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.

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 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
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
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
Not much. We normally consider the web of flanged things as resisting all of the shear.
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
"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
Dik
RE: control of existing RC beams
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
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).
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The name is a long story -- just call me Lo.
RE: control of existing RC beams
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
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.