Cutting Flat Plate 2-Way Slab

[StrEng007]

How do you all typically handle providing a new cut into an existing flat plate 2-way slab?

At first glance, I thought the best placement for the cut (1'x2' opening) would be located in the center of the column strip. Here, I'd be able to provide steel reinforcing angles to the underside of the slab to pick up the primary direction positive flexure reinforcing requirement. However on second thought, I wasn't sure how to handle the loads in the perpendicular direction's middle strip. In the perpendicular direction, there will exist a percentage of negative moment (ie the remaining percentage from this direction's column strip that handles negative moment) which will require top bars. Depending on which direction I consider, this column strip will have both positive and negative flexure at the center of the strip. Although, the positive will govern over the negative.

Any suggestions?

 

[JAE]

Our first preference for cut openings is within the middle strip boundary (center of the bay) and not allowed along column strips.

Having said that, depending on your bay sizes, a 12" x 24" opening may not be significant enough to worry about. You might cut several middle strip top bars but I would think at the midspan location you would have adequate ability to re-distribute the middle strip negative moment around the opening without seriously affecting overall strength.

If you know what the original reinforcement layout is you could see if there is any extra capacity in the -Mn design on either side of the opening.

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[StrEng007]

JAE,
Thanks for the response. When you cut at the middle strip center of bay, what's your typical reinforcing solution to keep flexural continuity? Frame the underside of the opening with (2) angles and run them back to the column strips on each adjacent side?

 

[JAE]

In many cases with small openings like that, we don't do anything if the opening is cut in the middle strip center area. So losing one or two bars isn't a concern, again, assuming the load is redistributed around the opening to the untouched adjacent bars.



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[KootK]

You'll find this helpful: PCA

I agree with JAE on all counts.

When you cut at the middle strip center of bay, what's your typical reinforcing solution to keep flexural continuity? Frame the underside of the opening with (2) angles and run them back to the column strips on each adjacent side?

1) Fabricate a reinforcement thingy from four steel plates, one each side of the opening if reinforcement is required in both directions. I don't see any advantage to using angles and they would be more obtrusive spatially.

2) Run the steel plates far enough past the opening on all sides such that you're happy that it effectively laps with the disrupted rebar.

3) Install some headed welded studs on top of the plates.

4) Cut cores into the slab where the studs will eventually be inserted.

5) Lift the reinforcement assembly into place and non-shrink grout the cored holes from the top, securing the studs.

6) Fireproof the reinforcing assembly as necessary.

Alternately, FRP reinforcement can be used. Either way, it gets costly and it's best to strive for a "do nothing" approach if possible. Another weapon in your arsenal is moment redistribution along the span. I believe that's already baked into the cake with the direct design method however.



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.

 

[StrEng007]

KootK,
Thanks for the information. That reinforcing option is nice and clean.

 

[KootK]

You're most welcome. Let me know if you decide to attempt that option. I've got a canned detail but it would probably take me 20 min to dig it up and post it. I don't want to invest the time unless I know that it would be of meaningful benefit to you. You know how it is in the world of billable hours...

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.

 

[StrEng007]

I understand what you mean... you can hold off on the detail for now. Thank you

 

[jike]

We prefer to core the corners of the opening and then sawcut the slab into the cored holes. This eliminates "saw run-by" which can be significant.