Stressed Bars - Anchorage to Concrete 3

[struggle67]

Hi Guys

I have a 47 mm dia bar that is required to stress up to 1300 kN to attach and anchor down precast cantilever element to CIS concrete pier. I have not done it before. The force could not be transferred to the concrete through bond due to embedment length constraint so there will huge concentrated load at the base plate. I have checked the end plate capacity and bursting links. And I presume that cone failure (RED LINE) is also possible. How do I check to prevent cone failure? How do I check required edge distance?
Concrete Shear Strength x Failure Cone Area > 1300 kN? or Add reinforcement(BLUE)?. Any other things that I need to consider?

Thanks

 

[Ingenuity]

struggle67

It looks like you have bearing/s at the 65mm gap under the precast cross head/top of the pier, so you need to consider that the application of prestress will pass-through these bearing/s, resulting in reactions that will produce tension forces in the top of the pier and also at the bottom of the crosshead.

I think you are best to look at this from a strut-and-tie method, and look at the flow of action/reaction forces for each half free-body.

Depending on how many bearings you have in the gap, you will also need to think about it in 3 dimensions - the angle of your 'struts' may be inclined about two axis.

If the pier has a circular section, you can take out the tension forces via perimeter hoop reinforcement too.

Similarly, for the crosshead - mirror image of the pier segment free-body.

Also, only a single PT bar central about C/L of pier? Not a lot of redundancy. I am not sure how you are casting the upper pier segment to engage the green-colored vertical rebar to the precast crosshead – maybe you do have redundancy – just hard to tell on when you engage the vertical rebar to the crosshead.

 

[KootK]

And I presume that cone failure (RED LINE) is also possible.

Ostensibly, there is no cone failure unless it pertains to loading conditions that you haven't yet shared with us. That said, this thread bears some similarity to the issues here. You'll be much less reliant on concrete tensile capacity however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[struggle67]

Thanks Guys

Apology for lack of info
Actually there are 3 numbers of bars. The forces (1300 kN each), positions (as shown below) etc.. were decided by someone else from the main structural consultant. And their calculation is as simply as applying some compression to the pier P(Required Force - 3x1300 kN)/A(Pier Area)=M(Unbalanced Cantilever Moment)/Z(Pier Section Modulus) We are just specialist and only required to determine the embedment length. But since we are the one who is going to do the work on site later and I am curious now. If you guys have work examples or code or other references about this in hand, could you please share with me.

 

[struggle67]

Hi
Please see this one and ignore the top image.

 

[Ingenuity]

An annular bearing - interesting - and has to be 'holed' to accommodate the PT bars and ducting. Looks messy.

An old but still relevant document that addresses related issues is VSL Technical Report #3 issued in 1991, authored by Dave Rogowsky (Canadian) and Peter Marti (Swiss) entitled: "Detailing for Post-Tensioning" Link or Link

It addresses general principles, local and general zones, with some numerical examples that you may be able to extrapolate to your somewhat unique application.

Curious - the green-colored vertical rebar, is it sleeved into the crosshead and post-grouted OR cast into the precast crosshead?

 

[struggle67]

Hi Ingenuity, KootK,
Thanks
And can I conclude as followed -

(1) Cone Failure is not possible.
(2) No strut and tie in my case as well since my strut has no angle.

And thanks for the links. I will definitely try to extrapolate from there. But at this moment, one last time. Please help me out.

(1) How I do determine bar embedment length? It seems to me like there is no control factors anymore as long as my struts have an angle less than 45 or 60.
(2) Should I concern about bars being too close to each other and to the edge 370 mm?

 

[KootK]

And their calculation is as simply as applying some compression to the pier P(Required Force - 3x1300 kN)/A(Pier Area)=M(Unbalanced Cantilever Moment)/Z(Pier Section Modulus)

This makes me think that breakout modes of failure are possible and that you should be designing according to principals similar to those set out in this article by Widianto. The design strategy seems to have been to prestress to avoid flexural tension in the concrete. The trouble with that is that, once you get to the lower level of the prestressing, that benefit comes to an end and you again need to deal with reinforcement for flexural tension again.

Our initial discussion pertained to just the forces induced by the prestressing. Dealing with applied connection loads complicates matters significantly.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.