cut opening in exist. flat slab elevated floor 2

[TedEP]

Owner wants to cut 8' x 8' opening in 10.5" tk exist. flat slab elevated floor. It occurs within intersection of the middle strips. Bays are 25' x 20'. As far as design aids, all I have been able to find are articles that are too general in nature. Yes, I know reinforcing will be required. I need references on how to design the reinforcing.
Thanks for everyones help.

 

[msquared48]

Is this PT?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

Why do you think reinforcing will be required? What will be placed in the opening?

BA

 

[frv]

BA.. the OP mentioned that the opening will be 8' wide and located within the middle strip. It's a pretty safe assumption that reinforcing will be required, IMO.

 

[Lion06]

I always thought the intersection of the middle strips was the best place to locate an opening.

I will be interested to read how to go about reinforcing the slab, though.

 

[BAretired]

Why would that be, frv? There is a substantial reduction in dead and live load more or less centered in the span, so I would think there is a good chance that there is enough reinforcing now unless something heavy is contemplated to be added in the vicinity of the hole.

If the slab is post-tensioned, it would be necessary to tie off any tendons coming in contact with the edges and re-stressing.

It would make sense to cut the opening a bit larger than needed and provide a steel channel framing around the periphery and anchored to all four edges.

BA

 

[hokie66]

Agree with BA about the panel in which the opening is located. An 8' square opening in a 10' x 12.5' M-M location is not likely to be a big problem for that panel. You would want to assess where the top bars stop to see if the edges are adequately reinforced.

You have to check the adjacent panels, as continuity will be removed. Need to analyze the remaining part of the slab. Hope you have the drawings.

If it is bonded post-tensioning, restressing would not be an issue, but if unbonded like used in the US, restressing would be required.

 

[TedEP]

msquared48, BAretired, frv, lion06, hokie66 - Thanks for the input.
The slab is not PT. The area encompassed by the opening will be a clear opening and will not deliver any load to the opening edges. The remaining area outside the opening will not have any unusual loadings.Yes, I do have the drawings.

After reading the replies, it makes sense reinforcing may not be required. Isn't the purpose of the middle strip reinforcing steel to deliver the middle strip load to the column strips? Thus, if there is no load (since there is no slab due to 8' x 8' opening), then there is no loading to deliver to the column strip?

BA - Was your comment "It would make sense to cut the opening a bit larger than needed and provide a steel channel framing around the periphery and anchored to all four edges" only applicable if the slab was PT (which it is not)? If you didn't just mean it for PT, then what would the design of the reinforcing be based on?

 

[ishvaaag]

Analysis will show if you need to reinforce or not. One of the ACI special publications remarked that the difference in required reinforcement for a solid slab and the same with one reasonable opening was not as bad as one could fear. And in the cases I have met whilst working, such has been in general the case. It is very well placed the observation of hokie66 on loss of continuity (as of course on retension if were posttensioned work).

If in your reach, providing an edge member (that completely replacing structurally the voided part was the -if soundly made- conservative old practice) may help to ensure you have better behaviour there, but now without such complete overtaking of the lost slab by the edge member, just what revealed it is needed.

 

[BAretired]

TedEP,

If there had been a load placed around the periphery of the opening, the channel would be designed to carry that load over a span of eight feet. As there is no such load, channels are redundant.

However, I would be concerned about overcutting at the corners of the opening such as you would expect from a circular saw. Overcutting should be avoided.

BA