Strut and tie bottle bursting stress?

[Tomfh]

AS3600 requires you to check for lateral bursting stress where the load can spread out. We find this often controls the design, requiring you to add ties/bursting-reinforcement, making an uglier design.

What we're struggling to understand is how a fatter concrete element can be somehow weaker than a smaller one? E.g. a 300x300 prismatice strut with 300x300 bearing area will be ok, but a 600x600 prism with 300x300 bearing area will fail under the same load. It doesn't make much sense to us.

Can someone comment? Can adding more concrete really cause an element to fail?

 

[Retrograde]

Did you ever find the answer to your question? I'm interested.

 

[cliff234]

Strut and tie design has always been confusing to me. In concept, it is simple – but the specifics (using ACI318) are not as simple to understand. Regarding bottle-shaped struts, my rule-of-thumb is that I never use them. I have found that if I need to rely on a bottle-shaped strut, then my beam is not big enough. If you cannot get a standard strut (with a uniform cross-section) to work, then you need to either make the beam wider, make the beam deeper, or use higher strength concrete. If I need to rely on a bottle-shaped strut, then I’ve found that the nodal zone strength will usually govern beam strength, and I therefore don’t get that much more capacity out of the beam using a bottle-shaped strut.

 

[KootK]

Two related threads that might be worth a read:

Link
Link

In what follows, I'll speak to my interpretation of the ACI codes. I'd need to get a look at the particulars of the AU clauses in order to weigh in on them with any confidence.

1) I'm not convinced that one actually has the option of neglecting load spread if desired. It seems to me that it's not something that a designer has much control over if the spatial possibility is there. That said, like you, the notion that more material is somehow worse than less is suspicious. More on that in a moment.

2) In ACI, there is a provision allowing designers to ignore bottle strut bursting stresses if the average compression stress on the strut is below a certain limit. One would hope that, whenever average compression stress is higher than the limit, only a bottle shaped strut would work anyhow. Whether or not that's actually the case, I can't say.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Tomfh]

Did you ever find the answer to your question? I'm interested.

Not yet!

I have found that if I need to rely on a bottle-shaped strut, then my beam is not big enough

Agree. The issue here is the Australian code says if the load can spread out then it MUST be designed that way. The cases we're looking at are pile caps. Big lumps of concrete. Prismatic struts are working, but the code says you need to model as bottles. Unless of course we've missed something...

Two related threads that might be worth a read:

Thanks. very relevant. Seems we're not the only ones to wonder about all this.

I'm not convinced that one actually has the option of neglecting load spread if desired. It seems to me that it's not something that a designer has much control over if the spatial possibility is there

Agree. If the load can spread it will. We're just wondering if it can actually be weaker than a physical prism.

That said, like you, the notion that more material is somehow worse than less is suspicious

Yes that's the problem we come against. These pile caps works as pure prisms. But when considered as a solid mass (as pile caps are), it doesn't work anymore - AS3600 says we need anti-burst ties.

 

[pikku14]

Agreed 3600 (and now the new 5100) isn't very clear on bursting reinforcement requirements for large members. You're correct that if the load can spread, it will, especially so at SLS and will tend towards a prismatic load path at ULS

I'm trying to dig up a resource which specifically spoke about pile caps, but can't seem to find it at the moment. The paper or guide recommended ignoring bursting reinforcement within the cap because cracking is usually controlled at the outer faces by side face reinforcement and there's increased strength from the confinement from the mass of concrete around the struts

 

[Settingsun]

There are cases where extra material reduces capacity but they're related to the extra material causing instability (always - at least as far as I'm aware).

I have a feeling that in most practical cases, code serviceability (crack width) requirements govern the bottle strut bursting reinforcement. The spread angle is 1:2 at serviceability compared with 1:5 at ultimate, which more than compensates for the difference in appliled load between SLS & ULS. In addition, you would limit the serviceability reinforcement stress moreso at SLS than ULS (similar to AS 3600 clause 12.7).

So, if we're talking about satisfying code requirements, ULS is probably non-governing with respect to bottle strut bursting reinforcement.

 

[Tomfh]

The spread angle is 1:2 at serviceability compared with 1:5 at ultimate, which more than compensates for the difference in appliled load between SLS & ULS. .............So, if we're talking about satisfying code requirements, ULS is probably non-governing with respect to bottle strut bursting reinforcement.

If we can flatten the bottle to 1:5 that would be great, but AS3600 seems to be saying we have to check ULS bursting using 1/2 not 1/5?

From clause 7.2.4: "If the calculated (ULS) bursting force (T*b) is greater than 0.5Tb.cr, with tanA taken as 1/2, then transverse reinforcement shall be provided..."

 

[Trenno]

To add fuel to the fire... Link

There's some people (highly qualified) that disagree with bottle shaped struts too...

 

[Settingsun]

I take the 1:2 angle at ULS to apply only to the test of whether the reinforcement can be omitted entirely. The reo can be omitted if the concrete won't crack even at ULS. So, the assessment must use ULS loads but an angle consistent with uncracked concrete (1:2).

If the check indicates that the concrete may crack (stress greater than half of nominal cracking stress), the design of the reinforcement then proceeds on the assumption of cracked concrete (1:5 at ULS).

The 1:2 angle for ULS was introduced in Amendment 2. I can easily imagine someone making a comment that the wording was unconservative, and a change being made to just this sentence without realisation that they had caused a discrepancy in the definition of Tb*. If my interpretation is correct, they should have used a different abbreviation for ULS-1:2 as Tb* already referred to ULS-1:5.

Edit: The commentary to AS 3600 does nothing to me except reinforce my interpretation.

 

[Settingsun]

As an example of the fallibility of the code writers, I've just checked AS5100.5 released this month (concrete bridges). In the corresponding section, they've copied the 1:2 angle at ULS from AS3600 Amendment 2, but not the Amendment 2 correction to equation 7.2.4(2) [T*b corrected to T*b.s].

I've also just noticed that Amendment 2 introduced a small error into equation 7.2.4(2). It now refers to T*b.cr instead of Tb.cr. This was not the case before Amendment 2 and happily has not been copied into AS5100.5.

 

[Tomfh]

So, the assessment must use ULS loads but an angle consistent with uncracked concrete (1:2).

Yes that's what we're doing, i.e. AS3600 says our lump of concrete is weaker than a significantly smaller internal prism. It's hard to believe that a cubic meter of concrete will fail but a 400x400 prism will be ok. If we documented the pile cap as formed up prismatic struts then AS3600 would say its fine.

 

[Tomfh]

There's some people (highly qualified) that disagree with bottle shaped struts too...

It makes sense that enlarging a prism won't weaken it.

 

[Settingsun]

Not sure the exact structure you're designing, but I don't think AS 3600 covers unreinforced concrete except in a few limited circumstances. So, your 400*400 strut would need longitudinal reinforcement. Then, being a reinforced compression member, would need ligatures to restrain the reinforcement. By that stage, it's getting close to the reinforcement for the bottle strut.

In summary, I think the bottle strut bursting reinforcement can be designed for ULS load at 1:5 angle, with serviceability requirements probably governing.

 

[Tomfh]

So, your 400*400 strut would need longitudinal reinforcement. Then, being a reinforced compression member, would need ligatures to restrain the reinforcement. By that stage, it's getting close to the reinforcement for the bottle strut.

Whether a strut is a reinforced concrete element is getting a bit off track.

My original query concerned the code saying a prismatic strut is ok, but saying that a significantly larger mass of concrete is not ok. I understand that the code says this, but I can't get my head around more concrete being weaker than less.