Special reinforcement around small openings @ concrete roof slab

[dcceecy]

The concrete roof slab will have 1'x2' and 1'x4' openings.

The slab is 8" thick two-way slab supported by concrete beams. the typical reinforcement is #4 @12" OC each way at bottom. some additional reinforcement at top along beam.

So around those openings, what kind of special reinforcement are needed?

some other engineer designed the slab but now the client wants openings. Any suggestions?

Thanks

 

[Ltwine]

"Anyway, surrounding with orthogonal and diagonal reinforcement is structurally sound practice that only has the problem of poor constructability, due to excess of rebar around small holes."

I Agree.

 

[GalileoG]

Interesting discussion.

I am curious though when a "small opening" becomes a "big opening" that requires consideration in the analysis stage. I understand for small openings, the steel blocked by the opening is placed on both edges of the opening. That seems simple enough, but when would I know that I have a large enough opening that I have to take into account the opening when I am determining my bending moments when performing my analysis.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[Ltwine]

I think ACI has something on opening around support regions. But judgement could be the most important factor.
Depending on the location, the strips on sides of the opening have to rely on adjacent members to pickup the loads, what is the picking up capacity of those members, what are the local effects...quite a few things to look at. There maybe rule of thumb I don't know about. I wouldn't worry about 1'x1', or 2'x2' openings, but for a small panel, these could be problemic too.

 

[miecz]

BARetired-

I like our idea of using the orthogonal bars. They are there anyway. It would relieve the congestion, allow for better concrete placement and so result in a stronger corner. I will just increase the size a wee bit to replace the diagonals.

Clansman-

Sometimes I see slabs designed by others and submitted to me for review that are riddled with large openings. These are always designed as though the openings weren't there, with additional bars added to the sides of the openings. These get a "revise and resubmit".

 

[BAretired]

miecz,

I don't think you need to increase the bar size to replace the diagonals. A #5 bar has an area of 0.31 in².

If placed diagonally across a corner of the opening, it provides 0.31 in² at right angles to a potential crack.

Two #5 bars placed orthogonally provide 1.414*0.31 = 0.44 in², not only at right angles to the potential crack but in every other direction as well.

I conclude that 4-#5 bars placed orthogonally, one each side of the opening are more effective in resisting corner cracking than 4-#5 bars placed diagonally, one at each corner of the opening.

BA

 

[asixth]

BAretired,

In your experience, have you noticed better performance using orthogonal bars as opposed to diagonal bars for suspended slabs. We have always called up 2-N12 diagonal bars 1500 long around each re-entrant corner (2 x 1/2" bars (72ksi) 5' long) for slabs on ground and typically provide orthogonal bars for suspended slabs.

All,

When the opening is located in lowly-stressed regions of the slab (i.e. regions common to both middle strips), I would simply place the additional reinforcement either side of the opening that was affected by the opening.

If the opening was a large opening in a highly stressed region of the slab (and I would classify a large opening as an opening which affects more than 1-bar), than I would definitely perform an analysis to ensure that the reduced cross-section can deal with the increase stresses.

dcceey,

Has the slab been poured and the client wants to core these openings.

 

[BAretired]

asixth,
No difference in performance noted. Both systems work fine. It is largely a matter of preference.

BA

 

[dcceecy]

The concrete has not been poured yet. But it will be a change of order. So cost will be more if we make too much change.

 

[hokie66]

Back to the original post, I hope this roof slab has a membrane. The typical reinforcement is quite light and will not control restraint shrinkage cracking.

 

[rowingengineer]

Asixth,
what if the slab was 400thick? would you still supply 2-N12?

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[asixth]

No, would only call up 2-N12 trimmer bars for slabs on ground which are reinforced with mesh. Don't generally call up 400 thick slabs on ground.

 

[rowingengineer]

ok Asixth so you may not have done many container slabs in the middle of nowhere, but not meaning to be picky but the point is that 2-N12's are not sufficient for every situation. I doubt 2-N12s would have much impact on a thicker slab, I believe the house code specifies (AS2870)3-N12’s as a minimum by the way.

I think the reinforcement whether diagonal or orthogonal should be in ratio to the situation, we could argue semantics about the differences but at the end of the day you need reinforcing to keep the crack small, this reinforcement needs to be or a size to match the problem. The thicker the slab the more reo required.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it