over their entire cross section, but that shear walls should be designed as columns using Section 10, and then checked for shear using Section 11.6.

 

[PeterPl]

That is what I am talking about - SHEAR

 

[Retrograde]

OK so I think it is Section 11.6 that you are more interested in.

Generally I have not found the shear strength to be a limiting factor in core wall design with flexure of more concern. But I take your point that axial compression should increase shear strength and therefore possibly reduce reinforcement.

Confusingly AS3600 defines Hw as the floor-to-floor height of the wall but the Commentary at Section C11.6 states:

"Note that throughout this Clause, Hw is the overall height of the wall."

 

[PeterPl]

The commentary you talking about is 1996 so it does not apply to 2001 or 2009 code. This is when they removed the overall height reference completely.
Axial compression is rarely a problem except for slender walls and than reinforcing does not help - most walls are minimum 0.0025 steel so not column either.
And don't forget tension reduces the shear strength (it can happen)

 

[Trenno]

Peter, there is now a commentary to our current 2009 code that was released in 2014.

 

[PeterPl]

I look before I replied on the SAA site there is nothing

 

[Trenno]

 

[IDS]

The Commentary is listed on the SAI Global web site,although it is below a superseded version for some reason.

I don't get involved with shear wall design, but it seems to me that all the issues raised should be addressed.

Presumably the strut and tie provisions could be applied in place of this section.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[PeterPl]

Yes, thanks all I have found it now, as you say Doug it is out of chronological sequence.

 

[degenn]

I think the wall section of the code was originally written by the same guys who built the pyramids. It is very simplistic and conservative.

The 0.8 factor for shear (and the extra 0.6 for compression) is to ensure that you are nowhere near failure and that everything is still very linear.

I think a useful approach is to look at the wall stresses as a Mohr Circle and provide horizontal and vertical reinforcement to suit, so that the "transformed" concrete is not in tension. This allows the actual axial stresses in the concrete to be considered. There are papers showing this method.

It is practice outside Australia and in some software packages to treat a composite wall as a single rectangular stress block section. This can be unconservative for slender walls with minimal reinforced and no ties and shouldn't in my opinion be done. It is best to treat walls as individual walls and keep them linear, particularly if you are using precast.

Take particular care with equation 11.6.3(2). Is says it is to be used if Hw/Lw>1 but if it is just over 1 then the value of Vuc is infinite which is obviously not right. The cut-over between the 2 equations should be at about 1.3 to get a smooth transition.

 

[Retrograde]

This allows the actual axial stresses in the concrete to be considered. There are papers showing this method.

I believe this is covered in fib Bulletin 45. Got any other references?

 

[degenn]

Stephen Foster at UNSW presented a paper a while back. Can't lay my hands on it just now.

 

[Trenno]

Peter, in regards to item b)

 

[PeterPl]

Trenno,
I am aware of other code provisions. My point was that AS3600 has none.
Certifying engineers will not accept ACI code in Australia.

 

[Trenno]

Sorry, no worries.

I thought we could cite something along the lines of "engineering judgement and best practice," which is allowed in the BCA?

Anyway.. I guess we can't hold our breath with a change to AS3600!

 

[dadomago75]

Hi everyone,
sorry for the late contribution, but I was looking for something else and I found this thread started by Peter, so....I couldn't resist!
(Hi Peter, hope everything is good )

AS3600 is quite semplicistic in many aspects, starting from columns reinforcement detailing to slab punching shear design......and on and on...

With regard to the concern raised, it is paramount to differenciate between 2 types of shear walls: 1) slender Hw>3L, 2) short Hw<0.3L

Type 1 is governed by flexural demand.
Horizontal reinforcement are required just to take shear, and vertical renforcement are required to take the bending. This walls can easily be designed as columns.

Type 2 is governed by shear.
For this type of wall both vertical and horizontal reinforcement are required to take the shear, because both V&H reinforcement are necessary to stitch the diagonal cracks that will develop from the cyclic demand. The cracks will develop along the diagonals in both directions due to the cyclic demand. If insufficient reinforcement is provided, the compressive struts may buckle and faildue to eccessive compressive stress.
An additional issue is the sliding shear, which is typical of stocky walls. Due to high bending moment in the wall, vertical bars can yield well beyond the 0.2% strain, developing a horizontal crack that will create the basis for the wall section to slide along the crack.
For this reason it is good practice to detail a crossed reinforcement to sew the horizontal cracks.

Furthermore, vertical load does not have any beneficial effect for the shear capacity other than reducing the stresses on the vertical reinforcement from the bending moment, hence reducing the risk of sliding shear.


For all of this I suggest to have a look to the Eurocode 8 (thank God now the Eurocodes are free, so you can find them on the web) or to the Italian Standard NTC08 (if you can read italian).
And if you want to become a master in seismic engineering, you can always read all the master dissertations (divided by year) on several topics Link

 

[PeterPl]

You should know Dadomago that trying to get something past PSV (the certifying engineers) to different code than AS is like pushing the "proverbial" up the hill.
Before ETABS introduced AS3600 I was using ACI318-9? code with AS phi factors and that was difficult too.
Wall shear is the most critical aspect in the wall design. All 4 items that I am concerned about are at best oversimplifications at worst straight out errors and oversights.
Why would you be limit shear to 64 MPa strength in everything except walls? Why would you totally change the definitions of wall heights from one year to another?

I have used various other sections of the code to paint myself out of a corner:

Beam shear - is good if there is plenty vertical reinforcing
or
shear friction

They both recognise benefit of vertical reinforcing and/or compression.