Strut And Tie - Tie Development At Nodes

[ProbingStructuralEng_03]

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 force shall be developed in each direction at the point where the centroid of the reinforcement in the tie leaves the extended nodal zone)except for ties extending from curved-bar nodes designed in accordance with 23.10. From quick numbers I have run the curved bar node method opens a can of worms itself.

How do you guys handle the anchorage requirements of internal ties? Its easy to picture end bearing plates for horizontal ties but what about the vertical ties? I don't believe the industry is seeing internal vertical ties constantly detailed with to be anchored by bearing plates so what's the methodology here? Is it really just the curved bar node theory to satisfy the anchorage requirements? I know I have seen folks say well put stirrups in and you are good but whats the logic there?

I appreciate you input on this. Strut and Tie is one method I feel engineers constantly try to engineer judge their way out of but I think ACI has made it pretty clear its a required analysis method for so many scenarios. I just want to find a way that this is all practically

detailed to help get every one on board. I have attached a picture of a situation I think may be a common scenario for folks. The example is essentially a wall or column supported on a a pile cap that has substantial moment that creates a tension load that needs resolved. You could even simplify this and just look at a column with a tension force in stead.

 

[KootK]

1) Firstly, great question. I don't know the answer with any certainty myself and I feel that this is an area of STM that is probably still in flux somewhat.

2) There are many, practical situations where one needs to design part of a member with STM and the rest with Bernoulli assumptions. Where you're back in Bernoulli country, regular stirrups make sense. After all, modern stirrup design itself is based on a truss analogy.

3) The tricky bit, always, is trying to figure out when to transition from STM to Bernoulli. You'll often see limits on shear span as guidance for this. A shear span of two for STM treatment is common. Like many others, I tend to stretch that where my judgment supports such stretching. When I look at your example, I see the struts fanning out as I've shown. This makes for a nice zone of stirrup love between the disturbed regions in my opinion.

 

[ProbingStructuralEng_03]

Here's my thoughts to your response:

1) That is unfortunate because I feel like I am banging my head against a wall every time this comes up!
2)I agree especially because ACI has provisions to make sure your stirrups are properly anchored.
3)You mention using shear span as an indicator. Why not use ACIs provisions on deep beams 4D? I do think the fanning struts as you have shown for a strut and tie analogy is a great way to significantly reduce the tie loads.

 

[KootK]

Why not use ACIs provisions on deep beams 4D?

I'm not familiar with that. Can you point me to the provision in a particular version of ACI?

I suspect that 4D for the whole beam is really just a dumbed down version of Full span = 2 X shear span = 2 X 2D = 4D. The shear span concept is more useful if one knows their craft well enough to make use of that knowledge.