New Opening in Existing Slab 2

[Eng_Struct]

Hi Group,

I have an existing slab-on-beam system that requires new MEP openings through the floor slab. We are looking at 6 - 5" dia openings in one row. The slab layout and opening location are shown in the image attached. I have created a FE model and have the localized bending moment and shear values. I want to avoid having to reinforce the slab and rely on the existing reinforcing bar.

My question is how far should I be distributing the concentrated forces around the openings? Are there any guidelines in the Canadian or American code? The existing slab works if I distribute the forces at least up to 500mm wide strip. Note that the slab is only 4.5" thick.

 

[manstrom]

I'd just GPR the slab and avoid cutting rebar. Save yourself a week of engineering time.

6x5" holes are not that much in a 12'x14'slab (I had to convert to freedom units)

To answer your question, I would distribute as much as you can justify by the amount of rebar in the slab. If that were a new 2-way slab design, you would have a column strip and a middle strip about 6' wide. You are not cutting out half the slab, just 30" of a 144" slab.

 

[Insert_Smurf_Here]

Depending on the code on your code of record and the requirements of this slab what you are proposing may or may not be acceptable. For example, if the slab needs to meet ACI 318-19 and it is part of your seismic force resisting system (Cat C through F) you would have to repair the cut rebar around the proposed opening so that they are fully developed (this would probably require over-demolition and some sort of U-bar detailing around the openings).

Additionally, I am not sure what your moments are on the bending plot but keep in mind that your moment capacity is 0 at locations where you cut existing straight bars through the slab (I don't know your current rebar layout).

 

[Eng_Struct]

@Manstrom; Thanks for the feedback. In my mind, the edge of the slab will crack due to the bending moment and it will extend inward engaging other bars (i.e., distributing the load) until the sufficient section is engaged with resistance greater than the demand. I am unclear on your comment regarding "as much as I can justify". I have #4 rebar in both directions at 7" spacing. The rebar is most likely to be tied together, I feel like I even take a 1-meter strip for the load distribution due to compatibility (i.e., the slab edge will not deform without pulling down on the portion next to it). Thoughts?

@Insert_Smurf_Here: Seismic is not a concern in my region. The max bending moment I am getting is 13kNm it is concentrated at the slab edge (elements are 150x150mm in the sketch above)

 

[jerseyshore]

If you can't get it to fit within the rebar, something we do in NYC a lot for small openings is to expose the end of the existing rebar after the opening is cut and then weld angles/bent plates to the rebar around all 4 sides. Pretty simple to make sure any forces are transferred around the opening.