Pile cap strut and tie - strut dimension and bursting reinforcement

[silver2164]

In designing a pile cap using strut and tie method, AS3600 notes the compression strut area, Ac, as the ‘smallest cross-sectional area of the concrete strut at any point along its length and measured normal to the line of action of the strut’ (cl.7.2.3). It is also shown in Figure 7.2.4(A) that the strut has uniform depth, dc. It does not provide any further guidance on how to calculate strut area, strut depth, and strut width (Ac, dc, bc respectively).

Figure 7.2.4(A) would infer that the strut dimensions are determined by the column size and strut angle, θ (this is assuming the column is smaller than the pile). This would make sense to me. In the figure attached, this is denoted as ‘dc1’.

At the bottom of the strut, the CCT node depth could be enlarged with an extended node per ACI method. This is denoted as ‘dc2’ in the attached. My interpretation is this depth could be used to calculate the stresses at the CCT node but not for the capacity of the strut.

I have seen some designers base the strut depth on the pile size only, not the column. This is denoted as ‘dc3’ in the attached. I do not think this is appropriate as there is insufficient area to get the load transfer into the column at the top.

Fourth approach is to keep increasing strut width (dc4) until strut capacity = demand, while still checking the CCC node for the smaller node widths.

Wanting to get some other designers’ opinion on how to calculate the strut dimensions. If I just ignore the column, the strut capacity is massive and easily sufficient. By basing it off the column size, the strut size and capacity becomes limiting and the column size ends up governing the pile cap design.

In addition, finding the strut area is simple for a 2-pile cap. For 3-pile and 4-pile caps, does the strut shape remain rectangular with dimensions dc x bc? I have seen an example, based on Eurocode, where the CCC node at the top becomes a pyramid shape and the struts are triangular. This would severely reduce the strut capacity.

Second query on bursting reinforcement. AS3600 Cl.7.2.4 requires bursting reinforcement to be provided when bursting force exceeds bursting capacity. This happens in most cases as the bursting capacity is quite low. However, I very rarely seen pile caps with bursting reinforcement. I have seen previous threads on this topic in this forum, with no clear resolution.

ACI318-19 allows for design of pile caps without bursting reinforcement. The strut capacity decreases but still gives reasonable capacity.

I’ve had designers say they provide side bars, and top and bottom bars, fully lapped ‘preventing the cap from bursting out’, and just ignore this clause, while keeping the full strut capacity.

My view is not providing bursting reinforcement is not compliant with AS3600. But other codes, or ‘rational method of analysis’ could be used to justify?

How do other designers deal with the AS3600 requirement for bursting reinforcement?

 

[bugbus]

I agree that your strut widths will be limited by your column dimensions, assuming your sketches are to scale. The struts need to line up with the edges of the column, so the middle option wouldn't work in my opinion. Figuring out the geometry can be pretty laborious, especially in 3D, so I can't provide much input on that. There are some papers out there that go through this exact scenario if you can find them. If the strut capacity is limiting, you have the option of adding longitudinal reinforcement within the struts, detailed as though they were mini columns.

For the bursting issue, the code says something along the lines of 'the bursting reinforcement needs to be evenly distributed across the struts'. I wouldn't count the perimeter reinforcement as being effective for that purpose, but I'm sure some would argue for it. There are too many instances of people ignoring what the code says because 'that's just the way it's always been done'. If the numbers say you need anti-burst, it should be there. It could be as simple as a few sheets of mesh suspended in the cage at various heights. I don't believe it needs to be N class but I'm not 100% sure without checking the code.

 

[rapt]

Wouldn't it be a fan shaped strut as per Fig 7.2.1(b)?

 

[Just Some Nerd]

Feeling awfully confused today, thought I'd made a reply to this thread earlier but must not have hit submit...

My line of thinking was in line with rapt's, a fan shaped strut between the column and pile faces, the critical cross section for the strut being the smaller of the areas rotated by the strut angle.

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Why yes, I do in fact have no idea what I'm talking about

 

[silver2164]

Thanks for the responses.

Assuming a fan-shape strut would remove the requirements for bursting reinforcement (according to AS 3600).

However there is no guidance on when to apply a fan-shape vs bottle shape. The commentary to AS3600 notes: "If the stress field along the strut axis can uninterruptedly diverge in a constant angle, a fan-type compression field results. If the compressive stress field, or strut, is free to diverge laterally but is forced by the geometry at both ends into narrower fields, a bottle-shaped strut develops".

The pile geometry is still narrowing the compressive stress field.

On one assumption, you don't need any bursting reinforcement. In the other assumption, you need lots.


@Bugbus, I agree. I'm constantly getting responses in the office with 'that's how its always done'. When I poke further I can't get a response.

 

[rapt]

The second sentence of C7.2.1 would appear to explain it.

If it cannot diverge a prismatic strut is formed, otherwise it is a bottle strut.

So according to that it would appear to be bottle shaped strut with different widths at each end matching the column above and the pile below.