RC Design - Octagonal Footing with Piles

[FreshGenZSE]

Hello Engineers,
First time posting and keen to get some design advices. Please see query as below.

Question:
I have an octagonal footing foundation design for wind turbines. What is the rationale of not having the stirrups (light blue) continuing into the “x” region marked in purple shown in the attached cross-section, 3 m thick?

For illustration purpose, please see the sketch below.


The governing shear force diagram is as below.



I say to provide stirrups myself but open to discussion as I may be overly conservative! Note that the shear demand is significantly larger than the concrete strength and the wind turbine tower is hollow.

Reason(s):

1. I have seen similar details in different projects with only mass concrete in the same region. I pointed it out to my manager and colleagues. They do believe it should be provided, but have no answer to my query so far.
2. I am uncomfortable to make the additional depth assumption as the tower is a hollow section. I am also recommended the same as it could be a risky call.
3. I have attempted to use strut-and-tie to justify that diagonal strut is viable alone. I resolved the overturning moment into axial tension and compression at the bolt connection, but found it impossible to have no tie in the region while having a model that is structurally stable!

For your information, I am a junior engineer with 2 year of working experience, and pretty unconfident with my own designs. If I have uttered something wrong, please do correct me so that I can learn!

Thanks in advance.

 

[HDStructural]

To be sure that I am understanding the foundation, the octagonal pad is supported by piles, I'm guessing at least 4 piles around the perimeter?

How much deeper would the middle section of the footing need to be to avoid the need for stirrups?

I would guess that they may be sized so that stirrups aren't needed in the middle portion, and the stirrups near the anchor bolts are only there to provide tension breakout reinforcement.

Any chance you are looking at the worst 1m section of concrete where the stresses are very high, but not looking at the section as a whole?
For example, when designing two way slabs we don't design the column strips for the stresses in the middle foot of the strip where the stresses are the highest, we design it for the total loads across the whole strip width and use the whole strip width to resist the loads.
That will often happen with FEA. If you design a simple spread footing using FEA and compare it to a hand calc, you will find that FEA requires much more rebar because of the higher stresses concentrated near the center of the footing while the hand calcs assume that the moments and shears are resisted by the entire width of the footing.