How to reinforce an existing reinforced concrete slab due to a creation of a future opening 1

[Structural-eng.]

Hello all. I am currently working on a project whereby we are required to evaluate and potentially reinforce an existing concrete slab due to the creation of a future 4 feet by 4 feet future opening for electrical conduits. We are dealing with a very old building that is in good shape (built in 1930). The challenge here, is that the rebar has been placed both orthogonally and diagonally (I am not sure what the reason is, we do not have any existing calculations). The future opening would therefore cut through what seems to be the main bottom bars. Can anyone please provide consultation on what should be done to reinforce the slab, and confirm that the diagonal bars are in fact the main column strip? Please see attached for the plan view snippet and the anticipated opening will be.
Eagerly awaiting your comments.

Thanks!

 

[hokie66]

I agree with your suggested placement of frp. But have you seen the slab soffit? Those two rectangles look like thickenings to me. If bottom bars, they are not called out.

 

[Ingenuity]

I think hokie66 has a point about the dashed lines that typically (at least in current drawing standards) represent slab soffit thickenings.

Additionally, consider that in the 27'8" E-W long-span direction there is only 7-5/8"Ø bars @ 7" in the half column-strip width of 5'3" (presumably 14-5/8"Ø @ 7" in 10'6" full column strip), however in the 22'4" N-S short-span direction column strips there are 15-5/8"Ø bars @ 9-[sup]1[/sup]/[sub]2[/sub]" to 9-[sup]3[/sup]/[sub]4[/sub]" centers over the full 11'2" column strip.

By more current design standards/methods, the total static panel design moment [L[sub]1[/sub][sup]2[/sup]*L[sub]2[/sub] vs L[sub]2[/sub][sup]2[/sup]*L[sub]1[/sub]] is about 125% more in the long-direction than the short-direction, but less rebar!

If you have not already done so, a site visit is worthwhile to review the slab soffit just to make 100% sure if there are soffit thickenings BEFORE you undertake further analysis or strengthening proposals.

 

[Structural-eng.]

@BWCSA @hokie66 @Ingenuity you are all absolutely right. I did in fact just notice the drop panel (band thickening) in the drawing. I will confirm it on site.
However, if I do the reinforcement of FRP or upturn beams on top of the slab, then we whould not interfere with the drop. Do you gents think that FRP stripes is a better solution than upturn beams? I am not sure how I would detail an upturn beam wihtout intervening destructively into the existing slab in order to have a monolithic connection?

Thanks in advance.
.

 

[RayGon]

The reinforcement layout is usually done in the very old days. What you will find when you start removing or cutting the concrete is that the center of the panel will have very little top steel or no top steel reinforcement at all. I would suggest to model the floor using a finite plate analysis. My hunch is it will not be a problem at all. However, like everyone suggested - PUSH BACK HARD AND GIVE THEM WHAT YOU WANT AND COMFORTABLE TO GIVE. Hope that helps.

 

[dik]

Can you just use structural steel?

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

-Dik