Strut Tie Modelling in AS3600

[rapt]

Nathan,

Replying to your queries below

- No mention on hot to resolve tangential shear stresses on the node. We need to refer to Stephen Foster presentation to have clarification on this

AS3600 is not a text book. It defines limits on the design methodology. Read a good text book on the topic, or Stephen Fosters presentation if you want.

- In the ACI the development of the tension tie start when the tie meet the concrete strut which make sense because in that region the concrete is in compression as such the tie is confined. Developing the tie before the node allow to have less congestion where the node is.

You cannot logically start the development before the node. The whole logic is that the full tension force reaches the node! There is a further argument that the development should start behind the node, past the end of the compression zone. ACI obviously does not agree, but that does not make them correct!

- Code does not cover confinement of the node if the node is overstressed. What do we do to confine the node? Follow the recommendations for columns?

This is covered in the new Commentary that hopefully will be released soon!

- Bursting of struts for pilecaps? Do we need bursting reo or not? I went to a conference and the lecture mentioned that bursting reo is not required for pile caps, however the core doesn't talk about this.

The Strut Tie chapter in AS3600 (chapter 7) is a general set of rules for the design of strut-tie and ties and their connections.

Pile Caps are a specific application which should be covered in Chapter 12, Design of Non-Flexural Members, but is not covered specifically. So I assume you will have to use some engineering judgement in deciding how to design it, or again refer to text books. But I do not think you can apply a single rule for all struts in all pile caps in this case. I would think it would depend on the arrangement and the location of the compression strut within the pile cap. A strut at the edge could easily burst, while an internal one that is fully confined by other concrete maybe is less likely to burst.

What i meant is that there are a lot of area that are not covered in AS3600. It would be good to have some clarifications.

Again, AS3600 is not a text book. It defines limits on the design methodology. You need to apply engineering judgement in applying the rules as you do for all areas of analysis and design.

Trenno,
Maybe the researcher mentioned is not up to date on S&T and needs to read up on it a little more before making comments like that!

 

[Ingenuity]

when one of the most renowned structural researchers made a comment in a seminar this year along the lines of, "Sorry to everyone who uses AS3600, but concrete just simply does not bottle like that..."

Maybe the researcher mentioned is not up to date on S&T and needs to read up on it a little more before making comments like that!

rapt,

I think the researcher that Trenno was referring was Prof Mike Collins who toured AU recently on S&T methods. He co-developed the Modified Compression Field Theory. Mike did his PhD in UNSW (I think) but has been at U of Toronto for a long time.

 

[Trenno]

Yes it was Collins & Mitchell ^^

 

[rapt]

In that case it would be interesting to see the actual context in which this was said. Both the ACI and Eurocode would indicate that compression struts do "bottle like that", judging by the diagrams in the respective codes.

 

[rapt]

The most common example of a bottle shaped strut used in design is in Prestressed Anchorage zone design to determine bursting forces for anchorage design. I certainly hope we are not going to change that logic!

Just because codes use an idealized prismatic strut to do the actual strut calculations does not mean that the compression does not actually spread in a "bottle like" shape as described in the codes (and would show in 3D finite element analysis). In the compression design of the strut, it is conservative to assume the idealized prismatic shape. But in terms of splitting, the bottle shape describes the bursting forces logically.

 

[rowingengineer]

This argument "concrete does behave that way" comes up about many parts of the code, generally it is used by people to suggest that the code provisions are not producing an exact true reflection and hence this means that the code provisions are doubtful. Truth is that the code is an idealization of most of the components that go into the make up of concrete structures, providing acceptable ways to produce the minimum design requirements for a building/structure for 95% of cases. If we are going to argue over the bottle the truss analysis in strut tie, than I think we can simply give up on any simple means of designs and everything shall be based on full non-linear deigns and final test values.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Trenno]

My comments didn't intend to offend anyone, as a young engineer I was just curious as to why he would say this...

To give a little bit of context he was at the time comparing the Australian and Canadian codes, so perhaps he was a little biased, who knows??

 

[rapt]

Trenno,

No offense received.

Technical experts should make sure that they make responsible comments when giving lectures. Apparently Prof Collins is known to be critical of the need to use Bottle Shape struts in design, but that is different to "concrete does not act that way". Concrete does act that way, but you can simplify it in design if you want. The compression design of the strut will be controlled by the smallest width available, which will normally be the width at the node. As you move away from the node, the compression stress reduces as it is spread over a wider width, if that extra width is available, so it is not as critical. But the compression stress will spread if it can. That is the basis of all concrete design when considering concentrated forces.

ACI code shows a bottle shape strut, and superimposed on it is an "Idealized prismatic strut". Eurocode shows curved compression trajectories in its section on transverse tension forces. You know what AS3600 shows. Canadian code implies prismatic struts! But is also requires a grid of "crack control" reinforcement in both faces for the full depth of the member.

 

[asixth]

Nice chat. I extend a couple of tie sets down the column starters into the pile cap thru the node zone as the design transitions from column to strut tie.

Most consultant drawings that I have seen call up 2 additional tie sets in the pile cap but I think this comes more from securing fixing column bars than anything.

 

[IDS]

I'm no expert on this subject, but I too heard the comment form Prof. Collins as quoted above during his Sydney seminar earlier this year. A quick search found:

http://www.icevirtuallibrary.com/content/article/10.1680/stbu.12.00029

Reported test results of reinforced concrete beams made with coarse recycled concrete aggregate have been re-analysed using strut-and-tie models and the efficiency factors of the recycled concrete aggregate concrete bottle-shaped struts compared with those made of natural coarse aggregate. The objective was to ascertain whether strut-and-tie model provisions for efficiency factors of bottle-shaped struts in current design codes and the recent literature, originally meant for natural coarse aggregate concrete, can be conservatively applied to recycled concrete aggregate concrete, or not. The results of the re-analysis indicate that the efficiency factors of the natural coarse aggregate as well as the recycled concrete aggregate concrete bottle-shaped struts were comparable and conservative when benchmarked against the predictions of CSA A23.3-04 and an efficiency factor model proposed by the authors. However, both ACI 318-08 and Eurocode 2 gave unsafe efficiency factor predictions for the recycled concrete aggregate as well as the natural coarse aggregate concrete bottle-shaped struts under investigation. This is attributed to the insensitivity of these two codes to strut inclination with adjoining tie(s) and does not per se indicate any inherent inferiority as far as recycled concrete aggregate concrete is concerned. This investigation indicates that recycled concrete aggregate concrete bottle-shaped struts can be designed using the efficiency factors recommended for natural coarse aggregate concrete bottle-shaped struts in CSA A23.3-04.

Full paper is 20 pounds to non-subscribers.

I haven't read the full paper yet (I have it downloaded), but the abstract suggests that not only does the Canadian code include requirements for bottle-shaped struts, it is more conservative than the ACI or Eurodode.

Regarding pile-caps, the suggestion that bursting reinforcement isn't required seems strange. Surely it depends on the specific pile cap, and can only be determined by doing the analysis.

For the development of tie-reinforcement, I presume there is considerable research on this. Does anyone have a link to a good paper?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/