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I'm reading this source about concrete strut and tie design.

Here is a picture from the site:

enter image description here

I'm curious, why isn't there a node at the point where the lower tie crosses the middle diagonal strut? In that position, shouldn't there be a 4-force node with two tension forces and two compression forces? This kind of node is not covered in any codes that I'm familiar with (for example, Eurocode deals mostly with 3-force nodes, such as CCT, CCC).

So why don't the forces meet at a node at this point, and why doesn't the capacity of concrete at that point need to be checked?

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3 Answers 3

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If they are physically joined then you would need a node, but if they are not joined then each will be analysed separately.

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  • $\begingroup$ How do you decide if they are physically connected or not? There is a strut and a tie, and they meet at a point. How are they different from those points that we have modeled as nodes? $\endgroup$
    – S. Rotos
    Apr 25, 2022 at 13:18
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    $\begingroup$ @S.Rotos by checking the design drawing to be sure - joints will be stated or drawn, whether clamps or welds etc. $\endgroup$
    – Solar Mike
    Apr 25, 2022 at 13:21
  • $\begingroup$ -1, this seems to be interpreting the diagram as an actual truss. This is just a strut-and-tie model describing the flow of tension and compression stresses in a monolithic concrete corbel. $\endgroup$
    – Wasabi
    Apr 25, 2022 at 22:14
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Who cares?

The reason there isn't a node there is because... who cares? The node is irrelevant and has no impact in the final result.

This is because all the members going through the "node" are continuous, and therefore the forces simply flow right through the node without any changes.

So, let's put a node there. Calculate that "truss" and what do we get? The exact same result as we'd get without the node. The horizontal tension to the left is exactly the same as to the right. Likewise, both diagonals are under equal compression.

enter image description here

That is: the horizontal tie and the diagonal strut do not transfer forces between them, so they each act as if the other doesn't exist.

The node is therefore irrelevant and can be safely forgotten about. Or included. Who cares?

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I've limited knowledge of S&T method, but the answer to your question is no. But, often, there is more than one way to come up with a realistic model, I think that was what the author had done (fine-tune the simple model).

enter image description here

Compare the model with/without the intermediate node:

enter image description here

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