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control of existing RC beams
3

control of existing RC beams

control of existing RC beams

(OP)
I posted something regarding this project in other topic. I dont have many experiences so im hoping for some insight regarding this.

Now im dealing with an existing structure - RC beams + slab at the bottom.

What do you guys usually do - do you control existing beam for bending moments and shear forces:
1) on a beam modeled as rectangular section
2) on a beam + slab (beff) - reversed "T beam"

I suspect that option 1 is more conservative?




Lets say that bending and/or shear control doesnt go through. Can I solve this problem by adding another layer of concrete on top (lets say h1 = 15 cm height). Im thinking about drilling holes in existing beam + epoxy and new stirrups + rebars. I would also control shear flow between existing and new/added concrete part and consider this when determining spacing between stirrups.






RE: control of existing RC beams

I wouldn't bother with T-beam action. There's really little benefit when the slab is in tension unless you have rebar congestion issues in the beam. As for the extra layer, see this thread for essentially the same problem: Link

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.

RE: control of existing RC beams

(OP)
Thank you KootK!

but I thought that the slab at the bottom of the beam provides benefit as far as shear strenght goes?

RE: control of existing RC beams

(OP)
one more question...

new stirrups are loaded with horizontal shear forces (because of the shear flow between existing beam and a new concrete part on top of it) + with vertical shear forces.

does that mean I have to consider interaction: shear + tension when designing stirrups and spacing between them?

RE: control of existing RC beams

You're very welcome.

Quote (OP)

but I thought that the slab at the bottom of the beam provides benefit as far as shear strenght goes?

Not much. We normally consider the web of flanged things as resisting all of the shear.

Quote (OP)

does that mean I have to consider interaction: shear + tension when designing stirrups and spacing between them?

I don't believe so. The forces are complementary rather than additive in this situation.

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.

RE: control of existing RC beams

Look at ACI 318-11, section 11.6.7.

"Net tension across shear plane shall be resisted by additional reinforcement..."

So if your original section (and stirrups) have an adequate ΦVn, equal to ΦVc + ΦVs, then your added beam depth above doesn't need to provide additional vertical shear capacity and your shear friction reinforcement crossing the plane between the two sections can be designed for only the horizontal shear.

However, if your original section ΦVn is not adequate and you need additional vertical shear capacity as well, you would then need to include new shear reinforcement for BOTH the horizontal shear and the additional ΦVs you need. These would be additive.

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RE: control of existing RC beams

I'd be putting additional dowels into the existing pilasters between the existing vertical bars and putting a completely new reinforcing cage in the pilaster extension. The dowels can be epoxied in place using Hilti Hit. Roughen the existing contact surface, saturate and use a latex bonding agent. For small percentages of reinforcing steel, a T beam does not provide much benefit.

Dik

RE: control of existing RC beams

(OP)
thanks guys you have been helpful!

dik - would you also put/anchor stirrups of completely new reinforcing cage in the existing beam or would you just put closed cage on top of existing beam and only dowels (in the middle) take care of proper connection? What kind of dowels did you have in mind - standard rebar with larger diameter or something else?

BTW how deep can you drill in existing concrete beam for adding dowels/new stirrups? Is 30 - 40 cm hard to drill?

RE: control of existing RC beams

As long as you can drill such that you're not cutting rebar, my guys on bridges drill 30-40cm routinely.

It's still doable if you hit bars -- but you need a steel cutting bit, and the hole tends to get sloppy (because it's hard to force the drill through).

----
The name is a long story -- just call me Lo.

RE: control of existing RC beams

If you are adding addition Vs to help with the vertical shear situation be sure to satisfy all the ACI requirements for shear reinforcement in terms of depth, etc.
This may require a very deep drilled hole and possibly even an anchor plate or something on the bottom of the beam.

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RE: control of existing RC beams

Quote (OP)

does that mean I have to consider interaction: shear + tension when designing stirrups and spacing between them?

Quote (KootK)

I don't believe so. The forces are complementary rather than additive in this situation.

In support of my previous statement, I submit the following:

1) The sketch below showing my understanding of the mechanics of the situation. As I see it, the truss action associated with vertical shear reinforcing creates a complementary compression field over plane on which shear friction capacity is desired. This is why I feel that shear stirrups can pull double duty as both vertical shear reinforcing and shear friction reinforcing. This is similar to what we do with the vertical bars in shear walls at the slab joints.

2) The clip below taken from PCA Notes 318-11 which appears to be in agreement with my interpretation. Calculation example 12.6 of the same document proceeds along similar lines.



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.

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