Creating new opening in Two-Way slab.

[chessiebear]

Greetings all-

Client asked me a while back about putting a new opening in an existing 2-way flat plate structure. I said 'stay within the mid-midlle and were going to be in good shape'. And they took that, literally and sold the alterations to the Owner.

Their intent is to remove the entire panel from column strip to columns strip. The whole mid-middle strip. There would need to be some new hangers from the slab to support the stair, but the net loading appears to be less than the combined dead and live load previously in place. I believe that it can be 'tricked' into redistributing the new loading pattern to the remaining slab.

What am trying to get my head around the requiement from ACI 13.4.2.1- "Openings of any size shall be permitted on the area common to the intesecting middle srips, provided total amount of reinforcemtn required for the panel without opening is maintained." Building was designed in 72-73 so I am guessing the '71 code. The same verbage is there under a different section number.
Since the entire panel is coming out, I am not seeing the reason for continuty as the bottom bars all end at point 0.15ln from the column centerline.

I see 2 different solutions but wanted to see what the community might advise;

1. Apply FRP to the perimeter to equal the capacity of the bottom bars in each direction? It would be equivalent to 15-#4 in each directions, so about 75k on each side of the opening.

2. Install steel beam grillage underneath, coring holes through the 9" slab and welding shear connectors to it making it minimum composite to take the bars strength in horizontal shear?

Using 0.22ln bar cutoff from face of support, I literally clear the mid-mid panel by 1".

Thanks in advance.

CB

 

[JedClampett]

I'm not a fan of FRP, but that's me. You're adding a heavy band of tension only reinforcing, presumably just below the slab. This seems inefficient and might have deflection issues.
I'm not sure what you mean by a grillage, but if you mean steel beams around the perimeter, I think you're on the right path. Just make sure they attach to a column or are long enough to transfer their shear to a load carrying member. An alternate is to build concrete beams under the slab.
You're also cutting development length of interior bars, so make sure that is considered.
It's very important to have your load carrying system in, whatever it is, before your demolish the section.

 

[JoshPlumSE]

Gosh, if your slab is 9" thick, I might use a 9" channel (or deeper if necessary) as my grillage and directly anchor into the SIDE of the slab rather than trying to attach to it underneath. Just does away with some of the coring and shear studs and such, replacing it with some simpler epoxy anchors.

 

[chessiebear]

Isn't that mid-middle panel hanging there off of the column strips? It would seem that removing the entire center panel would reduce the overall loads imposed by both the dead and live loads on the remaining column strips. Isn't it's function to aggregate surface loads to the column strips in two directions?

I am looking at punching out the ENTIRE mid-middle panel. The square in the lattice crust pie where the filling bubbles up.

If I was interrupting only a part of the slab, its easy to conceptualizing reinforcing for only the interupted bars, but this is the WHOLE panel going away. That is what I am struggling with.