Core Wall Base Pile Cap Starter Bars

[BacBac]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1723617656/tips/Core_Wall_Starter_Bars_1_degmkq.pdf[/url]

Hi All,

I'm a structural engineer working in a consulting firm in Australia.
There's a new manager in the company whom I'm working under at the moment, and he insisted that we should put double amount of vertical reinforcement as the starter bars at the pile cap for the walls especially when it's under tension due to ultimate earthquake loads.
He was from a national consulting firm in Australia which is famous for designing many tall buildings in Australia, and apparently, they always put more (double) amount of starter bars compared to the wall's vertical reinforcement.
Please see figure 2 on the pdf attachment for his reasoning for doubling the amount of starter bars based on the strut-tie principle (mainly because the reinforcement is not developed at the nodes).

I have some understanding on earthquake design as I've read couple textbooks by Moehle and Priestley, and I feel like this is not the correct approach from earthquake perspective as we don't want to over-reinforce the vertical bars for the plastic hinge to form, especially for ductility mu=2.
I've brought this up, but he said that the plastic hinge can form above the starter bars instead.

Could I have opinions from members in this forum regarding this design approach?
There appears to be a similar question here but seems like it's not answered.

https://www.eng-tips.com/viewthread.cfm?qid=513462

Thanks.

 

[JAE]

I'm not clear on why the hooked bars are said to only develop 50% of the strength of the starter bars.

But I would suggest consideration of the varied flexibility of the mat itself. Force follows stiffness.

As the concrete shaft is bent due to seismic or wind, the wall W1, as you indicate, goes into tension.
The tension in each bar along the wall would be theoretically equal if the mat below was infinitely stiff.

However, the mat isn't infinitely stiff as it spans (two way bending to some extent) from pier to pier.
More of the tension will flow into the corners of the shaft to be directly transferred to the piers below.
The bars pulling up on the mat between the piers will take less load as the mat will flex a bit there and shed the tension to the pier areas.

 

[BacBac]

Hi JAE,

Thanks for your insight.
As per the Australian Code (See excerpt from the code commentary on the image below), exactly at the end of the cogged/hooked bars, it's generally taken as only 50% developed.
i.e. in my figure 2, exactly at the node location, the bars are only developed by 50%.
This has been discussed in

https://www.eng-tips.com/viewthread.cfm?qid=504391

(Case 1 in Just Some Nerd screenshot)

Yes, I agree that as the mat is not infinitely stiff, the stress distribution will not be constant.
I simplify my sketch for the purpose of asking questions.
But assuming that all wall vertical reinforcements are designed at 100% of the demand force, does it mean that my starter bars have to be larger than my wall vertical reinforcements, as at the node location, the bars haven't developed yet?
I just feel conflicted that I have always put my starter bars the same size as my wall vertical reinforcement, but this other firm lead said that they always put more.
I wonder what other engineers do especially at high seismic areas, and what the justification to tackle the argument on my figure 2.

 

[Tomfh]

How deep is the pier cap? How much of the load are you trying to get into the piers?

I don’t fully understand the logic of doubling up. What failure is it aiming to prevent?

 

[BacBac]

Hi Tomfh,

This is a concept question that kinds of applicable to all cases.

Basically, it's to compensate that at the nodal zone, you aren't able to develop 100% (500MPa yield stress for Australia N Bars) of the anchorage of the wall vertical bars (see snapshot below), hence you put more bars with less anchorage (say 250MPa) to take the same amount of force which your vertical bars can take with 100% anchorage (500MPa yield stress).