Core compromised 2 way slab 1

[Lesali]

Q: If I add L's or C's below a raise slab to compensate for cut rebar, have I done any good? Assuming this is the tension face have I not just weakened it further by adding mechanical anchors.

I have a project at an old 1926 storage facility building (in a high seismic zone). A cellular carrier wants to expand an existing corded opening in a 8" 2-way slab. The opening is not in a column strip area, is 3" from the elevator opening, is 6" wide and projects 22" perpendicular from the corner of the elevator wall. In creating this existing opening the previous contractor cored thru the diagonals that would reinforce the corners of the elevator opening. Before I visited the site the carrier sent a contractor out to x-ray the floor but he only marked 2 cut diagonal bars (1/2" sq) so, we are sending them back to get a fuller picture of the rebar layout. At my site investigation I also did not see any other cut reinforcing bars that I would have expected to be present around a large slab opening. I thought the 3" elevator walls were just infill but could they be bearing? They appear to be a 3"x12"x24" concrete "bricks". So, no wall reinforcing, no floor ties etc.

I've never designed a floor such as this and thought I would put it into RISA Floor to investigate the plate stresses around these openings.

Thoughts of the sages would be appreciated.

 

[Archie264]

Maybe this will help: Link

 

[manstrom]

Yes, external reinforcing should help. The bond between the two is key. You may want to consider using epoxy to join the concrete to the new steel in order to make it composite. Otherwise, pretend the slab is fully cracked and design steel to support it.

FRP is probably the best bet, but this can get expensive and very specialized.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[KootK]

I imagine that those 3" walls are effectively bearing unless they're detailed to permit movement. It may be that the cut diagonal bars are just nominal crack control bars. If so, then I wouldn't bother trying to restore the lost flexural capacity. Unless you take special measures, like the epoxy mentioned above, a lot of movement needs to take place before the reinforcing really engages. Depending on where on the shaft your 6"x22" opening is, it might cause a punching shear issue.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.