Opening @ Existing Concrete Column

[BONILL]

I have a continuous reinforced concrete beam. An 8" Diameter opening was placed in the beam long after it was placed. The location of the opening is at 1/8 of the clear span of the beam. The design called for #4 @ 4" o.c. in that region. One has to assume then that the opening cut through (3) stirrups, making the spacing at that specific point equal to #4 @ 12"o.c. This obviously hurts the shear capacity of the beam, in addition to the fact that there is a probability of stress concentration and early cracking around the opening (since there is no special reinforcement to frame the opening).

A proposed solution is to fill the opening with non-shrink grout and recommend that the opening be made at the center of the beam (where stirrups are spaced at 8" o.c.). The beam will have less shear capacity still but at least it will have the concrete contribution of the shear resistance. Not sure it would solve my cracking problems around the opening though.

Any thoughts would be greatly appreciated.

 

[JAE]

Can you look into the 8" hole and see the stirrups - i.e. how many were actually cut?

This may not make any difference if at 2 cut (vs. 3) you still don't have the necessary shear capacity.

You could also do a re-analysis of the beam and check on the actual Vu required at the opening and compare it to the [φ]Vc with the cut stirrups.

 

[BONILL]

@JAE,

Since the opening is close to the column, the shear demand is too high for #4's @ 12" o.c. to work. I have runned the calcs.

 

[par060]

They have fiber glass wrap that can be used to increase shear capacity...

 

[dcarr82775]

I don't think the non-shrink grout even gets you back the original concrete alone shear strength. I do not think (especially in a cored hole) you will get enough bond strength between old/new for the tension stress.

Can you add fiber, external steel, or core vertically down the center and add threaded rods with plates/nuts top and bottom?

 

[bookowski]

I agree with dcarr, I wouldn't count on the grout if that's all you are doing. Installing new bars vertically that are properly anchored at each end and then grouting back the hole should do it. You might want to do something to improve the bond at new/old concrete. I would shore the beam first and maybe even jack out a portion of the dead load.

 

[DST148]

@BONILL: Relocating the opening would not help because the beam still needs to be upgraded for shear. Could you provide a rough sketch showing the details? location of the opening from face of the column, depth of the beam, location of opening in relation to depth of beam, thickness of the slab, width of the subject beam in relation to the width of column etc.....

There are different ways of upgrading an existing beam in shear..
As suggested above, FRP wrap is one of them. You could use steel plates / channels on each side of the beam and above / below the opening as required.
You could reduce the span of the beam by adding a diagonal brace or by steel beam bolted to the column and the beam on the other side. Of course, the capacity of the column also would have to be cheched.