Forming 6"x 6" opening in an existing RC Beam

[D.Jaya]

For a building refurbishment project ,it is required to provide 6”x 6” opening along RC beams of sectional size 9”x18”.
The existing building (03 storied) is about 25 years old and for the concrete structure, Grade 25 concrete has been used. According to the available structural drawings, the beams and the slab is being adequately reinforced. The span of the longest beams are is 21’-0” & 13’-9”

The previous use of the building is for a hotel and it will be refurbished for the same functional use.

I proposed the following methodology for the process of making 6”x6” openings in RC beams

1. After fixing proper steel props & other safety measures, 1/4 L of from the supporting column (Point of contraflexure region),a 6”x 6” opening on the beam shall be done using manual tools

2. The opening to be made with an extra 20 mm and a square sleeve fabricated by welding 8mm thick steel plates to be inserted & fixed in the opening by using high strength concrete grout.
( this arrangement will compensate for the loss of shear strength at the RC beam opening -but I don’t have a calculation to prove this)

I need your comments & opinions please

 

[Greenalleycat]

A few thoughts from me…

I am assuming the cross-section is 9” wide x 18” deep and the opening is going to be on the centreline of the beam depth?
Are you sure that your proposed cut location will not hit any existing steel, particularly stirrups?

I think, for a 6” hole on an 18” beam depth centreline, assuming no steel cut, you will likely have more of a shear issue than a moment issue. So I would be aiming for a point of zero shear, not zero moment.

I do not believe that your proposed solution will do anything for the shear capacity of the beam. So hopefully the shear capacity of the beam will be sufficient at the location of the hole.
Note that, in our code at least, we end up having to ~double the design shear force above and around penetrations, so you want to check your code provisions to make sure what you do is actually compliant with those clauses too if present

I would make sure your proposed hole stays the hell way from any yielding regions, if this is a seismic zone

Your hole will also generate additional bending forces due to the shear force acting at a lever arm equal to your hole width, so make sure you have sufficient moment capacity at the hole location to handle these extra bending forces (on top of the major bending component)

 

[HTURKAK]

6”x6” openings for 18” deep beam makes 33% of the depth so, can be considered as small opening. If possible, you may shift the location to around L/ 2 where shear is minimum and shift the location just under the slab .

 

[Greenalleycat]

Wow your code is a lot more opening friendly than ours... we are limited to 1000mm2 for a 'small opening' for a beam of these dimensions
6"x6" is more like 23,000mm2, so not even close to being considered a small opening

I would not be lifting it up near the slab, presumably this is going to be in a non-seismic region so it will be gravity-dominated
So at the midspan (assuming that is where the location is) the top of the beam will be in compression
You need concrete there to carry both the shear and the bending compression forces
Banging it in near the bottom (subject to avoiding steel and providing adequate clearances) would be a better option

 

[hokie66]

Agree with HTURKAK. I would place the opening at midspan, and hard up to bottom of slab.

 

[Greenalleycat]

@hokie, midspan I agree with, but why up against the slab?
I would not want to be removing concrete on my compression side

 

[dik]

It gives the widest expanse of concrete at the top of the hole and the greatest amount of concrete beneath the hole... With a T shaped section (even if not intentionally composite) the neutral axis is above the mid depth.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Greenalleycat]

Correct me if I am wrong here, I envisage a concrete T beam as being similar to a steel I-beam in that the web carries the vast majority of the vertical shear and the flanges contribute bugger all to that
In RC, it is more favourable to have solid concrete on the compression side than tension side for carrying vertical shear too, due to how concrete works
So, I still do not understand why we would advocate to remove the chunk of web concrete on the compression side of the member

 

[hokie66]

None of the web is in compression. The neutral axis is well above the slab soffit.

 

[Greenalleycat]

Sure but the principle is the same. Smaller cracks up the top so more shear carried there than near the bottom surely.

If the question is "this hole HAS to go here, can it be done?", then the answer may well be yes... But, all other things being equal, I don't understand why you would CHOOSE to put a hole right up near the compression side of the beam when you could equally just bang it in on the tension side.

 

[centondollar]

Assuming that the beam is designed for stirrups, the location of cracks have nothing to do with the shear capacity - it is the stirrups and concrete struts that carry the load. Eurocodes require stirrups and concrete struts to carry all the load, or for no stirrups to be required and for concrete/aggregate interlock along cracks/dowel effect of main rebar to carry all the load. I do not know what other codes say about this, but it is certainly not conservative to combine the shear resistance of stirrups with any other types of shear resistance mechanisms.

Furthermore, any location in the web is seldom close to the compression side in a simple-span T-beam, since the neutral axis is almost always in the flange in such beams; in other words, the entire web is the "tension side".

The best option is to make the cut where the shear is minimal and in the tensile zone; for the T-beam, this is the web at some distance from the supports.

 

[dik]

Greenalleycat... wot shear?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Greenalleycat]

Heh I've stitched myself up there haven't I.

Most frames I'm used to have pretty constant shear through the beam due to lateral loads, so even at midspan there is a large shear.

You're right that if this is gravity dominated it probably won't matter a damn.

 

[dik]

Don't think of it that way... As they note, you try to put the opening in the area of least shear. The opening is small dimension and the resulting moment is not great... a couple of 15M bars under it and you're likely OK (you still check to see if it's OK).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BAretired]

It may not be doable. A 6"x6" clear opening must be 8"x8" to allow for the steel plates and drypack grout. Any stirrups existing within the width of opening will be cut. In effect, there will be no stirrups in the width of opening, which means that a 9"x6" 'beam' of Grade 25 concrete must be adequate to resist the shear. Do we know the magnitude of shear at L/4?

BA

 

[hokie66]

I can't see the benefit of the steel plates. If you can cut the hole neat, forget about the steel box. Don't make the hole at L/4, make it at centre span.

 

[Ingenuity]

Practically speaking - and having chipped a lot of concrete over my lifetime - it is tough to concrete chip a small 6"x6" square hole through a 9" thick element, even with access to both sides.

I would concrete core a 6" hole.

 

[dik]

...and nibble away the edges, carefully. Do they have a mortising bit for concrete? Maybe it's a round duct/pipe?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BAretired]

Drilling an 8" diameter hole would be easier than the proposed scheme and would do less damage to the strength of the beam.

BA

 

[hokie66]

Coring a hole would definitely be the best way to go. But the size of the core would likely depend on the clearance for the drilling machine. I would be dubious about trying an 8" core. The vertical face of the beam below the slab is only about 13", and you don't want to get too close to the bottom bars.