Strut-Tie - Tie Development at Nodes (Starter Bars)

[BacBac]

Reviving this thread:

Strut And Tie - Tie Development At Nodes - Structural engineering general discussion

Hi All, Looking for some input on how you all handle ACI requiring tension ties anchorage at nodes. ACI Section 23.8.2 states Tie reinforcement shall be anchored by mechanical devices, post tensioning anchorage devices, standard hooks or straight bar development in accordance with 23.8.3 (Tie...

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Does anyone have any ideas on how to justify the starter bars reinforcement is developed at the node?
For the shear ties, reference from Australian Standards, it's said to be fully anchored if shear ties detail is followed (AS3600-2018 clause 12.2.1 that refers back to Clause 8.3.2.4 for anchorage of shear ties).

However, nothing is referred for the development of the starter bars of the column.
Any help would be appreciated
Thanks!.

 

[bugbus]

This is a problem I've come across in similar situations to your sketch.

AS3600 only requires that the tie extends beyond the node to achieve the design strength of the tie, not necessarily that it is fully developed beyond the node. It also says that you can get away with a minimum of 50% development beyond the node (i.e. a cog or hook), meaning you could place the node just in front of the cog/hook if these are present. The problem still is that, even with cogs/hooks, these would usually be placed above the bottom layer of reinforcement so the node would not really coincide with the bottom tension tie you have drawn. You might need to consider a separate load path from the base of the column bars to the other vertical bars in the pile cap/slab, which themselves can then be considered anchored at the bottom tie level.

 

[Just Some Nerd]

One way to resolve this is to apply the tension from the column at a node close to the top surface, and the portion in the slab be handled by shear reinforcement. It's more like the shear reinforcement is lapping with the column starters in that solution though as they're in a straight line.

I would check how often a column actually ends up in net tension due to a moment, in my experience (at least in this region) it's pretty rare to not have enough vertical load to stay in net compression under combined axial + bending.

 

[BacBac]

@bugbus:

AS3600 only requires that the tie extends beyond the node to achieve the design strength of the tie, not necessarily that it is fully developed beyond the node.

Agree with you. Does this mean that there'll be a case where you have more starter bars than your actual vertical reinforcement?
A coworker has suggested to always have double starter bars for this situation (assuming starter bars only developed by 50% at the nodes), but I don't think I have seen that kind of detail before.

It also says that you can get away with a minimum of 50% development beyond the node (i.e. a cog or hook), meaning you could place the node just in front of the cog/hook if these are present.

Are you saying that your interpretation of this sentence on the code is that if I have a cog or hook beyond the node, I can assume that bar is fully developed?


@Just Some Neds:

One way to resolve this is to apply the tension from the column at a node close to the top surface, and the portion in the slab be handled by shear reinforcement. It's more like the shear reinforcement is lapping with the column starters in that solution though as they're in a straight line.

The thing is, not all pile caps or transfer slab would have shear ties on it. Especially a thick pile cap supporting a core wall.
Do you typically put shear ties on the column strip of the said pile cap?
Moreover, for the case of a tension pile, I don't think we typically extend the shear ties into the top of the pile.

I would check how often a column actually ends up in net tension due to a moment, in my experience (at least in this region) it's pretty rare to not have enough vertical load to stay in net compression under combined axial + bending.

Agreed. This more of a case where lateral loads are applied to a cantilever column.
My actual issue is a case where a core wall/shear wall is supported on a pile cap.

Thanks all for the input.