Concrete Jacketing 2

[Blackstar123]

Due to increase in load on existing structure, I need to increase the size of the foundation (combined footing) and the concrete pedestal through concrete jacketing.
I am looking for ACI code provisions for design of shear keys such as diameter of holes, depth, spacing, shape of bar etc.
Any additional information or guidance on this topic will be greatly appreciated.


Euphoria is when you learn something new.

 

[HTURKAK]

You may look ASCE 41 for foundation retrofit and related commentaries, and FEMA 547/2006 Chapter 23. The link for chapter 23=

https://www.fema.gov/media-library-data/20130726-1555-20490-0332/fema547part3.pdf

 

[Blackstar123]

(A little bit late but i wanted to do some more research to gain some more knowledge before expressing my gratitude.)
Thank you so much for your response. I really appreciate your help.

Although, i dont have a copy of Asce41 but i did find a design guide for Asce41 on internet. Sadly, it didn't have the information i was looking for. Your second link, however, was very informational for retrofitting of foundation. (Even though the increased load is not due to seismic activity, but due to the addition of a small bin on the structure.)

I'm doing this kind of retrofitting (i.e., concrete jacketing) for the first time, so I wanted to make sure I was doing it right and as per code provisions.

I'm mostly concerned whether the anchorage of the shear connector in the existing concrete will be sufficient to provide a monolithic type of connection between the new and old surface.

The research reports I've read give very sparse information regarding the anchorage. Most just instructed in a paragraph to make sure the anchorage is properly designed to transfer the shear between old and new concrete and some mentioned to provide a min embedded length of 6db for connectors.
For a 12Dia bolt this give a min length of 72 mm. Which seems to be a very small length, even though the shear transfer by a rebar will not be very large and at maximum will be equals to the shear strength of the rebar.
But I do think that a rebar of this small length can easily slip. And, this will become a definite possiblity if the drilled holes are not properly cleaned before application of epoxy bonds.

On the other hand, if i provide a length equals to development length (approx equals to 28db), this seems like an unreasonably large depth.

From similar past projects designed by my firm, provided detail of concrete jackets have 60 ksi anchor rebars embedded in an epoxy filled hole, which has a diameter = db + 2mm and length of 200 mm. And so far no problem has arisen or brought to our notice from those projects.

For edge distances and spacing between connectors I'm following provisions of Aci 318 for shear force anchorage.


Euphoria is when you learn something new.

 

[Blackstar123]

One more thing, I saw a very strange thing on the internet today. And I'm a tiny bit afraid of asking this question, in fear that it may make me look like a complete idiot. But here it goes..
Some websites were calculating shear strength of rebar using Vn = (meu) Avf Fy. In this equation shouldn't it be 0.6Fy?
Meu is coeff. Of shear friction between old and new surface and essentially equals to 1.0 if old surface is roughned and cleaned.


Euphoria is when you learn something new.

 

[HTURKAK]

Yes…you are right fort he shear strength of rebar..…moreover, it is recommended that they be considered to have 50 percent of the effectiveness.

The retrofitting for shear strength deficiencies are difficult. Lets use strut and tie analogy… In order to develop diagonal compression strut, the existing bottom steel shall be tied to new with welding or coupling…and the new rebar anchored with 90 degree hook.
Other measures , increase the footing depth with concrete overlay…and the use of prestressing rod.
I will suggest u to look the worked examples at link =

https://www.fhwa.dot.gov/publications/research/infrastructure/bridge/06032/06032.pdf

 

[r13]

Some websites were calculating shear strength of rebar using Vn = (meu) Avf Fy. In this equation shouldn't it be 0.6Fy?

It is shear friction reinforcement, which is covered in ACI318 CH-11. Hilti provides epoxy grouted rebars with design information, you can check out on its website.

 

[Blackstar123]

Yes…you are right fort he shear strength of rebar..…moreover, it is recommended that they be considered to have 50 percent of the effectiveness.

But retired is also right. I checked the specified provision and code is allowing me to use Fy instead of 0.6Fy.

I found the answer on web for this. ACI is taking Fy instead of 0.6Fy because it assumes that the slipping faces will produce tension in the rebar. This is easy to imagine for a beam or suspended slab, but I am finding it hard to imagine that the tension produced due to slipping will be dominant than the shear stress due to loads.

If I think in terms of state of stress in the rebar for pure shear, both principal stresses will be equal in magnitude to maximum shear stress. And all the stresses will reach the yield stress at the same time. So it justified using Fy instead of 0.6Fy. But now the confusing part is, bolts of a shear connection will be in similar state of stress and yet we use shear yield stress = 0.6Fy. What am I missing?

Confusion cleared. My mistake in understanding the concept. As per the maximum shear stress theory, failure will occur when shear stress due to loads reach the maximum shear stress for a uniaxial tension test, which is Fy/2

 

[Blackstar123]

Hilti provides epoxy grouted rebars with design information, you can check out on its website.]

I found what I was looking for here. Thanks..

Euphoria is when you learn something new.

 

[r13]

Blackstar123,

You are welcome. Glade to be helpful.

 

[HTURKAK]

[I found the answer on web for this. ACI is taking Fy instead of 0.6Fy because it assumes that the slipping faces will produce tension in the rebar. This is easy to imagine for a beam or suspended slab, but I am finding it hard to imagine that the tension produced due to slipping will be dominant than the shear stress due to loads.

The concern is the lack of proper anchorage for the dowels..It is only recommendation to consider the 50 percent of the effectiveness of properly anchored stirrups, dowels..

Good Luck..