Pullout Strength of Tension Rebar 1

[Atooz]

Do you know how to calculate pullout strength of single tension bar usually used in pile caps. Enough distance from edges no worries about side face blowout. Some books say to use Un = 35*SQRT(Fc'). Based on ACI 318-14 Equation 25.4.2.3a, Ld would be quite large (ld=71*db) seems impractical. Pile cap thickness is 30 inch in my design. Will appreciate your response.

 

[DaveAtkins]

I have ACI 318-11, so I am at a little bit of a disadvantage.

First, make sure you are using the shortest ld allowed by Code. In ACI 318-11, there are two different formulas, and you are free to use either one.

This value of ld is what is required to develop the full yield strength of the bar. If your development length is less, you can calculate the capacity of the bar by multiplying the full yield strength by (development length available/ld). Note that this is only permitted when you need not develop the full yield strength.

Your other option is to hook the bar--development length of a hook will be much less.

DaveAtkins

 

[Atooz]

Thanks. Will appreciate if you could you please let me know which two equations you are talking about. I do not have ACI 318-11, is that Np = 8 * Abrg * fc' = 25 kips (for #8, Fc' =4000 psi.

 

[KootK]

I see this a little differently:

1) Development of the bar ensures that the rebar won't pull out from the concrete due to a bond failure.

2) Development of the bar does NOT ensure that a concrete breakout failure will not occur.

As such, I would do one of two things:

1) Effectively lap the rebar with the rebar in the elements above and below.

2) Use Appendix D provisions for post installed rebar to assess pullout capacity.

My opinion on this tends to be controversial so digest it with that in mind.

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.

 

[DaveAtkins]

Development of the bar does NOT ensure that a concrete breakout failure will not occur.

I see this differently from KootK.

For a reinforcing bar embedded in a mass of concrete, I think Chapter 12 (the development length chapter) of ACI 318-11 applies. Appendix D does not apply.

Using KootK's thinking, all reinforcing bars, in all situations, would need to be checked using Appendix D.

DaveAtkins

 

[Atooz]

Thanks for comments

There is no room for overlap from anywhere. Only where there are anchor bolts it may be possible. Usually practice is to leave it alone and try to satisfy any column tension. Question is how do I ensure the provided embedment length is good enough to take the axial tension. Assume that no bending/flexure involved.

Using (ld/db) = (3/40) * (fy/sqrt(fc') * (alpha*beta*gama) / ((c+Kct)/db)

and assuming Kct = 0 as there is only one bar going into the pile cap

getting ld = 23.72 inch (fy=60000,fc'=4000, db=1 in, alpha=beta=gama =1

Do you think is good

 

[KootK]

Using KootK's thinking, all reinforcing bars, in all situations, would need to be checked using Appendix D.

Not so. The vast majority of the time we are dealing with reinforced concrete principles whereby rebar tension is restrained by concrete struts/compression fields. It is only in cases such as the one being discussed here, where there is no concrete strut available to restrain the rebar, where we are forced to resort to using concrete tension (app D etc) to restrain rebar tension.

I argued my case for this concept extensively here: Link.

Question is how do I ensure the provided embedment length is good enough to take the axial tension.

If you post a detail showing your condition and the forces that you're looking to transfer, we'll be happy to review it.

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.

 

[JStephen]

They do make various couplers that enable you to splice rebar in a short distance. Can you use them or a similar product to put a "head" on the rebar similar to a headed anchor bolt? Just a thought. This would let you trim the rebar in place if necessary.

 

[humanengr]

ACI 318 applies a phi factor = 0.9 to tension controlled items, therefore, you should
use 0.9 X fy X Abar to calculate the design tension strength.

 

[Teguci]

CRSI has an example calc for just this. Also suggests 180 deg hooks over 90 deg hooks based on case studies. Don't forget to allow for pile installation tolerances (usually 3" min) and extend the development length that much further past the theoretical location of the pile. In my experience, even 3" is not enough, but it should catch the majority. There is also a study out there that suggests grouping the tension ties over each line of piles (not evenly distributed) - can't put my hand on it right now though.

 

[KootK]

@Teguci: I'd like to check out this CRSI procedure. Would you be able to point me to the specific document?

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.

 

[Teguci]

@KootK: I don't have access to the CRSI manual anymore and I can't find their overall design guide listed in their publications. Here is a link that references a new CRSI manual,

http://www.structuremag.org/?p=9343

.