Opening in Structural slab

[Ipetu]

Please see attached drawings and photo (multi-page PDF).

Existing slab is 10" thick, rebar is #6, f'c = 3000 psi. Rebar pattern in this area seems to suggest that the slab is spanning mainly in the left to right direction. No special rebar is called up around the existing opening. Client wants a new opening as shown. Approx. 20 ft between gridlines.

Using a conservative 60 psi for shear stress in the plain concrete (0.02f'c with f'c = 3000 psi), a 1 ft strip of slab with d = 8.875 in (d = h-cover-bar diameter/2)has V = 60 psi x 12 in x 8.875 in = 6390 lb / ft. Relatively speaking this is fairly high and can support a fair bit of load. So I have ruled out shear as a concern.

Analysing the effect of the proposed opening in flexure is proving to be a challenge. However, considering that the proposed opening seems to fall within a middle strip, I am leaning toward saying that it is acceptable.

Any thoughts, comments etc. appreciated.

 

[TXStructural]

Just some initial thoughts:
- I agree with the assessment that the slab is designed to shed loads right and left
- Obviously, the part inside the "L" is most vulnerable, and acts as a cantilever (supported in two directions, or on the diagonal)
- I would guess that in the existing hole, they moved some bars to the sides (rather than just stopping at the hole), but I could certainly be wrong.) Check for a typical detail with this. My concern is obviously that the expanded opening will cut any bars now supporting the perimeter of the existing opening.

 

[rscassar]

It does look like the opening is located in the area common to both middle strips but this zone is subject to high positive moments. So before I considered giving the structural okay to core through the slab, I would be ensure that the system can handle the redistribution of forces.

Also since the structural drawings are dated 1955 I would be doing some further investigation into integrity of the structural materials. 3000psi concrete isn't the most durable material by todays standards.

Any refurbishment requires a re-analysis of the new "proposed" structural configuration.

 

[csd72]

i agree with the above.

There are two directuions that need to be considered.

The reinforcing up and down the page has minimal effect as you are not actually widening the hole in this direction.

The left richt reinforcing will definately be compromised and you will need to check that it is sufficient with the new hole width and the load from this supported by strips either side.

One thing that does concern me is the reason for the hole. Is there going to be a big piece of equipment mounted at the top?

I would suggest that you use the google at the top of the page as the topic has come up several times before with some very good references and advice.